The material properties related to lead-free solder survival are typically coefficient of thermal expansion (CTE), glass transition temperature (Tg), decomposition temperature (Td) and moisture absorption.
As a general rule, the material should have a CTE of 70 ppm/°C or less and closer to the CTE of copper, about 17 ppm/°C, which is best. Other general rules suggest a Tg value higher than 170°C, Td greater than 300°C and moisture absorption less than 0.5%.
The high-frequency circuit material industry offers many laminates and prepregs (or bonding material) to choose from. Some of the laminates and bonding materials are robust for lead-free soldering, and others are not. Additionally, there are some bonding materials which are not intended to be used at lead-free soldering temperatures.
A broad classification of bonding materials can be thought of in two categories. Some bonding materials are thermoplastic, while others are thermoset. A thermoplastic bonding material has the attribute of being able to reflow or melt when subjected to certain elevated temperatures. The thermoset materials will not melt or reflow when subjected to elevated temperatures. However, at high temperatures, the concern can be related to material decomposition.