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In Solder Rework/Repair, Solder Training and Tools

Ask The Expert

 

Kris Roberson - BEST Expert Resource

"BEST's "Ask the Expert" corner, features helpful, educational and timely answers to some common and not-so-common questions.

"If you would like to see your question answered, you can email us at This email address is being protected from spambots. You need JavaScript enabled to view it."

 

 

Our company is experiencing major difficulties with the older 2 mm server boards. No matter how much heat we apply with a heat gun to the surface substrate of the PCB, we are not able to achieve sufficient semi wetting in the plated through hole to allow extraction of the solder with the desoldering unit. The thick server boards dissipate heat so rapidly that we cannot desolder the PTH's in order to remove and replace the defective components. ANSWER

When I try to tin my solder tip, the solder beads up and will not wet to the tip. What's wrong and how do I fix it? ANSWER

What is the best method for removing a small part with flat lug leads? ANSWER

A question with several parts.
1. Which is better at withstanding mechanical twist (Flexural loads), 132 pin MLF or 144 pin BGA?
2. Which of the above components typically has better Lead Free Reliability?
3. Which of the above components is typically better for ease of board singulation?
4. This product could potentially be produced in the 100,000 assembly range. ANSWER

Do you need a separate lead-free soldering station dedicated to lead-free soldering? ANSWER

I can't find the BGA I need in a tin/lead solder ball alloy. Can I use a lead free component alloy balled device in conjunction with a tin/lead process circuit board? ANSWER

How do I clean a rosin based flux from my board? ANSWER

Is there an industry standard regarding acceptability requirements of using lead free components in a leaded process? If not, has there been any testing performed to qualify this process? ANSWER

What is a Mesh Splice? ANSWER

Can a non-IPC certified individual posses the IPC-A-610 publication? After reading my Policies and procedures, I believe it is intended for someone who is seeking certification. Then once certified, the CIT or CIS can use the publication for inspections and so on. ANSWER

Can lead free solder be used to repair "old" solder joints (which were originally made with standard tin-lead solder)?

Can the new lead free solders be mixed with older tin-lead solder (in the same connection point)? --If the answer is 'No', what is your recommendation for repairing these older tin-lead solder joints? ANSWER

J-STD-001D paragraph 4.2.2 states that if the humidity drops below 30% the manufacturer shall verify that the ESD control is adequate. Does this mean that the humidity cannot drop below 30% and we have to implement environmental controls to keep that above 30%, or does this mean we have to take (other) special precautions? ANSWER

Coplanarity: We know that it is a defect condition, but what is the acceptable rework procedure? I think I remember you saying not to hold the part down and solder it. ANSWER

We have a potential customer that said it is normal practice to preheat PCB boards before SMT Automated assembly. I have never encountered this. Is it normal procedure? ANSWER

We build class 2 medical devices. Some of our vendor supplied products are now being tinned with Sac305. Our quality department has made the determination that mixing Sac305 with No-Clean 245 Solder Wire, .015" dia., Core Size-50 Sn63Pb37 (lead solder) is acceptable. ANSWER

However, there are issues with our products now since this change has occurred. Our solder joints are coming apart from the surface mount lands. At times, no pressure is applied to the product and the joints break apart. Is there anything in the IPC standards that states that you can't mix Tin Lead and Lead Free alloys? What else could cause this problem. ANSWER

What's the best procedure and tool for skimming a small bench top solder pot? ANSWER

We are having an internal debate regarding soldering irons and calibration. One faction says that on a regularly scheduled basis a soldering iron should be checked for tip to ground resistance and verify that the tip temperature is close to what the soldering station control dial indicates. The other faction claims that checking for tip to ground resistance is adequate.

I would be interested in your thoughts on this. Is there any industry standards that would give some guidance or specifications regarding solder iron calibration? ANSWER

I have a 6 layer pcb top layer slightly burned. I need to repair this pcb per customer. do I remove burnt fiber then clean this area and then do a very small fiber fill? and is there an IPC # for this repair. ANSWER

We use Alpha OM338 paste flux. Since it is No Clean flux should the flux be cleaned off after thru-hole hand soldering or any type of hand soldering? This has been a great debate here. ANSWER

I have a question for you. I have a Class three board that has a land and trace connected to a via. Could you please tell me what the IPC # would be on this. Would it be 7721/4.5.2 ? I would be using a film adhesive. ANSWER

I don't know if this is a valid question for your website. What would you recommend as far as equipment is concerned to do replacement of 144-pin surface-mount ICs? SiI0680 is the IC and they are on some server motherboards that need repairing. The IC's pins are fairly close together (I would say 25 mils) and are on all 4 sides. We have identified several possible pieces of equipment, however, they are in the $5000 price range. It would seem that a more cost-sensitive solution is available. These pieces of equipment do have the underneath of the board warming, but is a handheld device with the proper 144-pin attachment a suitable alternative? ANSWER

What gauge wire is required to ensure an ESD bench is properly grounded? Is there a Spec that is published that covers the Earth Ground to Work Bench requirements. ANSWER

I have a question for you. I have a Class three board that has a land and trace connected to a via. Could you please tell me what the IPC # would be on this. Would it be 7721/4.5.2 ? I would be using a film adhesive. ANSWER

