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January 2017 • SMT Magazine 37 dropped with the component facing downward so that the initial drop impact will record board surface tensile strains on each corner. Mounting Location Sensitivity Mechanical variation (post alignment, washer dimension, etc.) among the four board mounting fixtures on the table may impart some positional dependency in the drop shock impulse. The magnitude of this variation (and thus positional experimental error) is mea- sured through board bending strain measure- ments during drop events at each mounting po- sition. Four instrumented boards are mounted as shown as Figure 8. Individual strain moni- tor boards are identified as A, B, C, and D. After one instrumented drop with boards in the ini- tial position shown on the far left, the boards are cycled clockwise to the next mounting posi- tion. For example, board A is moved from NW location to NE. Board A is also rotated 90° such that the strain gage orientation will be still be radial, emanating from the center of the table. The other three boards are similarly rotated into new positions. The resultant strain measure- ments from successive drops are listed in Figure 8 for each of the four unique board placements. Tracking any given board through the four dif- ferent positions reveals the experimental strain variation. Positional variation exceeds the indi- vidual test board variation by an order of mag- nitude with the NW position consistently pro- ducing the highest strain (6% above the overall mean). Individual test boards are consistent to within 1% relative to the overall mean. Results and Discussion Weibull Distribution Plots of BGA Drop Failures Figure 9 shows Weibull failure rate distribu- tion plots by solder alloy for drop shock failures with (a) Cu-OSP board surface finish and (b) Im- mAg board surface finish. The drop lifetimes in- dicated are those for the first failing corner on each test board. The SAC105 alloy was limited to eight samples due to yield fallout at assembly. SAC305 can be seen to be the best perform- er for both board surface finishes. Noting the variability in drop lifetime (i.e., low Weibull shape factors, β), the other alloys can all be con- sidered to have lower but similar drop perfor- mance. In this experiment, the SAC-M alloy on the Cu-OSP finish failed with considerably low- er variability than all other experimental cells (β = 8.5). Figure 10 compares the characteristic drop lives of BGA interconnects among the five sol- der alloys between the two PCB surface finish- es. The effect of alloy silver content is seen to be similar for both finishes with the highest silver content (3%, SAC305) showing superior per- formance and drop lifetime generally decreas- ing with the silver alloying content. BGA com- Figure 8: Board strain measurement (maximum principal strain, in microstrain) by table mounting position. EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME