Finite Element Analysis (FEA)
FEA works by dividing the real object into a mesh consisting of millions of smaller, simple parts called the finite elements. The method approximates the real object unknown reaction to the conditions by generating simple mathematical equations that model each finite element. A computer then assembles each finite element into a large system of mathematical equations to predict the behavior of the real object. There are several FEA tools in the market, as dedicated software or as a part of the CAD software
FEA is used as the basis for simulating how the PCB assembly will behave under the forces generated inside the test fixture by showing areas of unbalances forces on the PCB assembly. These areas of tension are the product of unbalanced forces resulting from probes generating pressure while contacting the test points and push down rods and stoppers counterbalancing these forces.
T-25 Screen shoot or Photo of a computer monitor showing the preparation to run an FEA Analysis
In the process of designing a fixture, the distribution and position off probes are given by the PCB assembly test point layout. Push Down rods and stoppers are then distributed and position to counterbalance the forces generated by the probes. In principle, the pressure forces generated by the test probes are counterbalanced by locating one push down rod or stopper in the same position of each test probe, but on the opposite side. Bottom test probes are counterbalance with push down rods and top test probes with stoppers. This, however, is seldom the case due to the limited space available to set stoppers and push down rods. This limitation results in some areas of the PCB assembly, usually those with a high probe density, where the forces cannot be fully balanced. The unbalanced forces produce flexion on the PCBA board that may cause the component to break or solder joint to fracture. This situation is very critical when the PCB assemblies have BGAs or large ICs since these devices are easily damaged when board flex and they are generally accompanied by large amounts of small SMD components.
PCB-Test incorporates the FEA in our Fixture Design Process implementing an FEA tool as part of our CAD design platform. The outcome of the simulation is depicted via a color scale that shows the pressure distribution or flexion areas along with the PCB assembly. The scale goes from blue color to red color; red indicating areas with large flexion and blue indicating areas with very low or no flexion at all.
T-26 Photo of an FEA analysis run showing areas of stress
The information produced by the FEA permit us to:
Remove, add or relocate stoppers, push down rods and reinforcing element to improve force balance on the PCBA
Reduce to pressure on the PCBA by reducing probe force
Coordinate with board designer possible changes in the probe layout to reduce the number of probes in high-density areas and/or reduce component dimension to make space available for additional stoppers and push down rods.
The following picture sequence shows the addition and relocation of stoppers after running the FEA to improve force balance and reduce board flexion on a PCBA
First design: 6 Stopper, max flexion of 0,25 mm
T-27 Photo of the first run of an FEA Analysis showing the PCB with a stress area
Step 1: 1 pc Stopper added, max flexion 0,12 mm
T-28 Photo of a second run of an FEA Analysis after changes showing the PCB with a new stress area
Step 2: Stoppers added and shifted, max flex 0,06 mm
T-29 Imagine a desfășurării celei de-a treia analize FEA după modificările apărute, care evidențiază placa de circuit imprimat fără o zonă de tensiune
We recommend running FEA analyses in cases were:
PCB assemblies with high density on probes, especially in boards with BGAs and large ICs
Dense packed PCB assemblies with little space available for stoppers and push down rods
PCB assemblies with board thickness less than 1,5mm and a probes distribution that indicate possible flexion