Is the dwell time to achieve a good solder joint in the standard? I recall that it should be 2-5 seconds, but I cannot find it in the standard. Also, for a SN63 solder what is the recommended tip temperature? I would say around 700 degrees F. Is this anywhere in the standard. Engineers has 775 degrees specified. I think this may be to hot. The application is to solder a flex circuit to connector. ANSWER

What can you tell me about 'black pad' when it comes to BGA failure? I need to understand the cause of this dilemma, and thought I could ask the 'SME' subject matter expert. ANSWER

Is there any guidance/expectations as to how much coplanarity variation is ‘reasonable’ and expected? If a part coplanarity spec is .006, does this become the min solder paste thickness? Is there a standard that defines coplanarity expectations for various types of parts? ANSWER


 

 

 Our company is experiencing major difficulties with the older 2 mm server boards. No matter how much heat we apply with a heat gun to the surface substrate of the PCB, we are not able to achieve sufficient semi wetting in the plated through hole to allow extraction of the solder with the desoldering unit. The thick server boards dissipate heat so rapidly that we cannot desolder the PTH's in order to remove and replace the defective components.

Put down the heat gun and step away from the circuit board.

Hand held heat guns are useful in a very limited range of applications. Pre-heat of a large, thick circuit board is NOT one of those applications.
A large, thick server board is definitely going to dissipate a great deal of heat. As you have figured out, this is why you are having difficulty removing a Plated Through-hole component from your board. Pre-heat is going to be critical to the process. However, a heat gun concentrates the heat in a small area. The heat is then dissipated through out the board. Unfortunately, in order to get the entire board up to the required temperature (around 140o C – 160o C), the localized area when the heat gun is applied will be damaged.
So what is the solution?
The solution is a board pre-heater. Table top models are available from a variety of manufacturers. A pre-heater is designed to heat the entire assembly at the same time. A time/temperature profile should be made to ensure the proper heat ramp and soak time. To develop a T/T profile, attach thermal probes to the top and bottom of the board at various locations. Monitor the temperatures and adjust the heater settings as necessary to achieve a 2 – 4o /second ramp up. When all the probes indicate a steady 140 – 160o C, the board is ready for component removal.
If a board pre-heater is simply out of the question, an alternate method is to run the assembly through a reflow oven. Adjust the oven zones to achieve the required board temperature. Using heat resistant gloves remove the board from the oven and quickly rework the component. The major drawback to this method is the limited time for rework. As soon as the board is removed from the oven the board begins to cool.
By using either a board pre-heater or a reflow oven, large, thick boards can be reworked and repaired as easily as any other assembly.
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 When I try to tin my solder tip, the solder beads up and will not wet to the tip. What's wrong and how do I fix it?

The description of your difficulties sounds like tip oxidation. When a tip heats, the surface will oxidize (essentially rust) which will not take solder. This barrier forms very quickly in high heat applications (over 700 degrees F) and when using lead free solders.
So, what can you do to fix the problem and keep it from happening?
To tin an oxidized tip initially, use a brass brush to rub the oxidation off of the tip.
a. Heat the iron to around 600 degrees F (315 degrees C) and then brush the entire surface of the tip with the brush.
b. Tip the tip as usual with solder (Tin-lead or lead free)
c. Wipe the tip onto a damp sponge and repeat the tinning and brushing process until the oxidation is completely removed.
d.If, after the previous steps, the tip will still not tin, there are two other options.
i. Use a commercially available tip tinner. These are small containers of compound which, when heated, will remove the top layer of the tip metallization and reapply a new layer. Use this ONLY as a last resort.
ii. If nothing else works, the tip may be oxidized beyond repair.
2.Put the tip away "dirty"
a. After each soldering operation, make sure the tip is completely tinned with solder before placing it into the holder. The extra layer of solder will oxidize, but will be wiped away from the tip on the next use.
3. Use the lowest possible tip temperature.
a. One misconception commonly held in soldering is that higher temperatures means faster work. This is not true. Higher temperatures when they are not needed mean shorter tip life, higher oxidation rates and more possibility of damage to your soldered assembly.
b. Start with 600 degrees F (315 degrees C) and adjust only when needed.
4.Turn off the iron between uses.
a. Depending on the design of the iron, many tips heat within 5-10 seconds of powering on. Turning off the iron between each use will extend the tip life.
Keeping the heat low, tinning the tip between use and turning off the iron when not in use will extend help extend the tip life and reduce your consumables cost for your business.  
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 What is the best method for removing a small part with flat lug leads?


A component with flat lead leads can be treated the same as in the SOT or SOIC by IPC standards. While the component may be slightly taller than the typical SOT or SOIC the principles remain the same.

The following procedures are from the IPC- 7711

SOT Removal
Equipment required
Soldering system
Removal tips
Tweezer handpiece

1. Remove conformal coating, if any, and clean work area of any contamination, oxides or residues.
2. Install removal tip into tweezer handpiece.
3. Start with tip temperature of approximately 315° C and change as necessary.
4. Apply flux to all the lead/land areas.
5. Clean residue from tips with damp sponge.
6. Tin tips. ( Authors note: if you notice very little solder on the leads at this point you may add solder as necessary.)
7. Lower tips over component and squeeze Handpiece contacting all leads with tips.
8. Confirm solder melt of all joints and lift component from PCB.
9. Release component from tips by wiping on a heat resistant surface.
10. Prepare lands for component replacement.

