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36 SMT Magazine • April 2017 colored solder were seen within the voids, such as in the third image of the 10-gram series and the first image of the 30-gram series. The spot- ty solder islands were attributed to the once- formed liquid joints, but were wiped out par- tially by expanding voids. In most images, concentric ring textures can be seen clearly, indicating the Cu coupons may be warped. Also, the joint appeared to be thick- er toward the center, and the solidification may have been developed stepwise from the edge to- ward the center. 1. Bondline Thickness (BLT): The 100- gram weight samples showed a much lighter shade of color than the other three weight sam- ples, suggesting much thinner solder BLT data for SAC305 joints under various weights. The sandwich samples were cross-sectioned, as ex- emplified in Figure 2, with the BLT measured at both the edges and the center. The average val- ue was calculated to represent the BLT of a giv- en weight condition, as shown in Table 1. 2. Voiding vs. Weight Analysis: The actual voiding data of SAC305 under various weights are presented in Figure 3 and Figure 4. Other than the 100-gram condition show- ing a wider voiding distribution, no clear trend can be concluded. The lack of an obvious trend could be attributed to data scattering. It might VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING also be possible that weight may not be a di- rect governing parameter. Similar to all other liquids, liquid solder is expected to follow flu- id dynamics, with a greater difficulty to have la- mellar flow under a small clearance. Since void- ing involves void coalescence and escaping, and a higher BLT will facilitate both, it makes sense to investigate voiding behavior around the bon- dline thickness. Figure 5 shows that with in- creasing weight, the BLT maintained constant around 0.18 mm at first, then decreased at a weight higher than 50 grams, and then reached 0.04 mm at 100 grams of weight. Figure 2: Edge view of cross-sectioned samples under 30 grams of weight. Figure 3: Actual voiding data of SAC305 under various weights. Table 1.