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24 SMT Magazine • June 2016 collapsed BGA/paste molten alloy to coat and react with the edges of the pad. This results in a stronger solder joint. Figure 3 illustrates this difference. Also, the coefficient of thermal expansion (CTE) mismatch between solder mask and SAC alloy will cause stress to the solder joint during thermal cycling, which can lead to premature failure. Another common issue with mask defined pads is the increased potential for mid-chip sol- der balls (MCSBs). Mid-chip solder balls are de- fined as large (>100 µ) beads of solder found in between the leads of a passive component. Reducing the volume of paste deposited is one method of reducing mid-chip solder balls, but eliminating the solder mask between the passive component terminations is also an ef- fective resolution to the problem. Using both techniques is generally the best recommenda- tion. Figure 5 shows an example of how a global contract manufacturer solved a mid-chip solder ball problem by eliminating the solder mask be- tween passive component leads without reduc- ing the paste deposit volume. Essentially, this was a conversion from mask defined to pad de- fined design rules. Random solder balls can also be reduced with the use of pad defined joints. Slight mis- registration between circuit boards and stencil apertures can commonly result in solder paste being printed slightly off the pad. If the pad is PAD DEFINED VS. MASK DEFINED: WHICH METHOD IS OPTIMAL? Figure 2: 3D photos of both mask defined and pad defined circles of the same size. Figure 3: Transfer efficiency and solder paste volume variability in both mask defined and pad defined boards.

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