Fatigue crack growth in solder joints at high strain rate
Chapter 9 investigates the fatigue resistance of solder joints through fatigue crack growth. While the expression of fatigue resistance in the form of strain-life facilitates fatigue design, the study of fatigue crack growth in solder joints provides valuable insights that could aid in the robust design of solder joints. Through the use of a sophisticatedly designed test vehicle and delicate instrumentation, the electronic resistance across a critical solder joint could be monitored to a nano-ohm resolution, which translates to a micrometer resolution for fatigue crack growth. Fatigue crack in the solder joint has been observed to form within a single board-level drop-shock without a period of crack initiation. When cycled at constant strain range at an elevated strain rate, the fatigue crack path in most solder joints tends to cut through a mixture of bulk solder and intermetallic compounds, resulting in a mix-mode fracture, although the ductile–brittle mixity differs between solder joints of different material system. Significantly lower fatigue crack growth rate (d a /d N ) was observed when the crack propagated within the ductile bulk solder; solder joints with higher fatigue resistance generally have longer fatigue crack propagation path within the bulk solder. The rate of fatigue crack growth (d a /d N ) increased at small crack lengths and reached a peak before declining gradually. Using three-dimensional fracture mechanics, the magnitude of the J-integral around the crack tip was found to correspond with the observed rate of fatigue crack growth. The declining J-integral at longer fatigue crack lengths is attributable to the increased compliance of the solder joint. Thus, the fatigue resistance of a solder joint can be improved by increasing the compliance of critical solder joints.