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44 SMT Magazine • November 2017 Another common mistake is using 217°C as the liquidus temperature for SAC 305 and SAC 387. The liquidus temperature for SAC 305 and SAC 387 is 220°C and it is the solidus that is 217°C. This can lead to some confusion when set- ting the specification limits for the reflow pro- file. If the plateau is between 219-220°C, this can lead to severe flux exhaustion resulting in graping and HIP in hip-scale packages (CSP) and package-on-package (PoP). Inspection Tools for Voiding, Graping, and HIP There is no set requirement for the QFN voiding limit, and each company has their own specification. The range can vary between 15- 50% for standard applications and down to 5% for special applications. X-ray is the most common way to verify the voiding levels, and most modern X-ray equip- ment will do an accurate void calculation. The issue is that many production lines worldwide do not have adequate high-magnifi- cation microscopes to validate that the void re- duction don't have a negative impact on solder joint quality. IPC 610 clearly specifies that the minimum magnification required for solder joints <0.25 mm should be 20X, and in many cases, only a 2-4X magnification glass is available; a graping issue would easily be missed when using 2-4X magnification glass. Conclusion It is very important to understand design, materials, and process since they are very close- ly connected. There are many ways to achieve high-density assemblies, and it is crucial to have a "toolbox of technologies" to be able to fulfill various requirements. It is also neces- sary to consider the interaction between mul- tiple technologies in all areas during develop- ment and deployment since several advanced technologies will, in most cases, be used on the same product. Depending on the end product, several options can be considered and the selec- tion should be based on data and not assump- tions. SMT Editor's Note: This article was originally pub- lished in the proceedings of SMTA International. References 1. IPC-A-610E-2010: Acceptability of Elec- tronic Assemblies 2. Jensen, T.S., "The Graping Phenomenon: Improving Pb-Free Solder Coalescence through Process and Material Optimization," Indium Corporation, 2008. 3. Briggs, Ed, IMAPS, Orlando, Florida (2015). Jonas Sigfrid Sjoberg is the technical manager for Asia at Indium Corporation. Wisdom Qu is a senior technical support engineer in for Indium Corporation in eastern China. David Sbiroli is technical manager for Indium Corporation's Global Accounts. Chris Nash is the product manager of PCB assembly materials at Indium Corporation. Table 4: Liquidus and solidus temperatures for SAC 305 and SAC 387. DFX ON HIGH-DENSITY ASSEMBLIES