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8 SMT Magazine • December 2017 In my recent conversation with process en- gineers at an EMS company here, they said one of the critical processes that determine yield in their line is the solder paste printing process. According to them, one of the key reasons for this is the incorrect printer set up, which re- sults in issues such as insufficient solder or sol- der bridging. Of the three elements involved in the process—stencil, solder paste, and printer— the stencil is considered one of the major fac- tors affecting the transfer efficiency, accuracy, and consistency, of solder pastes into the pads, especially with the continuing trend towards miniaturization. Indeed, in our latest survey on solder paste printing, a majority of the respondents high- lighted stencils as one of their key challenges. They mentioned the quality of the stencils; get- ting the right stencils—their stencils are done by a third party; aperture design; and stencil wear, among others, as issues around this part of the process. This is made more challenging because of the finer pitch and spacing in PCB designs. Specific problems in this regard include complete filling of apertures, paste release, and the large range of component types and sizes and the solder paste thickness requirement on the same design. Other main issues include the accuracy and repeatability of the equipment, and the charac- teristics of the solder pastes being used. Which brings me to our topic for this month's issue of SMT Magazine. Many stud- ies over the years have found that up to 70% of PCB assembly defects come from the solder paste printing operation. In this issue, we look at the critical issues in the solder paste print- ing process, and how assemblers can address these challenges to help improve their yield and quality. E DITOR'S NOTE

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