Any time a structure experiences stress reversal, part of its life is used up. The two most common types of stress cycles that cause the most damage in electronic equipment are thermal/temperature cycling and vibration cycling. These two types of cycling can occur separately or they can be combined. Alternating stress cycles can be generated with a zero mean stress or it can also be superimposed on a steady stress.
The stresses may be thermal or vibration and sometimes shock. In electronic and photonic systems various stresses are often generated together, but they can be generated separately as well. Most of the time the steady-state stress is thermal and the alternating stress is vibration. Predicting the fatigue life of electronic equipment exposed to any combination of an alternating stress superimposed on a steady stress is very difficult, even with the use of a computer program. The fatigue life analysis made with any computer evaluation, no matter how sophisticated, should not be trusted unless there are test data from well-instrumented prototype test models to prove the accuracy of the data. Thermal cycling and vibration fatigue damage test data on printed circuit board (PCB) through-hole pin grid arrays (PGAs) showed no damage when the vibration tests were performed at 95 and 25 โข C. However, when the tests were performed at -55 โข C, many of the PGA wires fractured. The tests were always run with new assemblies to avoid any problems with possible fatigue damage. Some of the conclusions from these tests were shown as follows. Solder creep at elevated temperatures, and even at room temperature, allows the PGA wires to relax. This reduces the magnitude of the bending moments and bending stresses on the PGA wires. This increases the fatigue life of the wires. Solder creep at low temperatures is sharply reduced, so high thermal stresses are locked in the PGA lead wires. When the vibration is imposed at low temperatures, the PGA wires experience an alternating vibration stress superimposed on the sustained thermal stress. This increases the magnitude of the maximum stress acting on the PGA wires, which reduces the fatigue life of the wires. Slow thermal cycling events allow the solder to creep and to relax stresses in a manner that results in higher solder joint stresses and strains than rapid thermal cycle events. Slow thermal cycling therefore will result in more solder joint failures than rapid thermal cycling over the same temperature range. The slower solder is cycled the weaker it gets because of the creep and stress relaxation properties of solder.
Solder creep at higher temperatures presents a problem in trying to establish a laboratory thermal cycling test program that produces the same type of solder joint failures that are