Mechanical Force
Internal energy stored in a solid material undergoing angular deformation affects the mechanical durability of solder joints. The shear strain energy represents the accumulated stress that occurs when adjacent board components experience differential displacement. This energy builds up primarily during thermal cycles where materials with different expansion rates are joined.
Thermal Stress
Temperature fluctuations cause printed circuit board assemblies to expand and contract at different rates because of mismatched coefficients of thermal expansion. In a typical assembly, the silicon chip, the plastic package, and the FR4 substrate expand differently, which generates shear forces on the interconnecting solder balls. The resulting shear strain energy accumulates within the solder joint with each thermal cycle, driving microstructural changes and the initiation of microcracks.
This energy concentration is highest at the corners of large packages where the displacement is greatest.
Joint Failure
Fatigue life and reliability of the board assembly are directly tied to the management of these localized strain energies. When the accumulated shear strain energy exceeds the cohesive strength of the solder alloy, a crack propagates through the joint, leading to an open circuit. Mechanical testing, including thermal cycling and vibration profiling, is used to quantify the strain energy and predict the time to failure.
Designers minimize this hazard by using underfill materials to distribute the strain more evenly or by selecting laminate substrates with matched thermal expansion properties. This preventative measure ensures assembly survivability.
