What Is a Strain Gauge?
A strain gauge measures the changes in electrical resistance when force is applied. Using a strain gauge in testing allows us to see how forces impact physical material such as a PCB, allowing engineers to make informed design decisions. The test allows us to get a clear image of how a material or board is reacting under load pressure. Too much physical stress can damage a PBC in the testing process, therefore its important to get the key areas of the design correct.
In Testing, What Is a Strain Gauge Used For?
In the ICT test, a fixture uses various mechanical mechanisms, such as pneumatic or vacuum systems, to attach the test points of a PCB to the test probes of the fixture, ultimately creating a connection between the PCB and the ICT test machine. The number of pins, their density, and their dispersion on the PCB being tested, along with the compression force of the pin, directly affect the amount of pressure that develops in different areas of the circuit. During the test of the fixture, possible damage can occur to the printed circuit or its components due to the pressure mechanism during the test process.
What Strain Gauge Tests Are Carried out?
We use Finite Element Analysis (FEA) software before constructing any test fixture to predict and simulate the forces exerted on the PCB during the In-Circuit Testing (ICT) process. FEA allows us to model how mechanical forces, such as pressure from test probes, will interact with various components of the PCB. However, since this is only a simulation, to ensure that these forces are accurately reflected in the real world, strain gauge rosettes are placed on the fixture.
Strain Gauge Rosette Testing
To carry out a real-world test, the solder rosettes are placed onto the test fixture. The major point of fault that can occur is the strain endured by BGA devices. With a high concentration of probes below a BGA and limited ways of counteracting this force, fractures can occur along the solder balls. The images below give a clear example of how the fractures appear.
In Summary
Forwessun combines FEA (finite element analysis) software, strain gauge rosettes and data acquisition hardware to measure the strain of the UUT during test. Each rosette is placed in an ideal position to measure either the strain of areas measured high during the FEA simulation, or around high sensitive components such as BGA’s.
FAQs
What is strain gauge testing?
Strain gauge testing measures the mechanical stress applied to a PCB during testing.
It helps identify bending, flexing, or deformation that could lead to cracked solder joints or component failures.
Why is strain gauge testing important?
PCBs can be damaged by excessive mechanical force during clamping, depanelisation, or ICT/FCT testing.
Strain gauge testing ensures the board remains within safe stress limits, improving long‑term reliability and reducing field failures.
How does a strain gauge work?
A strain gauge is a thin sensor bonded to the PCB surface. When the board bends or flexes, the gauge changes resistance. This change is measured and converted into precise strain data, allowing engineers to monitor stress in real time.
Where are strain gauges placed on a PCB?
Can strain gauges be used in ICT or FCT fixtures?
Yes. Strain gauges are often integrated into ICT and FCT fixtures to monitor the force applied when the PCB is pressed onto test probes. This ensures the fixture does not introduce excessive stress that could damage the board.
What problems can strain gauge testing help prevent?
When should manufacturers use strain gauge testing?
Strain gauge testing is recommended when launching a new product, validating a new fixture or analysing field failures.
Does strain gauge testing slow down production?
No. Strain gauge testing is typically performed during validation, not during every production cycle.
Once the fixture and process are verified as safe, normal testing continues without strain gauges attached.