SOIC Removal – Solder wrap method
Equipment required
Soldering system
Removal tip
Chisel Tip

1. Remove conformal coating, if any, and clean work area of any animation, oxides or residues.
2. Install Chisel-Tip into soldering Handpiece.
3. Start with tip temperature of approximately 315° C and change as necessary.
4. Tack solder to one of the corners of component leads using soldering Handpiece with Chisel Tip installed. (Author's note: Wrap the solder wire around the component.) Repeat for the other side.
5. Replace Chisel Tip and soldering Handpiece with removal tip.
6. Remove all are solder from the tip and thermal shock tip with damp sponge.
7. Tin bottom and inside edges of the chip with solder.
8. Lower tip over component contacting all leads with the tip.
9. Confirm solder melt of all joints and lift component from the PCB. ( Surface tension of the tip should lift the component from the board. If this does not occur, use of tweezers to lift the component is optional.)
10. Release the component from the tip by wiping it on a heat resistant service.
11. Re-tin tip with solder.
12. Prepare lands for component replacement.

Between these two procedures you should be able to easily remove a flat lug lead component.

Note: It is possible to use the tweezers for the larger components as well. Simply follow the same procedure for the SOT above.
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 A question with several parts.

1. Which is better at withstanding mechanical twist (Flexural loads), 132 pin MLF or 144 pin BGA?
2. Which of the above components typically has better Lead Free Reliability?
3 .Which of the above components is typically better for ease of board singulation?
4. This product could potentially be produced in the 100,000 assembly range.


A number of studies have been done along the lines of mechanical twist or Flexural load reliability between MLF and BGA components. The conditions of the study typically range between 5° and 9° of twist. After 100,000 flexing operations, neither the MLF or BGA components showed significant loss of adhesion or solder joint degradation. These results show that board singulation by either a board press/ die cutting operation or through a router operation would both be feasible options.
If size or real estate on the board is a major concern, the MLF is definitely the way to go. The MLF will typically use much less real estate than a comparable BGA.
As far as ease of manufacture and lead-free reliability, BGA is shown to be very easy to work with and have a long established history in production as compared to the MLF. If the MLF is chosen think carefully about the board pattern (size of pads, thermal vias in the ground, size and masking) and paste printing is critical (for example "window pane" apertures for the stencil).
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 Do you need a separate lead-free soldering station dedicated to lead-free soldering?

The short answer to this question is, "No". Let me prove my point by pointing you to a study BEST Inc. completed which answers this question.
To summarize the study, BEST found that a series of three "rinses" reduces the lead content of a tip sample to a level below the requirement for ROHS compliance. This is a savings in new equipment cost, depending on the size of the company and the number of stations involved in soldering.
So, how is the "rinsing" accomplished? Just like using soap to remove dirt from a person's hands, using lead free solders to rinse a solder tip dissolves and removes the lead from the tip. Much the same way as soap holds the dirt in suspension until it can be removed from a person's hands by water, the tin dissolves the lead that was previously deposited onto the solder tip and holds that lead in suspension until it can be wiped from the tip.
To accomplish the rinsing is a simple series of steps.
1) Mount and heat the appropriate solder tip.
2) Apply an ample amount of lead free solder to the heated tip. (What is ample? Enough to cover the entire heated area of the tip.)
3) Using a clean, damp sponge, thoroughly wipe the solder tip.
4) Repeat steps 2 and 3, two more times.
5) That's it!
This rinsing process can be used on any tip previously used for leaded soldering.
During the transition time that all companies will or are facing between leaded and lead-free operations, rinsing means that the same stations can be used interchangeably to complete the rework process.
For more information on rinsing, you can find the full report on the BEST, Inc website here
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 I can't find the BGA I need in a tin/lead solder ball alloy. Can I use a lead free component alloy balled device in conjunction with a tin/lead process circuit board?

Using lead free components in a tin/lead process is acceptable and can be processed.
However, since the balls on the BGA contain no lead, there will be a different thermal profile required in the rework process. You will need to conduct tests and evaluations to be sure that the proper heating, cooling, and reflow times are being utilized.
A basic tin/lead thermal profile should increase the temperature (ramp) of the board, component, and solder no more than 2-4 degrees per second. The peak reflow temperature can be anywhere from 190o to 225 o C.
I suggest referring to the solder paste manufacturers' data sheet. A lead free thermal profile has the same ramp of 2-4 degrees per second. The peak reflow range increases however to 245 o to 260 o C. This increase in temperature can cause damage to the tin/lead components which have been designed for the lower peak temperatures.
One option is to use a selective soldering process that will not affect the surrounding components on the lead free BGA after the tin/lead components have been placed and reflowed. A second option is to use a slightly higher than tin/lead thermal profile. The BGA balls have a melting point of 217° C. A profile of 226 o to 228 o C for 45- 60 seconds should be sufficient to allow full melt of the BGA balls while not seriously damaging the tin/lead components.
Unfortunately, the tight temperature range can be very tricky to obtain and maintain.
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 How do I clean a rosin based flux from my board?

