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70 SMT Magazine • November 2017 However, the experimental results from this investigation are still pending as the data col- lection and interpretation is still on-going. Design Guidelines The test vehicle experimental data discussed previously has provided a good baseline under- standing and insight into the parameters influ- encing this solder separation failure mode. Be- cause this solder separation failure mode can re- sult in an electrical failure either at manufactur- ing test or prematurely during field use, it poses a high risk for product launch in terms of solder joint quality and reliability. This establishes the need for specific design guidelines with the use of VIPPO pads. Based on the experimental data thus far, mixed VIPPO and non-VIPPO pad de- signs within a single BGA footprint are prohib- ited for applications having a secondary SMT process. For designs having only a single SMT reflow process, the rework process of an adja- cent component can still pose a high-risk for solder separation of the adjacent non-reworked component having a mixed VIPPO and non- VIPPO BGA footprint. Therefore, a validated mitigation strategy is required for manufactur- ing production release of such designs. There is also on-going work to establish more data with the VIPPO and VIPPO + backdrill pad designs within a single BGA footprint in order to de- fine an acceptable backdrill depth limit which does not induce the solder joint separation fail- ure mechanism. Summary and Conclusions This work has evaluated various BGA pack- aging and PCB design parameters in order to characterize their influence on this double-re- flow solder separation failure mode for mixed VIPPO BGA pad footprints. It was observed that this failure mode is not dependent on the BGA pitch and can occur for ≤ 1.0 mm pitch compo- nents. It was also found to be independent of the VIPPO drill hole size, occurring with both 9.8 and 7.9 mil drill hole sizes. However, this failure mode has been shown to be sensitive to the BGA package body size, with the risk of oc- currence decreasing for large package body siz- es. It is also suspected that the package weight, BGA density and ratio and location of VIPPO- to-non-VIPPO pads within the BGA array may also play a role along with the package body size. As previously noted, the current guidance recommends not to mix the VIPPO pad struc- tures with non-VIPPO pads or deep-backdrill VIPPO structures within a single BGA footprint. Further investigations are still needed to estab- lish more specific guidance regarding the us- age of these VIPPO structures with non-VIP- PO pad designs or deep-backdrill VIPPO struc- tures. As performance requirements continue to advance, these types of PCB structures and de- signs will become a necessity for future genera- tions of products. Hence, these design guidelines provide only a short-term solution to address this fail- ure mode. More detailed understanding of the mechanisms driving this failure mode and how to control them are needed to develop a long- term solution that can allow implementation of these mixed VIPPO designs. Future Work As discussed in this paper, this work has helped to establish PCB design guidelines and best practices in order to prevent this defect VIA-IN-PAD PLATED OVER DESIGN CONSIDERATIONS Figure 15: VIPPO stub length after backdrill.