Meaning
Determination of internal stresses remaining in a material after processing reveals the latent forces that can cause warpage or failure. Residual stress measurement quantifies the locked-in forces within printed circuit boards and solder joints after reflow. These internal stresses result from mismatched thermal expansion between metal traces, glass-reinforced laminates, and silicon dies.
The measurement identifies the baseline stress state prior to any external mechanical or thermal loading in the field.
Experimental Technique
Diffraction methods measure the change in atomic lattice spacing to calculate internal stress without destroying the sample. X-ray diffraction calculates the strain by measuring how the Bragg angle shifts across the surface of the solder joint. For deeper analysis of internal layers, neutron diffraction or hole-drilling strain gauge methods are deployed.
Hole drilling involves removing a small amount of material and measuring the deformation of the surrounding area as stress relaxes. This destructive method is used to calibrate and validate the non-destructive diffraction scans.
Manufacturing Impact
High internal stresses cause printed circuit assemblies to warp during subsequent assembly steps. Warpage leads to open connections or bridging of adjacent contacts on high-density ball grid arrays. Measuring these stresses helps adjust the cooling rates in reflow ovens to minimize gradients.
By optimizing the process parameters, factories reduce the residual forces and improve the long-term reliability of the finished boards.
Stress Redistribution
Over time, residual stresses relieve themselves through plastic flow, especially at elevated operating temperatures. This relaxation can warp the package or initiate microcracks in the solder joints.
