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PCB007-Aug2018

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54 PCB007 MAGAZINE I AUGUST 2018 OSP coat. Baking also accelerates solid diffu- sion between metals and increases intermetal- lic growth. This can lead to a "weak knee" or other solderability issues if the intermetallic layer reaches the surface and oxidizes. Effects upon other finishes (immersion tin, immersion silver, ENIG) are further detailed in the guide- lines. IPC-1601A (2016 revision) also states: "If process controls are ineffective, and printed boards have absorbed excessive moisture, bak- ing is the most practical remedy." It goes on to state, "However, baking not only increases cost and cycle time, it can also degrade sol- derability of the printed board which requires extra handling and increases the likelihood of handling damage or contamination. In general, both the printed board fabricator and the user should strive to avoid baking by practicing ef- fective handling, packaging, storage, and pro- cess controls…" In addition to moisture management at key steps in the fabrication process, IPC-1601A also makes clear that boards should be protective- ly packaged to limit their exposure to ambient humidity during processing and storage. And, importantly, packaged only after determining that their moisture content is below the max- imum acceptable moisture content (MAMC) level, which is typically between 0.1% and 0.5% moisture weight to resin weight. Just as with components, 125°C baking tem- peratures degrade the solderability of PCBs. IPC-1601A warns that as little as 4-6 hours at that temperature can render HASL finished boards unsolderable. Over the decades that passed since the J-STD-033 standard was cre- ated, new technologies were developed and proven to safely reset component floor life us- ing low temperatures and ultra-low humidity without requiring extensive time. These 40- 60°C and <1% methods were first adopted in Europe, and their recognition and use has now spread to North America. The same methods were applied to PCBs, and engineers from the company SMT and Hybrid GmbH published their findings in "Production of Printed Circuit Boards and Systems" [1] . They Figure 1: Comparison of the various drying processes using QFP100 as an example.

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