Effect of steady-state operating temperature on power cycling durability of electronic assemblies

Temperature (thermal) cycling is a major cause of failures of electronic systems. This paper discusses the effect of the steady-state operating temperature, as controlled by parameters such as the coolant inlet temperature, on the fatigue life of electronic assemblies subjected to thermal cycling resulting from off-on power cycling. The power cycling fatigue life depends on the steady-state operating temperature T-s, as well as on the temperature range Delta T in the thermal cycle. The power cycling durability of copper plated through holes in circuit boards is very sensitive to the steady-state PTH operating temperature, because the strain ranges are in the portion of the copper S-N-f(strain vs. number of cycles to failure) curve that is nearly horizontal. If the power is turned on at the same time the coolant now is started, reducing the steady-state operating temperature can improve the durability. If the coolant is turned on for a substantial period before the power is turned on, reducing the coolant inlet temperature may degrade the durability for operation on hot days.