SMT007 Magazine

SMT-Apr2018

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40 SMT007 MAGAZINE I APRIL 2018 Crack growth was assessed using cross-section and dye-and-pry techniques on unmonitored assemblies that were removed from the chambers at fixed readpoints. Conclusions on the impact of the parameters were determined based on the totality of electrical test and crack growth data. EXPERIMENTAL: Design The package attributes are summarized in Table 1. Those highlighted in yellow were varied in the experiment. The substrate dielec- tric details are in Table 2. BGA arrays are shown in Figure 1. The baseline SMD design in Figure 1a contained only SMD pads. Hybrid- A in Figure 1b used the same footprint, but the outer four rings were substituted with NSMD pads, while the pads at the die edge were main- tained as SMD. By contrast, the outer six rings were NSMD for Hybrid-B, encompassing the die edge. In all cases, the SMD pad SRO (solder resist opening) was 0.45mm. To compensate for solder wetting down the pad sidewall, the NSMD pads on the hybrid designs were slightly smaller to produce a similar ball height. These packages were daisy-chain test vehi- cles with pairs of solder joints electrically connected as illustrated in Figure 2. A complete circuit was created by connecting pairs on the PCB side that were skipped on the package. All solder joints were monitored as one "net." A failure on any solder joint meant the remain- ing solder joints could no longer be electrically monitored. Except where parameters were intentionally varied, the daisy-chain packages were mechan- ically similar to the final products: same die Table 1: Package details. DOE variables in yellow. Table 2: Substrate dielectric mechanical properties. Figure 1: BGA footprint showing arrangement of SMD and NSMD pads for the three different designs. Figure 2: A daisy-chain connection representation between package substrate bottom metal and printed circuit board top metal. The red dashed line illustrates the electrical path.

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