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Strain rates

The strain rate is the increase in strain per second (microstrain/sec). The Bansal, Yoon & Mahadev study (2) showed the relationship between BGA failures and strain rates. The faster a force is applied to the PCB, the less force is required to damage the UUT. A weak force applied very quickly does more damage than a strong force applied slowly. The study shows that it is possible to repeat  BGA  fractures in production during high-speed flexion tests (>5000 microstrain/sec). At low speeds (about 500 microstrain/sec), they showed that some components even withstood up to 6800 microstrain. However, at high speeds, a strain of 1000 microstrains was enough to damage the components. We suggest a maximum strain of 600 microstrains at all times.

Strain rate versus time

In the above graph, the points beyond |5000| should be considered as an indication to a zone where the strain should be less than 600 microstrains. Between the seventh and eighth second, we can see that the strain decelerates to a speed of about -6200 microstrain/sec. In the next graph, we can confirm that these strain values are under the established limit.

Strain versus strain rate

It is thus essential to measure and control the strain rates. The speed at which the probe and vacuum forces are applied to the UUT can be easily controlled by using a vacuum regulator or using more or less supportive springs between the fixture's base and support plates. Only appropriate equipment using strain gages can measure strain rates.

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