Cavity Board SMT Assembly Challenges (Part 1)

Estimated reading time: 10 minutes

It would be expected that since the fiducial and surface BGA are created in the same fabrication steps, there should be minimal difference in their locations relative to the CAD database. However, given that the cavity features are three layers below the surface, it would be possible for layer-to-layer imaging tolerances to accumulate as the PCB layers are built-up. The results of these measurement calculations are shown in Figure 12.

There has previously been no design rule in place for this surface to cavity registration tolerance, so the effort with the test boards has been to bound the amount of mismatch and determine if it is impactful to the SMT process. In these test boards, the worst-case surface to cavity registration tolerance was

Figure 12: Measured vs. drawn registration error.

Cavity Opening Tolerance

The tolerance on the size of the cavity opening was initially deemed important as it was unknown at the time of the TV design how tight a fit the stencil would require to the cavity opening. A surface profile tolerance of 150 μm was added to the cavity X-Y dimensions in order to minimize the variation.

Table 1. Cavity dimensions' tolerance.

None of the suppliers met the 150 μm surface tolerance as all the cavities measured were smaller than allowed by the surface tolerance.

While the dimensions were smaller than the surface tolerance allowed, it was noted that within a given supplier, the X-Y size was generally repeatable. This is shown in Table 1. But to have a design specification that would work across multiple fabricators, a larger nominal surface tolerance of 400 μm would be more achievable.

Editor’s Note: Stay tuned for the second part of this paper, which will focus on the SMT assembly of components into the cavity and the impact of the PCB design on assembly.

This article was originally published in the proceedings of SMTA International 2018.

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