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40 SMT Magazine • January 2017 drop impact imposes a tensile load on the 'out- er' side of the joint at the base of the BGA pad, initiating and propagating a crack into the lam- inate. The upward rebound board flexure then imposes a tensile impulse on the 'inner' side of the BGA pad and compression on the out- er side. The magnitude of the second impulse is necessarily smaller than the initial impact loading. The magnitude of the rebound flexure is reduced through dampening. Moreover, the board cannot flex away from the plane of the package on the inner side as readily as it can on the outer side because the inner side board is constrained by the adjacent solder joints at- tached to the package above. That's not to say however the outer crack will always preferen- tially produce a failure. Different levels of frac- ture toughness are encountered along the two crack paths as well as different path lengths be- ing required to produce a failure. Depending on solder alloy and interfacial toughness con- siderations either the solder/IMC crack path or the laminate crack path under the pad may win out, transecting the interconnect to produce an electrical open. For certain alloy:finish combinations, pad cratering proved to be a relatively common failure mode. In these instances, the laminate crack shown in the diagonal cross-section of Figures 11–13 propagated to meet another op- posing laminate crack from the other direc- tion. When viewed from a section taken in the plane of board (Z-section) this failure mode is characterized by a circular crack propagating into the plane of the BGA pad from the perim- eter of the solder mask opening until the cen- tral laminate crater separates from the pad. An example is shown in Figure 15. All pad crater- ing events that produced electrical failures did so with such circular cracks through the BGA pad. No failures through the input copper traces were observed. This mode of circular pad crater- ing causes the two input channels appear to be open simultaneously (Figure 3), the same fail- ure signature as the IMC/solder failures. Thus, the event detector could detect failure but not uniquely identify the failure mode as anticipat- ed. All failures must be cross-sectioned to iden- tify the failure mode. The observed failure modes of the BGA256 package in 900G drop shock are summarized in Table 3 for all solder alloys tested. For each EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME Figure 14: Illustration of crack path competition in corner BGA solder joints due to cyclic oscilla- tions after drop. Stresses due to (a) downward board deflection at initial drop impact and (b) upward rebound deflection. Figure 15: Z-direction section of failed corner joint with pad cratering. Table 3: Failure Modes of BGA256 in Drop.