Rosin based fluxes are still in use because they do a good job of cleaning surfaces and allowing for solder to wet solderable surfaces well. Any remaining residue from the fluxing and soldering process must be removed to avoid dendrite growth and oxidation.
A draw back to the use of rosin flux is the need for an aggressive cleaner to properly remove any residue left on the board after the soldering process. Check with the manufacturer of the flux being used for the proper solvent for each flux used. Once a determination has been made as to the correct solvent the cleaning can begin.
For a hand soldering operation, there are some simple steps that will ensure good cleaning.
1) Isolate the area. Some solvents can be very detrimental if they are allowed get onto and under the surrounding components. Use a static and lint free wipe to surround the component and area where the flux was used.
2) Use an acid brush or other static free, stiff bristle brush to scrub the area. Use small, circular motions to spread the solvent to the area being cleaned. Scrub for a few seconds (a 10 count is sufficient) to ensure that the solvent has a chance to bond with and break down the remaining rosin residue.
3) Immediately wipe the area. Using same wipe utilized in isolating the area, quickly wipe the area dry. As stated before, solvents left on the board can be as detrimental as flux left on the board.
4) Use a compressed air source to blow out solvent that has seeped under the components. A central compressed air source (factory air) or cans of compressed air are both sufficient. Remove any dislodged solvent with a wipe.
5) Inspect the area for any remaining flux or solvent and repeat the above steps as necessary until the area is completely free of flux, flux residue, solvent or solvent residue.

With these simple steps, the person performing the work can ensure an acceptably clean operation.
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 Is there an industry standard regarding acceptability requirements of using lead free components in a leaded process? If not, has there been any testing performed to qualify this process?


To answer your first question, there isn't a separate standard addressing the use of Lead-Free components in a Tin-lead process. All IPC standards apply to both tin-lead and lead-free. The only difference between Tin-lead and Lead-free acceptability is cosmetic (See IPC-A-610 Section 5)
To answer your second question, Lead free components have been used in leaded processes for much longer than you are probably aware. Many manufacturers have completely switched to Lead-free finishes in order to comply with the RoHS and WEEE directives. To avoid duplicate processes, one for lead-free and one for tin-lead, most components on the market today are RoHS and WEEE compliant. This has actually cause problems for some U.S. Government and Military contractors who have had to send parts out to be tinned with Tin-lead solder due to military demands.
However, in general, Lead-free components will solder very well with Tin-lead solder. Numerous studies have been done and are available on the internet. www.greensupplyline.com and www.smt.net are just two websites which have published articles on this subject. The caveat to this is that there may be difficulties in soldering Lead-free BGA's with tin lead solder. The higher reflow temperatures required for some Lead-free alloys is not compatible with the requirements of some lead-free BGA's.
The best recommendation is to check with the individual manufacturer of the components in question and ask about backward compatibility. The manufacturer should have all the information about the components and solderability.
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 What is a Mesh Splice?

The Mesh Splice is one of 4 IPC recommended methods of connection 2 wires. The 4 methods of wore connection in the spec are the mesh, hook, lap, and wrap techniques. This column will specifically address the mesh splice method as given in the IPC 7711A.
1. Install soldering iron tip
2. Start with tip temperature approximately 260oC and change as necessary.
3. Form the mesh splice by fanning the wire strands on both untinned wires into a cone shape.
4. Gently begin meshing the wires together a minimum of 1.3cm so that the strands interlace evenly and of equal length.
5. Twist the wires slowly using a slight pulling motion to restore the original lay of the wire. Do not over twist.
6. Select appropriate heating element to establish a heat bridge and minimize the effect of solder wicking beneath insulation. Solder as needed.
7. Clean and inspect
8. Position insulation sleeve / tubing over the spliced area, apply heat to shrink to a snug fit over the splice and wire insulation.
The splice is the only one of the 4 recommended methods in which the wires are NOT tinned prior to splicing.
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 Can a non-IPC certified individual posses the IPC-A-610 publication? After reading my Policies and procedures, I believe it is intended for someone who is seeking certification. Then once certified, the CIT or CIS can use the publication for inspections and so on.

IPC does not require membership or training to purchase or use any of the published standards. However, training in the appropriate standard is recommended.The training policies and procedures document states:
The goals of the IPC Professional Training and Certification Programs are to:
a. provide a standardized set of industry developed, recognized, approved and traceable training programs to enhance understanding and appropriate application of criteria in the standard.
b. provide an understanding of accept/reject criteria to enhance an individual's motivation and ability to consistently and correctly apply the technical discrimination criteria.
c. teach methods and processes to improve an individual's skills and ability to achieve acceptable workmanship.
d. teach methods and techniques how to use, navigate, locate and to apply the criteria contained in a document to the appropriate class or classes of production.
Completing an IPC training course will increase your understanding and ability to navigate and locate information in each of the published standards. IPC certification is recognized worldwide. The certificate is a personal and portable credential. That means that where ever you go, your certification goes with you.
For more information about available IPC training, please contact Katy Radcliff at This email address is being protected from spambots. You need JavaScript enabled to view it..
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 Can lead free solder be used to repair "old" solder joints (which were originally made with standard tin-lead solder)?
Can the new lead free solders be mixed with older tin-lead solder (in the same connection point)? --If the answer is 'No', what is your recommendation for repairing these older tin-lead solder joints?

