Strain gages

Strain gages are small devices used to measure surface strain. Perfectly adhering to a PCB’s surface, the sensor will stretch according to the PCB surface strain at a precise location.

These devices are made from a very thin metallic element with electrical resistance that varies according to stretching or compression. As in Image 3, a sensor installed at point A will stretch and indicate a positive value (e.g.: 600 microstrains). However, a sensor installed at point B will contract and indicate a negative value (e.g.: -600 microstrains). The strain gages are unidirectional; i.e. show strain on one axis. Certain superimposed models, as in Image 4, can show strain on several axes simultaneously. When used appropriately, the strain gage is a very precise and indispensable tool.

When installed inside a fixture during a vacuum test, strain gages let us obtain a real-time reading of the strain involved on the PCB. The Strain vs. Time Graph (graph above) that we provide with each stress analysis represents the strain over a period of 10 seconds. During this period, vacuum is applied to and removed from the fixture. The red, yellow and blue curves in the above graph represent the three different axes of the sensors used. The black curve represents the maximum diagonal strain calculated according to the following equation:

MAX( |e1+e2-e3|, |e3| )

Typically, we recommend that the maximum strain be less than 600 microstrains at all times. Strain levels are directly proportional to vacuum levels. All our tests are performed at a maximum vacuum level, between 28 and 29 inches of mercury.

Strain gages can be installed at any place where high strains are expected from probe concentrations. An IPC guideline for strain gage testing (3) recommends that all BGA components with a body size equal to or larger than 1 inch should be tested using strain gages. One strain gage should be installed on all four corners of the component. If all the BGAs on the PCB are smaller than 1 inch, it is recommended to test the three largest devices.