What is the Ideal Coefficient of Thermal Expansion for a Substrate Used for Surface Mounting

INTRODUCTION: Thermal expansion mismatch is a major problem in surface mounted technology. Especially when large and leadless components are soldered to a substrate, there can arise large stresses and strains in the joining medium, usually solder. Formally analogous problems arise in flip-chip mounting as well, so these considerations apply equally to that case. The large number of recent papers on this subject attests to its relevance and also to its complexity. The answer to the question posed in the title, of course, depends on what you are going to do to the assembly. We consider here six situations, more-or-less in increasing order of sophistication. Each situation considered by itself calls for a different CTE (coefficient of thermal expansion.) Some generalizations, however, can be made, based on engineering judgment.