Let's dive right in. To answer your first question, Yes, lead free solder can be used to repair any solder connection. The criteria stated for acceptability in IPC-A-610 are identical. The only differences between the two solder joints would be their cosmetic appearance. (Tin lead = shiny, smooth, "Hersey's kiss".
Lead free = grainy, gray, more round / higher contact angle). Techniques used to solder tin lead and lead free solders are the same. You can even use the same soldering iron and tips.
(see www.solder.net/technical/rinsing)
To the second question, I wouldn't recommend mixing tin lead and lead free solders. In fact, I wouldn't recommend mixing old solder with newly applied solder at all, even new tin lead with old tin lead. As solder sits and is used over the years is does degrade. The surface oxidizes and corrodes and the bond between component and solder slowly weaken. You will need to remove the old solder completely and apply new solder, whether you are using tin lead or lead free solder.
The procedure would be:
-Remove any existing solder using either a solder extractor (spring loaded or vacuum type) or solder wick.
-Clean the area with isopropyl alcohol.
-Apply new solder and flux as appropriate.
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 J-STD-001D paragraph 4.2.2 states that if the humidity drops below 30% the manufacturer shall verify that the ESD control is adequate. Does this mean that the humidity cannot drop below 30% and we have to implement environmental controls to keep that above 30%, or does this mean we have to take (other) special precautions?

J-STD-001D paragraph 4.2.2 states' "When the humidity decreases to a level of 30% or lower, the manufacturer shall verify that ESD control is adequate, and the range of humidity in the assembly area is sufficient to allow soldering in assembly materials to function correctly in the process, based on vendor recommendations or documented evidence of process performance. "
The comfortable range for human comfort is between 30% and 70% relative humidity. Above 70%, the humidity is uncomfortable for most people and increases the likelihood of oxidation and corrosion on the components. Below 30% relative humidity the danger is the increase of electrostatic discharge and electrical overstress damage. The standard makes clear that the manufacturer must have a static control system in place. Below 30% this system is even more critical than when the humidity is in the range of 30% to 70% relative humidity. That doesn't mean that when the humidity is between 30% and 70% that no ESD control system is needed.
Below 30% relative humidity a manufacturer needs to verify that the operators are using heal straps and/or wrist straps and are testing them at the documented and approved intervals. How often should they be tested? That depends on the ESD control system in use. Most recommendations say at least once per shift. I would take that one step further, and say they need to be tested any time the contact is removed from the person wearing the strap. If the operator goes to break and simply un-clips the ESD strap, the contact is not lost. If the operator removes the strap completely from their body, the operator needs to retest upon returning to their station and putting the strap back on.
The manufacturer also needs to verify but the static dissipative flooring is intact and properly operating. Any grounding straps between carts and the floor or machinery and earth grounds also must be tested and be verified of working properly.
Section 4.2.2 is not necessarily about the environmental controls, but rather verification that the manufacturers ESD practices are in place, are in use, and are sufficient to control electrical static discharge and electrical overstress damage that occurs from ESD.
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 Coplanarity: We know that it is a defect condition, but what is the acceptable rework procedure? I think I remember you saying not to hold the part down and solder it.

Coplanarity misalignment that causes an improper solder joint is a defect. Simply push the lead back into alignment to correct this condition. How much? How far can you bend it back? That is dependent on a company decision in coordination with the device manufacturer. I know some companies that specifically state that no lead correction is allowed. Others place limits on how much or how many leads can be re-formed.
The IPC-A-610D standard section 8.2.5.8, page 8-53 states that the defect isn't the bent lead itself, the defect the improper solder connection. I believe the intent here is to point out that if lead is bent so far out to cause an improper solder joint, it is a defect. It is not to set a limit on how far is can be bent or prevent the correction of a bent lead. For many package and lead types there are JEDEC guidelines on lead coplanarity as well as other lead criteria.
JSTD-001D section 7.1, page 18 states that leads shall be formed to the final configuration before mounting. If that means bending them back into the proper configuration, it is acceptable. The same section states that the forming shall be performed in such a manner as to ensure that there is no damage to the lead, lead seal, or body. Finally, Section 7.5, page 19 states that leads shall not be held under tension during solder solidification. However, this is ONLY a defect in Class 3 operations. There is nothing established prohibiting holding a lead down during solder solidification in Class 1 or 2 operations.
SO, the short version is that you CAN hold the lead down in class 1 and 2 operations. You CAN form the lead back into position as long and the forming method doesn't do any damage. The limit to the correction is a company decision.
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 We have a potential customer that said it is normal practice to preheat PCB boards before SMT Automated assembly. I have never encountered this. Is it normal procedure?

