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X-ray Uncovers the Best and Worst of BGA Assembly Continued from page 70
the BGA, which may cause it to delaminate or “popcorn” during removal.
It is advisable to use a rework station that has a bottom-side heat nozzle to help bring the whole BGA to the correct temperature. Some rework stations also have an IR plane to bring the entire board to temperature, which also helps mini- mize warpage of the sample. The correct nozzle in the rework station will apply localized heat to the BGA package and run the tem- perature profile. As soon as the sol- der is molten, lift the package with a vacuum wand or tweezers. To avoid an extra heat cycle to the board,
September, 2017
clean the BGA site by removing the excess solder left on the pads using a solder vacuum tool or solder wicking braid. The goal is to end up with a solderable array of pads while avoid- ing damage to the board or pads.
Inspecting BGAs for Defects Creative Electron’s TruView void measuring software.
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BGAs hide their problems very well. The best solution is to have an X-ray system to look through the package and assess the quality of the assembly. Figure 1 shows an exam- ple of the best —and also the worst — image of a BGA assembly. It is the worst because it has every single assembly problem that can be found in a BGA assembly. It is also the best because it allows us to see all these problems in a single image, making it a great educational tool. This pic- ture was taken with a TruView™ Fusion X-ray system at 110 kV and 100 μA, with the sensor tilted at 55°. Figure 2, however, is an exam- ple of a good BGA assembly. All the balls are properly collapsed, there are no opens or shorts and voiding is minimal. There is some shape defor- mation, but not enough to cause any trouble. In short, this is the image we want to see of a BGA assembly.
Coplanarity. Imagine two planes, like two sheets of paper, perfectly flat.
Now imagine placing these two sheets of paper on top of each other. The four corners of the top sheet will touch the four corners of the bottom sheet. They are coplanar; they are perfectly parallel to each other. Now gently lift one of the corners of the top sheet. This creates a coplanarity problem, a common issue in BGA assembly caused by either assembly misalignment or by “popcorning” due to trapped moisture in the plastic BGA package. Coplanarity is one of the trickiest problems to diagnose because it requires the inspection of the entire BGA.
Short Circuit. Short circuits, or “shorts,” are identified by a solder bridge between two or more solder balls. This defect is easily picked out in an X-ray image. The root cause of this problem is diverse. It can be caused by incorrect solder deposition, either if too much solder was deposit- ed, or if it was misplaced in between the pads. Misalignment between the BGA and board can also cause a short. In addition, if solder mask is not properly deposited onto the board, some of the solder on the ball can run onto the trace. This can eventually touch a neighboring ball, causing a short.
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Head-in-Pillow. Perhaps the most challenging BGA assembly problem to identify, head-in-pillow has haunt- ed engineers since the early days of BGA technology. Under special cir- cumstances the solder ball collapses, though not entirely. This partial col- lapse allows the solder ball to rest on top of the pad and to make electrical contact.
Available exclusively through Technica, U.S.A. 1-800-909-8697 •
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Since the contact between sol- der ball and board pad is mechanical and not metallurgical, the integrity of this electrical contact will depend on how the board is flexed. This is a case where the finished board passes all electrical tests in the facility, but fails at the customer’s site. Temper- ature differences that cause the BGA to move, even slightly, can cause the
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