The dressing of the pads can be done by one of several methods. The methods for solder removal include the solder wick or solder vacuum techniques. The solder vacuum process has less chance of damaging the pads while being somewhat slow. The braid technique, while faster, can lead to lifted pads or otherwise damaged solder mask areas if not done properly or with the right size of wick.
After solder removal from the pads of the BGA site cleaning is performed or not performed depending on whether a water-soluble or no-clean paste flux has been used. Cleaning parameters would follow those used in the initial assembly of the product.
Solder must be selectively applied to the PCB site of the BGA in order to attach the solder balls of the BGA to the PCB. Solder paste is preferred as it provides "tacking" functionality for the device placement. In addition it provides compensation for height differentials while assisting to minimize warpage of the PCB. Numerous studies have also concluded that the application of solder paste in BGA rework is critical in obtaining a highly reliable product. There are several methods which can be used to apply solder paste as they are explained in detail below:
SELECTIVE SOLDER PASTE DEPOSITION METHODS
"Course: BGA Rework-A Primer-Site Prep"
While there are many ways to replace the area array devices using flux, solder paste, or solder bumps, the four most common methods of solder paste for rework are discussed below: Dispense The technology of dispensing solder paste has made many advancements and improvements. Most of the advancements have occurred in the fully automated systems and include changes to the needle configuration, the pump mechanism, and the programming. Although it is possible to manually dispense solder paste with a syringe and needle, it would result in too many inaccuracies and excessive cycle time. The use of semi-automated dispensing systems which control the quantity of paste deposited by adjusting air and timer settings provides greater control over the paste quantity, but will not improve accuracy. A fully automated dispensing system usually consists of an X-Y table and a computer controlled dispense head. These systems are programmed with the coordinate data for the locations requiring solder paste and then sequentially dispense the paste at the desired locations. They are very accurate in the paste deposit placement providing very consistent paste deposit quantities. However, one of the drawbacks of using a dispensing system is the requirement to use a solder paste with a lower metal content. Higher metal contents (87%) usually result in frequent clogging of the nozzle. With this lower metal content, the paste deposit is more prone to slump and separation than higher metal contents. Applying paste to the same locations on many boards is fairly easy due to the programming of the system and fixturing of the PCB. The major drawback to the dispense system is throughput. For larger pin count BGAs, cycle time is limited by the maximum speed of the pump and may take as long as five minutes when set-up time is included. From a rework point of view, the program and set-up time must be included for every different BGA to be processed.
Metal stencils for depositing solder paste have been in use for decades. Metal stencils for selective (a particular device, not the entire PCB) solder paste deposition have been in use almost as long. One of the greatest yield detractors in the SMT manufacturing process is the solder paste application process and the same holds true for the solder paste application process for rework. The process steps for using a component specific stencil for selective solder paste deposition for rework and a full PCB stencil for the original SMT manufacturing are almost identical. The major difference between the original stencil printing process and the selective solder paste printing process used for rework is the amount of process control. During the original solder paste stenciling process there are machine controls for the squeegee speed and pressure, snap-off distance, removal of the stencil from the board, and in some cases stencil alignment. The selective solder paste stenciling process is very dependent on the skill level of the technician to manually control these same variables. The metal stencil printing process begins with the stencil being aligned with the land patterns on the PCB, then the stencil must be held in place in a manner that ensures intimate contact with the PCB. Next, a squeegee is used to roll a bead of solder paste across and down through the apertures of the stencil. Finally, the stencil is lifted from the PCB surface resulting in finely defined solder paste deposits. If the stencil printing operation runs perfectly, the cycle time is very fast. Cleaning the stencil is a critical process step to ensure the stencil will provide acceptable results on the next BGA to be reworked. One variation on the metal stencil process is to print the solder paste directly onto the solder balls instead of the BGA lands on the PCB. This method provides the advantage of printing solder paste where placing a stencil on the board would be impossible due to space considerations.
Flexible Removable Stencils
A relatively new material/process for deposition of solder paste is the use of flexible solder paste stencils. These stencils are laser cut from a polymer film with a residue-free adhesive backing that allows for easy removal. The first step to using the flexible stencil is to remove the paper to expose the adhesive backing. The stencil must then be manually aligned with the land patterns on the PCB and firmly pressed down in place on the PCB surface. A squeegee is then used to roll a bead of solder paste across and down through the apertures of the stencil. The stencil must then be carefully removed from the PCB surface. There is no cleaning step as the stencil is disposable
An alternative method of stenciling solder paste on the PCB is to use a stencil that remains in place on the site location and becomes an integral component of the PCB. The stencil material is a polyimide film with a high temperature adhesive covered with a paper backing. This material combination has been used on PCBs for many years now in the form of bar code labels and Kapton tape. This material is available in the standard formulation or in an ESD safe variety. Like the removable stencil, the semi-permanent stencil is laser cut and can be provided in a number of different configurations. When using the semi-permanent stencil, the paper backing material is removed exposing the high temperature adhesive. The stencil is then manually aligned with the land patterns on the PCB and pressed into place. Again a squeegee is used to roll a bead of solder paste across and down through the apertures of the stencil. At this point the paste application process is complete. The stencil is not removed from the PCB and therefore no stencil cleaning step is required
The following comparison chart compares and contrasts the various BGA rework stenciling technologies:
Comparison of (3) Most Common BGA Rework Stenciling Technologies
|BGA Rework Stencil Techniques||StencilQuik™ Permanent Stencil||Flexible, Plastic, Polymer Mini-Stencil||Traditional Flat or Formed Metal Mini-Stencil|
|Total Cost for Use||Lowest||Moderate||Highest|
|Damaged Solder Mask Repairs Required||No||Yes||Yes|
|Stencil Cleaning Cost||None||Moderate||High|
|Stencil Material Cost||Lowest||Moderate||High|
|Equipment Impact||For larger pitch components the need for high end rework systems may be eliminated||None||None|
|Operator Skill Level Requirement||Low||Moderate||High|
|Insulation in between balls||Excellent-StencilQuik™ acts as insulator between balls 10X increase in surface resistance||Adequate if site prepared properly||Adequate if site prepared properly|
|Volume Consistency of Paste Deposition||Fair - Dependent on stencil/circuit board flatness and operator squeegee skills.||Good - Flexible polymer with adhesive sealed apertures ensures consistent volume.||Fair - Dependent on stencil/circuit board flatness and operator squeegee skills.|
|Circuit Board and/or Device Planarity Requirements||Circuit board and stencil must be very flat.||Non-dependent on circuit board or component flatness.||Circuit board and stencil must be very flat.|
|Print Registration Accuracy||Excellent||Fair - Stencil must be carefully taped to circuit board surface||Fair Stencil must be carefully taped to circuit board surface.|
|Risk of Component Damage||None||None||None|
|Component Site Access||Good - Component perimeter clearance .025" - .075" typical.||Good - Component perimeter clearance .025" - .075" typical.||Fair - Component perimeter clearance .075" - .150" typical.|
|Stencil Maintenance & Cleaning Requirements||NOT REQUIRED. Stencils are affixed to the circuit board underneath the BGA||None - Stencils are disposable after single or multi-use.||Moderate - Flatness must be maintained and apertures well cleaned between uses.|