Preheating of SMT boards is not an unusual practice IF the boards are multi-layer or have difficult/large ground (thermal) planes attached to any components. There's nothing inherently wrong with pre-heating of any circuit board before rework or repair. In many cases preheat is a valuable and PCB saving step of the process. But, it can be an extra and unnecessary expense if the individual repair does not require pre-heat.
It's possible that the customer is concerned with the possibility of heat stress cracks in SMT capacitors. SMT capacitors have been known to fracture if they are soldered (removed or replaced) without preheat. However, the possibility is extremely remote. Even if the capacitor does fracture, they are less than 1 cent each and the replacement time is very minimal. The cost and time of pre-heating would be more than the time required to replace a fractured capacitor in the off chance one does crack.
The difficulties involved in preheating before automated placement of SMT components are:
1.Preheating will cause a shorter working life for the paste after the PCB is printed. You will get excessive slump (basically the paste won't stay in brick form as needed) and early activation of the flux.
2.The surface tension of the paste is going to be reduced causing the components to possibly skew or shift out of position during the assembly operation.
3.From a cost standpoint, you would need and extra oven conveyer/board handling of some kind to heat the boards. This will mean extra floor space, electrical use and time.
4.If the preheat is going to be a batch process rather than an inline process there will be additional personnel needed to transport and handle the heated boards.
5.Boards that are heated in a batch process will need a way to keep them heated to the proper temperature until processing.
6.The PCBs will not maintain heat for very long after pre-heat. By the end of the paste process most of the heat will have dissipated. Before the PCB even gets to the reflow oven the board will have cooled to nearly room temperature.
7.During preheating the board will expand. The expansion isn't a huge amount (depending on the PCB) this will cause a difference in placement position from the Gerber/board data. Again, it may not be more that a fraction of a millimeter but it might be enough to throw the placements off.
Unless there is a very specific reason (unique assembly need) preheating before automated SMT placement really is not required.
Having said all that, the customer is the most important entity in our business. If the customer would like a preheat of the boards before rework, it may be a good idea to quote the customer the price with (higher) and without (lower) the extra steps and time for the preheat operation.
Work with the customer and educate them in a way that they will be able to better weigh the options before contracting with you to build.
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 We build class 2 medical devices. Some of our vendor supplied products are now being tinned with Sac305. Our quality department has made the determination that mixing Sac305 with No-Clean 245 Solder Wire, .015" dia., Core Size-50 Sn63Pb37 (lead solder) is acceptable.

However, there are issues with our products now since this change has occurred. Our solder joints are coming apart from the surface mount lands. At times, no pressure is applied to the product and the joints break apart. Is there anything in the IPC standards that states that you can't mix Tin Lead and Lead Free alloys? What else could cause this problem.

There is nothing in the IPC standards that states you cannot mix alloys. It really depends on your process. Most components on the market are tinned with Lead Free in order to save money and time on the component manufacturing end. There are a number of studies that have been done (I don't have a direct link to any of them) that are testing the long term reliability of mixing alloys. Most have shown that, while not as reliable as straight tin lead, a mixed alloy joint should have a reasonable life expectancy in most cases. I don't think that the alloy mixing is what is directly causing the difficulties with your process. All of the parts we use in class and in our manufacturing operations are tinned with Lead Free and we do not have any difficulties with the mix of tin lead and lead free.
My suspicion is this…
When the lead free parts were introduced to the process there was no change in the Time -Temperature profile of your reflow process. What's happening is an incomplete reflow of the solder and the result is weak solder joints. A second possibility an contributing factor may be insufficient flux activity. While most flux compositions can be used interchangeably with lead free and tin lead not all will work with the same results.
My advice is to look at your TTP on the reflow oven and determine how much of an increase in temp or time is needed to achieve full reflow. Seconds, go through the qualification process (begin be checking with the component and solder manufacturers) to be sure that the flux you have in your process is sufficient for the needs of the mixed alloy.
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 What's the best procedure and tool for skimming a small bench top solder pot?

There's no real trick to skimming any solder pot, bench top or full size. Simple is best. If your solder pot has a paddle to clean the surface of the pot before use, collect the dross into one area with that paddle. Otherwise, use a stainless steel slotted spoon to collect the dross in one area and scoop out the dross to a heat resistance container. After it's removed, follow factory and local laws for disposal or reclamation. Be sure to use heat resistant gloves and a breathing mask available. Something as simple as an over the counter allergen mask will work fine unless the operator is particularly sensitive. If more breathing protection is needed there are any number of masks on the market which will protect the user from dross dust (but follow any safety and/or OSHA regulations which are in place at your facility.)
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 We are having an internal debate regarding soldering irons and calibration. One faction says that on a regularly scheduled basis a soldering iron should be checked for tip to ground resistance and verify that the tip temperature is close to what the soldering station control dial indicates. The other faction claims that checking for tip to ground resistance is adequate.

I would be interested in your thoughts on this. Is there any industry standards that would give some guidance or specifications regarding solder iron calibration?

The only real guidance is in J-STD-001 Appendix B. Remember that these are only GUIDELINES and not requirements. We suggest that resistance should be measured periodically. Operators should verify temperature controls through daily performance tests (does it work?). With the tip coatings breaking down more frequently with lead free high tin content solders, watch out as temperature readings can be misleading.
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 I have a 6 layer pcb top layer slightly burned. I need to repair this pcb per customer. do I remove burnt fiber then clean this area and then do a very small fiber fill? and is there an IPC # for this repair.

