Strain Gage Testing is the practical assessment of flexion on specific areas of a body while it is under stress.
Strain Gauge Test is used to measure PCB assembly board flexion when they are tested on a Test Fixture. When PCB assemblies are tested they are set under stress due to unbalanced forces acting on the PCBA. These forces result when pressure from probes contacting the test points cannot be fully balanced by the pushdown rods and stoppers supporting the PCB assembly. As a consequence, the PCB assembly board flexes in many directions.
T-30 Close up photo of a test fixture showing probes and pushrods before actuation
PCB assembly flexion is measured using the principle and methods of Strain Measurement. When PCB assembly flexes, a change in length or strain on its substrate occurs. This change of length can be measured using sensors called Strain Gauge. Strain Gauges are a conductive metal strip with an electrical resistance that changes its value according to the compression or stretching under the device is set. When the device is stretched, the metal strip becomes thinner and longer and its resistance increase; when it is compressed, the metal strip will get thicker and shorter and its resistance will decrease. The most common strain gauge sensor used for the Strain Gauge Test on PCB assemblies is the Rosette. A rosette is a stack of three strain gauge sensors set at 0o, 45o and 90o angles that allow measuring strain in three directions on the same spot.
T-31 Photo of a Rosette stamp
A scanner connected to the strain gauges is continuously measuring their resistance while the PCB assembly is set under stress. This method allows measuring changes in the Strain Gauge resistance and also the speed at which these changes are occurring.
T32 Photo showing the Strain Gauge Test results in a graphic
We run a Strain Gauge Test when a customer includes it in the requested scope of supply or when the PCB assembly contains BGA or big ICs. Our Strain Gauge Test process is carried out under the IPC/JEDEC-9704A Printed Circuit Assembly Strain Gauge Test Guidelines. This document describes methodologies for strain gauge placement and sets specific guidelines for strain gauge testing, evaluation, and reporting.
T-33 Photo of a PCB prepared for an SGT test placed on a fixture