In the Chapter 3 on "Thermal & Mechanical Stress," Harry Charles mentions a "homologous point" in 3.5. We are unfamiliar with this term. What is this point? What are the implications of solders in operating environments below their homologous point?
The homologous point is the temperature midway betweeen the melting temperature (of the solder) and absolute zero. Below this point the solders are harder, stronger, and more resistant to fatigue. As solders are exposed to higher temperatures above the homologous point they become softer and more subject to fatigue and solder joint failure.