Pad Cratering Susceptibility Testing with Acoustic Emission

Estimated reading time: 16 minutes

Figure 7 shows that Laminate F is a frequent AE emitter as evidenced by the significant number of AE events with amplitude greater than 40 dB that occurred when both the applied load at 30lbs and 50lbs compared to other five laminates. Figure 8 shows that Laminate F released significantly more energy than other laminates when both the applied load at 30lbs and 50lbs. Both the number of AE events at given applied load and the cumulative energy of AE events at given applied load indicates that Laminate F is prone to pad cratering.

Our empirical experience with laminates is that Laminate F is inferior and that Laminate A, our control laminate that is dicy-cured, is superior. Laminate F proved to be a frequent AE emitter, confirming our experience. The expected superiority of Laminate A is not evident; this may be because of degradation due to incompatibility of this laminate with the multiple high temperature reflows. The degradation with additional reflow cycles seen in the other test methods reported in our earlier paper [8] was not found, although with this test method we compared only 5 and 7 reflows.

Verification of pad cratering was done with cross-sections. To enhance the visibility of the pad craters, two methods were tried: powder filling and polarized light. Polarized light produced much more visible pad craters. The vast majority of the pad crater failures were, not surprisingly, at the corners of the largest BGA. Fig. 9 shows examples of pad cratering.

As indicated in Figure 5, not all AE events are associated with pad cratering because some AE events are located outside of the package. Figure 10 shows two examples of laminate cracks indicated by AE events, but not associated with pad cratering. The cross-sectioning results confirm this finding.

Table 2 compares results of various test methods for evaluating pad cratering. Note that the results of six different laminates’ resistance to pad cratering using the ball pull, ball shear, and hot pin pull tests (please refer to our earlier paper [8]). This ranking of laminate materials shows quite a bit of inconsistency of results among the methods. No conclusion can be drawn at this time as to which test method is better able to distinguish the laminate materials’ resistance to pad cratering.

Figure 6: (a) The applied load at first locatable AE event with amplitude greater than 40dB, (b) Average applied load when first four AE events with amplitude greater than 40dB.

Figure 7: (a) Number of AE events with amplitude greater than 40dB when the applied load is 30 lbs, (b) Number of AE events with amplitude greater than 40dB when the applied load is 50lbs.

Figure 8: (a) natural log of cumulative energy of all AE events with amplitude greater than 40 dB when the applied load is less than 30lbs, (b) natural log of cumulative energy of all AE events with amplitude greater than 40dB when the applied load is less than 50lbs.

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