In order to accomplish this repair see our soldering tip "PCB Corner Repair" or our complete tutorial here
IPC7721 3.5.1 Base Material Repair, Epoxy Method is the referenced method for the repair of base board laminate.
The basics are (assuming that there are no interlayers impacted):
1. Use dremel to grind away all burnet areas
2. Undercut around edges of the ground out areas
3. Fill with epoxy/colorant
4. Wet sand when cured
5. Thin out epoxy for final coating on service
6. Wet sand when cured
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 We use Alpha OM338 paste flux. Since it is No Clean flux should the flux be cleaned off after thru-hole hand soldering or any type of hand soldering? This has been a great debate here.

We demonstrate the iso of ultra isopropyl alcohol when cleaning no-cleans. Other solutions are out there from vendors such as MicroCare =VeriCare(we do not sell any cleaners as we are purely a teaching institution.
There are a number of reasons for white residues from cleaning with IPA. Let me list just a few, and see if any of these hit a nerve:
Purity. There are many grades and purity levels of IPA. The best choice is to get a high-purity great of IPA, 99.9%+ water-free.
Storage. Alcohol is hygroscopic as it attracts contamination (including humidity) out of the air. If the alcohol has been sitting around a while, or stored improperly, it can actually dilute itself with water out of the air, as well as dust and other contamination. Alcohol should always be stored in sealed containers, out of bright sunlight, in cool, dry conditions. I Process. Many technicians use alcohol with a brush, to scrub the residues and loosen them from the PCB. The problem is that this process makes it very difficult, if not impossible, to rinse the contamination off the board. You can tell if some contamination remains simply by touching the board: if it's sticky, it's not clean.
Proper cleaning processes require wetting, scrubbing, rinsing and drying. Usually one is eliminated for some reason and causes problems.
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 I have a question for you. I have a Class three board that has a land and trace connected to a via. Could you please tell me what the IPC # would be on this. Would it be 7721/4.5.2 ? I would be using a film adhesive.

The R/R guidelines for IPC 7711/21 are just that-guideline procedures. When we perform a via in pad repair we generally:............
1.Remove the lifted pad cutting off at a convenient spot, namely a trace
2.Repair the via with high temp solder a small eyelet or even a fine wire
3.Replace the pad per the procedure you citied (although we would use an epoxy bonded pad repair circuit frame as the bond strength is greater)
4.Clean, test and inspect

Single Via After
Single Via After
Single Via Before
Single Via After

Above are some pictures of sample before vs after work!
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 I don't know if this is a valid question for your website. What would you recommend as far as equipment is concerned to do replacement of 144-pin surface-mount ICs? SiI0680 is the IC and they are on some server motherboards that need repairing. The IC's pins are fairly close together (I would say 25 mils) and are on all 4 sides. We have identified several possible pieces of equipment, however, they are in the $5000 price range. It would seem that a more cost-sensitive solution is available. These pieces of equipment do have the underneath of the board warming, but is a handheld device with the proper 144-pin attachment a suitable alternative? I would be using a film adhesive.

In our rework and repair business we would use the following to remove/replace a component such as this:
1. Bottom heater. We do not sell equipment but we use:
http://www.okinternational.com/product_convection/pct_1000
2.Soldering Iron with a small chisel tip and use the pt-pt or drag soldering method. See more on methods here:
http://www.solder.net/technical/tips.asp
The above solution is approx $2,000.00
Another method would be the removal using a hot air system like you are describing. We would recommend a bottom heater for LF soldering. This method is a more controlled but you have to use a lot more dexterity with the rework stencil. This solution is about $5-6K new and 1/2 as much used.
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 Engineering be just that has come out into manufacturing and removed 2 components that are on a lead free board IPC Class 3, with my leaded solder and iron. Is this board scrap or can I remove the solder, clean the lands and reapply new components with lead free solder?

You would need to "rinse" the board and the tips at least 3x. See http://www.solder.net/technical/tip21.asp The tin solubilizes the lead.
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 What gauge wire is required to ensure an ESD bench is properly grounded? Is there a Spec that is published that covers the Earth Ground to Work Bench requirements.

Regarding your question on wire gauge size for grounding workstations, the ESD association's ANSI 20.20 document does not reference a wire size, rather that the wire is strong and insulated. The document is a free download from the ESD Association's web site. From my memory, most of the suppliers of grounding cables for workstations use 18 gauge stranded wire. One thing to keep in mind is that you ground all the workstations and operators the same. If you are using electrical ground (conduit), which is preferred, then everything must be grounded the same. If you use earth ground (a driven ground rod) this must be your only way to ground. If you use both, there is the possibility of a serious potential difference that can injure operators, and damage assemblies and equipment.
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 Is the dwell time to achieve a good solder joint in the standard? I recall that it should be 2-5 seconds, but I cannot find it in the standard. Also, for a SN63 solder what is the recommended tip temperature? I would say around 700 degrees F. Is this anywhere in the standard. Engineers has 775 degrees specified. I think this may be to hot. The application is to solder a flex circuit to connector.

