Effects of combined cyclic thermal and mechanical loading on fatigue of solder joints

In the past, laboratory tests have been used to examine surface mount solder joint fatigue by means of an accelerated test to failure and to establish correlations between thermal amplitude and cycles to failure of a specimen. In this work, mechanical loading is applied in addition to thermal loading. Test coupons populated with components are tested in an apparatus enclosed in a thermal chamber that applies cyclic four-point bending to printed circuit boards (PCBs) simultaneously with synchronized thermal cycles. Bending is regulated to a given surface strain measured by a thermally insensitive strain gauge circuit mounted on the surface of the PCB. During a cycle the temperature varied nearly linearly between -50°C and 150°C in 15 minutes and had 15 minute dwells at each extreme. The bending cycle was fully reversed and applied either in or out of phase with the temperature cycle. Electrical continuity and resistance in the solder joints is monitored, and the specimens are tested until failure criteria are met. As expected, the addition of mechanical strain to the thermal load results in shorter fatigue life. At a surface strain of 1000 με the fatigue life is significantly reduced in capacitors. In addition, the failure shifted from the solder to the component with visible cracks forming in the capacitor and at the interfaces between the capacitor and solder.