Finite Element Analysis (FEA) & Strain Gauge
At Forwessun, full Finite Element Analysis (FEA) checks are performed on every test fixture during the design stage. Physical Strain Gauge tests can also be performed prior to shipment, if required.
Why FEA / Strain Gauge?
To run the In-Circuit Test (ICT), a test fixture uses mechanical mechanisms such as pneumatics or a vacuum; the function of which is to attach the test points of the Unit Under Test (UUT) to the test probes in the fixture. Ultimately this creates a connection between the UUT and the ICT machine.
The number of probes and their dispersion on the UUT, along with the compression force of the probe, directly affects the amount of force that develops in different areas of the UUT.
If the test fixture isn’t designed correctly, damage could occur to the UUT or its components as a result of the forces exerted during the testing process.
Simulation
To help us simulate what is happening inside the test fixture during the testing process, we use SolidWorks Simulation to simulate the forces exerted on the UUT.
Test fixture designs are first prepared using 2D CAD software. This allows us to place the components needed in the fixture; including the UUT, Test Probes, Feet and Pips. When the first draft of designs are complete an electronic drawing is generated so we can start simulations in SolidWorks Simulation.
When using SolidWorks Simulation the test fixture designer can visually see if there might be any potential problems with the design of the fixture. This will allow them to pin-point any problem areas, and then redesign aspects of the test fixture before it gets built.
Once the simulation passes, it will give both us and our customer the confidence that no UUTs will be damaged during testing.
Results
Once the simulation has been produced, there are a variety of results to interpret.
Stress:
This takes into account the von Mises stress occurring on the UUT. The failure rate is defined by the material used and failure at this section means the fixture may cause total structural failure and damage the UUT itself.
Displacement:
The displacement takes into account possible movement and bending of the UUT inside the fixture. Our standard highest possible limits are at 0.1mm movement across the whole UUT, but we normally try to keep this below 0.08mm to ensure the least possible movement from the board.
Strain:
We also simulate the equivalent strain expressed on the UUT and try to limit the strain the board is under to below 500 με (microstrains). The points of interest we look at are the strain that is situated around components and most importantly BGAs. We can also simulate the placement of Strain Gauge rosettes and cross reference with our own physical strain tests carried out after the fixture is built.
All results can be customised to your specifications for limits and standards.
Physical Strain Gauge Tests
Using FEA is only a simulation, to make sure that these results are reflected in the real world we can perform a physical Strain Gauge test, if required. For this, we attach strain gauge rosettes to a sample UUT and take measurements when the UUT is in the test fixture.