The JSTD doesn't specifically call out a temperature or dwell time for any soldering application. Appendix B does give tolerance ranges for Benchtop and hand soldering systems, but that is a range within "...the idle tip temperature".
For dwell time, again there is no specific time called out. Section 3.11 states, "Tools and equipment used SHALL be selected and maintained such that no damage or degredation that would be detrimental to the designed function of parts of assemblies results from their use. Soldering irons... SHAL be chosen and employed to provide temperature control...".
As a rule of thumb I use between 2-5 seconds dwell on the connection. BUT that depends on the thermal mass of the object being soldered, the thermal sensitivity (are heat sinks required) and the temperature set on the iron.
More specifics on rework cam be found in the IPC-7711B/7721B. None of the procedures are specific to Flexible circuit soldering, but most of the temperature settings for hand soldering in general range around 600 deg F (318 C).
My personal experience is that 600 F (318 C) is sufficiently hot to complete most of the solder connections required in hand soldering. SN63 typically reflows at about 360 F (183 C). 775 does seem to be a little hotter than necessary, especially for Flex-circuits. By increasing the tip temperature you are effectively reducing the working time your operator has before damage to the circuit occurs. My opinion is that if the tip is reduced to around 600 F (318 C) you will see a decline in overheated solder connections, damaged circuits and other damage. You should also see better solder connections because, as your operators learn that they have a little more time, they are more willing to leave the iron in place thus allowing sufficient flow time for the solder to completely cover the connection.
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 What can you tell me about 'black pad' when it comes to BGA failure? I need to understand the cause of this dilemma, and thought I could ask the 'SME' subject matter expert.

Black Pad is a corrosion of the nickel plating layer, causing it to appear in black in color. The problem is caused by the BOARD CONSTRUCTION and NOT BY ASSEMBLY.
When confirming whether or not black pad is present......
1. Make sure that the PCB is affected uniformly
2. Confirm the presence of a thinner nickel deposition measured by XRF/cross-section analysis;
3. No intermetallic layer formed during final soldering process -this can be confirmed via cross sectioning
The black pad problem seen with electroless nickel immersion gold (ENIG) causes premature solder joint failures and low shear forces after the board has been assembled. This makes it a troubling problem-as the problem area usually accepts solder and forms solder joints. However, the joints are weaker than the surrounding normal joints, making it difficult to test for, find, and eliminate.
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 The past few weeks we have had non- soldered leads on SMT multi-leaded parts due to the interaction between leads out of alignment (coplanarity) and the solder paste thickness applies by us (our screen thickness).

Example: Samtec .025 square connector leads found with chronic unsoldered leads on a board with .004 paste.  Upon inquiry, Samtec informed us they can have as much as .006 coplanarity and they recommend at least .006 paste thickness.
Example: Samtec .010 ‘L’ leads (20 per side) with chronic unsoldered leads on a board with .004 paste.  Samtec informed us they spec .006 coplanarity and recommend .006 paste thickness.
Example: SM transformer with 8 ‘L’ leads with random unsoldered leads on a board with .006 leaded solder paste.  We found random leads with a NoLead solder bump .007 to .012 on the bottom of leads from a supplier tinning operation (the NoLead solder bumps do not melt in our leaded temperature process).  Supplier has no coplanarity spec and there is an argument about if the parts can be rejected without a spec

Our design department does not know or control the lead thickness/co-planarity specs of our mixed technology parts/boards.
Our manufacturing engineering group somewhat arbitrarily chooses screen thicknesses based mainly on experience with solder bridges due to close lead pitch and ‘hope for the best’ with parts that have large coplanarity variations that are within tolerance.

Is there any guidance/expectations as to how much coplanarity variation is ‘reasonable’ and expected?
If a part coplanarity spec is .006, does this become the min solder paste thickness?
Is there a standard that defines coplanarity expectations for various types of parts?

Thanks for the good question re: gull wing coplanarity.
As a background IPC-A-610 8.3.5.8 states that if coplanarity impacts the formation of the solder fillet per the gull wing solder fillet requirements that that it is a defect condition for all 3 classes. Here are some suggestions……

  1. Coplanarity of the parts equal to or exceeding  0.003” I would rework
  2. I would institute incoming sample inspection for fine pitch components on new projects and component suppliers
  3. I would institute a 100 percent incoming lead integrity screening of components with a known incoming quality problem that has not been corrected by the supplier. The parts vendor should then be able to rework the leads and get them back in specification (tape and reel houses perform this service)
  4. Institute a requirement of component handling training of all personnel who handle components. The greater these are handled the greater the potential problem.
  5. Institute a requirement of trayed devices to come in full tray quantities. This eliminates the manual handling required of the components not in full trays.

In order to understand this problem I would recommend getting some data on the coplanarity of these “problem” components and performing a correlation to its impact on it being a yield deterrent. Get this from your tape/reel vendor. They should have the JEDEC part requirements for the type of part you are talking about.

I take it that you are seeing these defects via AOI post assembly?
We build a lot of stencils for fine pitched devices (you did not tell us the pitch of the connectors) and we NEVER see a modern board with 6 mil thick stencils. This is NOT the way to solve this problem rather it is a means to push it elsewhere (shorts).


 

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