Effect of Lead-free Soldering on Key Material Properties of FR-4 Printed Circuit Board Laminates

摘要

The high temperature exposures associated with lead-free soldering of printed circuit boards (PCBs) can alter the laminate material properties thereby creating a shift in the performance and reliability of the PCB and entire electronic assembly. The knowledge of PCB laminate material properties and their dependence on the material constituents, combined with their possible variations due to lead-free soldering exposures, is an essential input in the selection of laminates for appropriate applications.An experimental study is conducted on fourteen types of commercially available PCB laminate materials to assess the effects of lead-free soldering process on key thermomechanical and physical properties. The laminates are classified on the basis of their glass transition temperature (high, mid and low), type of curing agents (dicyandiamide (DICY) and phenolic), type of flame retardants (halogenated and halogen-free), and presence or absence of fillers. Laminate material properties [glass transition temperature (T_g), coefficient of thermal expansion (CTE), decomposition temperature (T_d), time-to-delamination (T-260), and water absorption] are measured as per the appropriate IPC-TM-650 test methods before and after subjecting to multiple lead-free soldering cycles (namely, three reflow cycles, six reflow cycles, and a combination of one wave and two reflow cycles).The lead-free soldering exposures resulted in variations in the material properties of certain FR-4 laminate material types. The extent of variations in the thermomechanical and physical properties under investigation are discussed as a function of material constituents. It was found that the type of curing agent has a more pronounced effect on the response of materials to exposures than the type of flame retardant or presence of fillers. For example, a significant variation in the T_g and CTE of certain DICY-cured materials is observed after the exposures. Also, time-to-delamination of DICY-cured materials decreased whereas phenolic-cured materials could retain their thermal stability even after exposures. An increase in water absorption after the exposures is observed in most of the materials. The exposures did not affect the laminate materials to an extent of changing their decomposition temperatures.