Board level solder reliability vs. ramp rate & dwell time during temperature cycling

The effects of ramp rate and dwell time are studied through laboratory results and Design Of Experiments (DOE) using Finite Element Analysis (FEA) incorporating stress/strain and plastic work history. Results demonstrate that solder joint fatigue life is more sensitive to dwell time than ramp rate during thermal cycling. A non-linear relationship exists between solder fatigue life (Mean Time To Failure, MTTF) and dwell time. Increasing dwell time past 20 minutes has a minimal effect on the acceleration factor or lifetime. This study also reveals that a dual chamber test presents harsher stress conditions than a single chamber test (when comparing the same number of cycles/hour) for solder joints because of an associated longer dwell time (faster ramp rate has a lesser effect). Modeling shows that the acceleration factor increases by a factor of 1.23-fold for a specific BGA assembly when the test condition changes from single chamber to dual chamber. Experiments were performed to validate the FEA modeling. Different ramp time/dwell times were achieved by modulating the temperature profile in a single chamber oven. If dwell time changes from 5 minutes to 10 minutes with a ramp time of 5 minutes, apparent MTTF decreases by 55%. However, MTTF remains practically the same as the ramp time increases from 5 to 10 minutes, if dwell time is 10 minutes. These test results are consistent with modeling predictions. The focus of temperature cycling tests should NOT be on number of chambers, nor upon ramp rate, but upon dwell time. We recommend 8 minutes to 10 minutes dwell at a high temperature of 125/spl deg/C.