In Circuit Fixture Spring Probe Force Balancing on Ultra Fine Pitch Assemblies
Balancing micro-probe forces with precision push-fingers prevents substrate strain over 500 microstrain, protecting fine-pitch BGAs and MLCCs from fixture damage.
Fixture Planarity
Surface Leveling
Geometric surface alignment ensures that all contact points on a test or assembly fixture sit within the same horizontal plane. Proper fixture planarity prevents the uneven application of force when the tool engages with a printed circuit board. This uniformity is achieved through precision milling and the use of flat base materials such as aluminum or high-pressure laminates.
Measurement of the surface height at multiple points confirms that the fixture meets the design specifications.
Contact Uniformity
Pressure distribution depends on the relative height of the individual probes or push fingers. If the fixture planarity is poor, some probes may apply excessive force while others fail to make a connection. This leads to false failures during electrical testing or physical damage to the pads on the board.
Shimming or adjustable mounts are used to correct small deviations in the alignment of the top and bottom plates.
Stress Prevention
Mechanical integrity of the circuit board is protected when the fixture remains perfectly flat. When fixture planarity is maintained, the board does not flex or bow during the press cycle. Excessive bending can fracture ceramic capacitors or break the internal copper barrels of the through holes.
Maintaining a flat interface is a requirement for high-volume production of thin or fragile substrates.