Effects of Thermal Cycling and PCB Substrate Type on Reliability of Solder Joints

This study aimed to analyze the reliability of solder joints on different types of printed circuit board (PCB) substrates. Conventional glass-epoxy (FR-4) laminate and aluminum substrate with glass-epoxy dielectric layer were included in the evaluation as the rigid types. Tested flexible PCBs were made from polyimide. Moreover, FR-4 substrates varied in used surface finishes. Soldering pads were protected by hot air solder leveling (HASL) or organic solder preservative (OSP). Components mounting was accomplished using low- temperature solder alloy Sn42Bi57Ag1 and consequent convection reflow soldering. Assembled PCBs were subjected to 500, 1000, 1500, and 2000 cycles in the thermal shock chamber. The first evaluation approach was the measurement of electrical resistance during and after the thermal cycling. Shear strength of solder joints was also assessed. A thickness of intermetallic layers (IML) was observed in relation to the mechanical properties of solder joints. Thermomechanical analysis (TMA) was employed to determine the glass transition temperature (T
g
) of organic substrates and to obtain the coefficient of thermal expansion (CTE) of all tested substrates. Solder joints on the substrate with an aluminum base layer exhibited the greatest degradation due to thermal cycling. The electrical resistance of the joints on this substrate type increased significantly, and the occurrence of extensive cracks was very frequent. A substantial increase in the width of the IML reduced the shear strength.