Soldering Materials and Metallurgy

J.-P. Clech, Review and Analysis of Lead-Free Solder Material Properties, to appear in lEEEAViley NEMFs Lead-Free Project, 2004. On-line version can be viewed at http //www.metallurgy.nist.gov/solder/ clech/... 

J. Liang, N. Dariavach, and D. Shangguan, Metallurgy, Processes and Reliability of Lead-Free Solder Interconnects, to appear in Micro- and Opto-Electronic Materials and Structures Physics, Mechanics, Design, Reliability, Packaging, E. Suhir, C.P. 

Material such as plastic and ceramic component bodies, lead and pad metallurgies, solder, printed circuit board laminate, solder paste flux, and adhesives all absorb infrared radiation at different rates. Therefore, the direct radiative approach is known to cause overheating of some components and underheating of others. For this reason, direct IR heating is no longer the preferred method for reflow. 

In the end, it s understandable to feel victimized by the change to lead-free solders and the fact that metallurgy choices may not be yours to make. The change most likely will come suddenly, inconveniently and piecemeal, making the management of separate materials a headache. There is abundant lead-free information online and quality support available to educate technicians and engineers for the change. When it comes to rework and repair, there are economical and simple steps that can be taken to permit a smooth phase-in to the lead-free zone. Go forth boldly and prosper. 

The importance of understanding the physical metallurgy of the Pb-Sn alloy system stems from the microstructure-properties relationship. That is, the microstructure of a Pb-Sn alloy, or any material for that matter, determines its mechanical properties. In the case of Pb-Sn solders, this relationship is particularly critical because (1) the microstructure of these materials is sensitive to the cooling rate used in an assembly process, and (2) the microstructure can be readily altered during the course of the service life experienced by an interconnection. 

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