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BGA Rework and Reballing Technique | BGA Stencil Rework Process
This BEST developed reballing technique for area array devices is the designed
for speed and fast turn-around of small to medium volume quantities. It is
used as other high volume tooling and fixturing is being designed and fabricated
for higher volume reballing. The stencils are custom made for your patterned
device on the BEST laser. After applying flux to the bottom of the part, a polyimide stencil
corresponding to the size of the balls and part pads is aligned over the part.
The properly sized solder balls are then poured into the apertures with the fixture keeping the
StencilQuik™ stencil affixed to and aligned with the part. The assembly is then reflowed, cleaned and inspected. It is the most time-consuming of the reballing techniques used at BEST.
This technique is one that is straightforward and is used when a moderate number
of the same part pattern reballing is required. The tooling and stenciling holds
multiple reballs.
Materials Required
- stencils
- Solder balls
- Tacky flux
- Reball fixture
- Reflow oven
- Stereo microscope
- Soft brush
Mechanical Fixture and Stencil Reballing Process Flow
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Prepare Device
The dressing of the pads can be done by one of several methods. The methods for the removal of solder from the package include the use of solder wick, solder vacuum and low temperature wave solder or solder pot 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. The use of a low temperature wave soldering machine or solder pot require the ability of the equipment to maintain temperature while making sure that you know the type of alloy that the balls are made from in order not to contaminate the solder pot.
After the balls have been removed and the site prepped it is good practice to leave some solder on the pads in order to make reballing easier. Clean the bottom of the package with isopropyl alcohol -do this after the balls have been removed as this will make cleaning up of the flux residue easier.
Be sure to inspect the solder mask in between the pads to insure that its integrity
has been maintained. This inspection should be performed using a stereo microscope
or other optical inspection tool. Make sure all of the solder balls have
been removed.
Choose Reballing Materials
Make sure that the fixtures and the miniature stencils are in useable shape and clean.
Insure that the proper solder alloy and ball size has been chosen. Insure that the properly-specified "tacky flux" has been chosen.
Apply Flux to Part
Tacky flux is now spread onto the bottom of the part after being dispensed via a paste flux syringe. The flux should have the following characteristics:
- It must be a paste or "tacky": or "sticky" flux as this physical property is essential for ball re-attachment
- It should contain a mild to medium active organic acid
- It should be water soluble in order to make the clean up process simpler
Loading of Solder Balls using a StencilQuik Stencil
The "loading up" of solder balls onto the underside of the device is done in a
polyimide stencil. The stencil is aligned with the pads on the bottom of the device and solder balls are poured into the various apertures of the stencil using a mechanical fixture which "captures" the excess solder balls. An ESD-safe soft bristled brush is used to assist the "sweeping" of the balls into the apertures. Excess balls are collected in the "catch basin" portion of the fixture which allows the solder balls to be later poured back into original solder ball container.
Reflowing the Part
As with all soldering processes the temperature profile used is one of the key ingredients for success.
Every package type may require a different thermal profile setting. Variables that may impact the profile settings include the package materials, package mass, solder alloy and package size.
Place the fixture into the hot air convection oven or hot air reballing station and start the reflow heat cycle.
Cleaning
The reballing process used by BEST includes the use of water soluble "tacky
flux" prior to placing the balls back onto the device substrate. This water
soluble flux insures that the sites are properly prepared and that wetting
action occurs on all the solderable surfaces.
While these water soluble processes have many processing benefits, their more
aggressive nature and their potentially harmful by-products must be cleaned off
the BGA or CSP substrate after having the solder ball attached. BEST implements
a rigorous batch cleaning process to remove residues from the underside of the
device. This insures that no matter the end use of the reballed device, they are
sent back cleaned and with the confidence that there will not be any product
reliability issues related to ionic contaminants or residues left on the BGA/CSP
after reballing by BEST.
In order to insure the level of cleanliness is obtained with the BEST
cleaning processes, BEST cleans reballed devices to a programmed cleanliness
level. Our closed-loop cleaning system washes devices to a pre-selected
resistivity level using only DI water. During the wash cycle, the water is
continuously purified by a series of filters. Each batch of reballed devices
cleaned in this manner comes with a cleanliness document verifying that the
parts meet a given ionic contamination level equivalence. IPC J-STD-001
specifications for ionic cleanliness are met for all cleaned devices.
Ionic Cleanliness Testing
An ion chromatograph (IC) is the most common tool for precision testing of
device cleanliness. This method can quantify and identify specific ionic species
that are present on an electronic device. The most common test method followed
is the IPC TM-650 2.3.28. The device is placed into an ionically-clean bag (e.g.
Kapak), and is immersed in an extract solution of 75% isopropyl alcohol (IPA) to
25% de-ionized water. It is important to be sure that the sampling procedures do
not introduce any ionic contaminates.
The PCB is extracted in the solution for one hour at 80 degrees centigrade,
the resulting extract is then injected into the ion chromatograph. The IC then
separates and detects each individual ion for which it was calibrated.
Click here to see an
Ionic Cleanliness Testing Report (PDF).
Baking
While BEST follows the JSTD-033 guidelines with respect to controlling the
moisture absorption into plastic packages, there are cases where the MSD level
of the parts are unknown by customers. In these instances, BEST assumes the
worst case MSD level for the given part and bake the parts for a period of 48
hours at 125 degrees centigrade.
Inspection
Inspect the reballed device for contamination, missing balls and flux residue.
These
inspections should be performed under a stereo microscope
or other optical inspection tool.
Marking
All parts (components) which have been reballed by BEST are marked with a small
yellow "dot" using a heat resistant marking pen which indicates the device has
been reballed by BEST, Inc.
Other marking options, including the use of a
laser etching system, are also available.
General BGA/CSP Reballing Process
Handling of BGAs
StencilQuik™ Fixturing Technique
EZReball (TM) Preform Technique
High Volume Automated Machinery
Dry Packaging
Ceramic Package Reballing
Reballing Challenges
Optional Services
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