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June 2016 • SMT Magazine 25 mask defined, solder paste may be printed di- rectly on solder mask. Paste on mask does not form an intermetallic and frequently results in random solder balls, as shown in Figure 6. In summary, mask defined design rules lead to better solder paste print deposit volume and volume repeatability. Because the vast majority of SMT defects are a result of printing issues, there is a strong argument for minimizing the variability in the printing process. Producing stronger board-to-solder connec- tions is also a highly desirable outcome. Pad de- fined joints help reach this objective. Reducing mid-chip and random solder balls is also a key goal for many circuit assemblers. The answer to the question is not as simple as it might first seem. Perhaps the key is to have very good solder paste printing and pad defined design rules, as this will lead to greater electrical reliability in board assemblies. SMT Mitch Holtzer is global director of customer technical service (CTS) for Alpha Assembly Solutions. To reach Holtzer, click here. PAD DEFINED VS. MASK DEFINED: WHICH METHOD IS OPTIMAL? Figure 4: An example of mid-chip solder balls on a mask defined assembly. Figure 6: Random solder balls due to paste deposited on solder mask. Figure 5: An example of how a mid-chip solder ball problem was solved by eliminating the solder mask between passive component leads without reducing the paste deposit volume.

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