SMT007 Magazine

SMT-July2018

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70 SMT007 MAGAZINE I JULY 2018 Feature by Brian Morrison VEXOS CORP. Over the years, best practices have evolved and will continue to evolve with the changing environments, company needs and challenges and what may work for one company may not necessarily be best for another. A common area sometimes overlooked versus surface mount is wave solder, which can commonly introduce significant touch up and rework if not appro- priately managed. The following recommendations are based on experiences and best practices and is not intended to be considered hard and fast rules, but rather guidelines—your situation will dictate which practices work best for your company. This article will be focusing on common wave defects and best practices to both address, predict and proactively prevent these issues from reoccurring. Contributing elements such as component selection/ considerations, design, tooling and process will be discussed. By far the most common wave defect is bridging, which is the unwanted formation of solder between conductors. Defect contribu- tors include component, design, tooling and process. Solder Bridging Component Considerations Lead length: Specification of component lead length in the design versus the PCB thick- ness provides the respective protrusion of the lead into the solder during this process. Ensur- ing the lead length is neither too short (i.e., solder cannot reach the pin to achieve capaci- tary action) or too long (i.e., provides a path- way for webbing from one pin to the adjacent) can prorogate bridging for the assembly. Best practice when specifying the compo- nent lead length is to ensure the lead length is long enough to provide the necessary heat transfer for proper wicking to provide sufficient barrel fill while neither exceeding the maxi- mum protrusion specified as per IPC-A-610. A good rule of thumb is the length should not Best Practices in Manufacturing: Wave Soldering

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