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November 2017 • SMT Magazine 63 VIA-IN-PAD PLATED OVER DESIGN CONSIDERATIONS type of failure mode has also been identified on a component having a full VIPPO BGA pad pat- tern on the PCB when there is also a pattern of VIPPO with deep backdrill (BD) within the foot- print. Hence, the deep-backdrill VIPPO struc- tures seem to mimic the behavior of the non- VIPPO pads so that it becomes comparable to a mixed VIPPO/non-VIPPO BGA pad footprint and again, induces solder separation in the sol- der joints on a VIPPO pad when subjected to a secondary reflow. Since the separated solder ball shape is rounded near the open or partially open inter- face, this indicates that the solder joint under- went reflow subsequent to the separation. Fur- thermore, since the separation is between the bulk solder and the IMC and does not reflect a brittle fracture, it is suspected that the separa- tion occurred after the solder has softened and is nearly molten. Figure 6 illustrates a brittle solder joint failure in which the fracture occurs within the IMC itself and exhibits more of a flat surface indicative of crack propagation. Failure Mechanism There seem to be two primary effects that are occurring which contribute to this failure mechanism. First, there is a CTE mismatch be- tween the VIPPO structure and the non-VIP- PO, or deep-backdrill VIPPO, structure, that re- sults in a greater expansion of the PCB beneath the non-VIPPO BGA pad, or the deep-backdrill VIPPO pad, as compared with the VIPPO BGA pad. Secondly, the higher thermal conductivi- ty of the VIPPO structure as compared with the non-VIPPO, or deep-backdrill VIPPO structure, allows the VIPPO solder joints to reach liquidus before the adjacent solder joints having a non- VIPPO, or deep-backdrill VIPPO pad. Therefore, during a secondary reflow process, when the adjacent non-VIPPO solder joints are still solid, tensile stresses are induced in the VIPPO solder joints as the adjacent non-VIPPO solder joints are pushed up due to the greater out-of-plane PCB expansion beneath those pads. Subsequently, when the VIPPO solder joints become molten, these high stresses are relieved Figure 4: Example of brittle fracture. Figure 5: Partial solder separation. Figure 6: Complete solder separation.