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38 SMT Magazine • April 2017 3. Radius of Curvature: The relationship observed between BLT and weight can be un- derstood by reviewing the radius of curvature effect. For a curved surface, any point on the surface can be specified by two principal radii of curvature, as shown in Figure 6. R1 is the radi- us of curvature in the plane of the paper and R2 is the radius of curvature perpendicular to the plane of the paper. Equation 1 This equation describes the pressure differ- ence across a curved interface (DP) in terms of the surface tension of the interface (g) and the two principal radii of curvature at a point on the surface [1] . The surface tension of SAC305 has been reported to be 0.568 N/m [2] . In the sandwich structure discussed here, R2 was the Cu cou- pon radius, 0.906 inch (0.0115 m). By setting an R1 value being half of the BLT, or 0.003 inch (0.000076 m), the pressure difference across the liquid solder surface is 765,000 g-f/m2. The Cu coupon area used in this study should be able to maintain a constant BLT with a weight placed on top of the sandwich up to 318 g-f. In this study, liquid solder was squeezed out at 100 g, lower than the calculated 318 g-f. This was probably attributed to the uneven BLT caused by warpage of the Cu coupon, as shown in Table 1. 4. Lamellar Flow Effect: Voiding is a function of BLT thickness (Figure 7), where the voiding maintained at about 32% area ratio at the thin bondline, then decreased gradually when the BLT was greater than 0.15 mm. How- ever, if the voiding was expressed as volume by multiplying the voiding area % with BLT, the re- lationship between voiding and BLT can be seen in Table 2 and Figure 8. Here, all void shapes were approximated as pancake, and the volume was expressed as an arbitrary unit. Figure 4: Voiding data of SAC305 under various weights. Figure 5: Relationship between weight and BLT for SAC305. Figure 6: Radius of curvature [1] . VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING