Fatigue crack propagation behavior of underfill materials in microelectronic packaging

Cracks formed in underfill materials under loading are often observed to continue their propagation into traces in the substrate, which causes electrical failures in microelectronic devices. Hence, a fracture mechanics-based technique was used to characterize the fatigue crack propagation behavior of six different underfill materials under two different environmental conditions, i.e., ambient and 85°C/85% relative humidity (RH). Under the ambient condition, there was a well-defined threshold in each material studied, and the fatigue crack growth rate was found to have a power-law dependence on stress intensity range. However, under the 85°C/85% RH condition, near-threshold instability was observed. The crack growth rate at the near threshold region was found to oscillate from low to high after a period of time during cycling. The near-threshold instability is believed to be the result of interaction between the crack tip and moisture.