A cure for BGA's warped sense of humor: rework can result in BGA warpage--find out how it can be prevented.
The Components of Warpage
Some minor distortion occurs in BGA components and circuit boards when heated, due to the thermal coefficient of expansion (TCE) of the materials. In a BGA component, distortion is greatest at the outermost edges--the furthest area from the neutral point, or center of the BGA package. The larger the BGA, the greater the deflection can be at the outer corners. The warp is very slight--the center of the BGA will bow upwards, and the corners down--but such warping is often enough to cause the corners to move closer to the circuit board surface, putting extra pressure on the corner solder joints as they flow.
As a result, the solder balls at the corners pancake to the point that they may touch one another--consequently bridging (Figure 1). Once bridged or shorted, the solder balls remain that way due to the surface tension inherent in the molten solder. The problem is usually not detected until after rework, during x-ray inspection. At that point, one realizes that the rework procedure was unsuccessful and must be performed again, subjecting the site to additional thermal cycles--risking pad degradation and other problems.
[FIGURE 1 OMITTED]
Also consider that the circuit board may warp slightly on its own, and that this warp, if combined with that of the BGA, can amplify the problem. The solution involves providing the needed support for the outer corners of the BGA during the critical time in which the BGA solder balls are molten. Measure the clearance between the circuit board surface and the underside of the corner or edge of a properly soldered BGA--one that is exactly like the BGA to be reworked. Cut small pieces of non-solderable material to serve as spacers--made of various temperature and solder resistant materials. These spacers should be slightly less than the thickness needed, as the spacers will be held in place with tape, accounting for the full thickness.
Once the old BGA has been removed, the site prepared and solder paste applied to the pads on the board surface, tape these spacers at the four corners of the BGA site using high-temperature, heat-resistant tape. The edge of the spacer will fit just under the edge of the BGA corner. The spacer should not penetrate beneath the BGA far enough to contact or disturb any solder connections. Remember to take into account the thickness of the tape plus the thickness of the spacer--thickness should always equal the desired final clearance or slightly less, but never exceed final clearance. Distance can be gauged visually to prevent contact between the spacer and the solder connection.
As the new BGA is ramped up to reflow temperature, under a hot gas re flow machine nozzle, the spacers prevent the corners from flattening the balls underneath (Figure 2). As the BGA cools, and the warp decreases, the outer edge balls solidify. Solder connections beneath the center solidify as well. To answer the question probably forming in your mind, I have seen no wetting or connection problems under the inner periphery of BGA connections (those that would theoretically be raised by slight warp). The "squishing" is the problem, and the spacers correct that problem.
[FIGURE 2 OMITTED]
These spacers may be easily removed once the BGA and board have cooled to ambient temperature.
Jeff Ferry is president of Circuit Technology Center, Haver hill, MA; (978) 374-5000; www.circuittechctr.com.
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|Title Annotation:||Rework and Repair Depot|
|Date:||May 1, 2002|
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