Fixture Strain and Spring Force Deflection Limits on High Density Assemblies
Fixture spring force and deflection limits enforce maximum micro-strain thresholds to prevent latent BGA pad cratering and ceramic capacitor fracture during testing.
Spring Force Limits
Probe Pressure
Mechanical constraints for test probes define the maximum and minimum pressure allowed to ensure reliable electrical contact without puncturing the target pad. Spring force limits prevent the excessive wear of gold plating on pads while maintaining enough force to break through surface oxides. These limits are specified in grams or Newtons and are determined by the diameter and tip style of the probe.
Contact Reliability
If the pressure is too low, the connection may be intermittent and lead to false failures during the electrical test. Maintaining the spring force limits ensures that the probe makes a solid metal-to-metal contact with the pad every time the fixture is closed. The force must be high enough to handle any contamination on the surface but low enough to avoid deforming the board or the components.
Surface Protection
High-density boards with small pads are especially sensitive to the force applied by the test pins. Engineers select probes that operate within the spring force limits to avoid leaving deep indentations on the soft copper or gold surfaces. Regular testing of the probe springs is necessary to ensure they have not weakened over time and fallen below the required minimum force.
Adhering to these limits extends the life of both the test fixture and the boards being tested.