This article deals the occasional tendency of larger (and thinner) BGA components to warp during rework.
The type of warping addressed here can cause bridging and shorts at the outer corners of the device during replacement. This problem can require more rework at the BGA site, surely no fun at all.
It's generally understood that some minor distortion occurs in BGA components and circuit boards when heated, due to the varying thermal coefficient of expansion (TCE) of all the different materials. In the BGA component, this effect is greatest (or at least appears greatest) at the outermost edges. The larger the BGA package, the greater the warp at the outer corners.
Figure 1: Solders balls at the corners of BGA components can pancake to the point that they may touch one another consequently bridging.
The warping can be very slight - in this case, the center of the BGA will bow upwards, and the corners down; but it's often enough to cause the corners to move closer to the circuit board surface, putting extra pressure on the corner solder joints.
The result can be that the solder balls at the corners pancake to the point that they may touch and bridge with one another. (See Figure 1). It's worthwhile noting that the warping may also occur in the opposite direction with the corners curling upward.
Once bridged or shorted, solder balls may remain in that condition due to the surface tension inherent in the molten solder. This problem is usually not detected until after rework, during X-ray inspection. At that point, you know the rework procedure was unsuccessful and must be done again, subjecting the site to more thermal cycles, risking pad degradation and other problems.
Also, the circuit board itself may warp slightly, and this additional warp - if combined with that of the BGA - can amplify the problem. Of course, the first thing to look at is the profile itself, but even when profiles are tweaked there can still be the occasional problem at the corners due to inherent limitations of the rework process.
The solution to this problem is a simple but effective one that we employ whenever we see the need. It merely involves providing the needed "support" for the outer corners of the BGA during the critical time that the BGA solder balls are molten.
Our method involves measuring the clearance between the circuit board surface and the underside of the corner of a properly soldered BGA like the one that we are going to rework. We then select small pieces of non-solderable material to serve as spacers.
The thickness of these spacers is slightly less than the component stand-off height in its post-reflow state. Remember, the spacer is made to prevent bridging and not to hold the component at an arbitrary height above the board.
Once the old BGA has been removed, the site prepared, and solder paste applied to the pads on the board surface, we place these spacers at the four corners of the BGA site. They are held in place with a low tack, heat resistant, no-residue adhesive.
The edge of the spacer will fit under the edge of the BGA corner. You would not want it to penetrate beneath the BGA far enough for the spacer to disturb any solder connections. It's easy to place the spacers precisely at that point, since the pad area and paste print volume are easy to see and distance can be gauged visually to prevent contact between spacer and solder connection.
Figure 2: Spacers prevent the corners from flattening the balls underneath.
As the new BGA is ramped up to reflow temperature under a hot gas reflow machine nozzle, the spacers prevent the corners from flattening the balls underneath. (See Figure 2) As the BGA cools and the warp decreases, the outer edge balls solidify, and the solder connections beneath the center solidify.
In answer to the question of whether curling of the edges causes issues with interior connections, we have seen no connection problems under the inner periphery of BGA connections (those that would theoretically be raised by slight warp).
The shorting is the most pressing problem, and the spacers correct it. These spacers can be easily removed once the BGA and board have cooled to ambient temperature. Depending on your applications, equipment or engineering support, you may or may not see much of this condition, but if you do, now you have a solution.
Several members of the Circuit Technology Center team contributed to this feature story.
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