Meaning
Mechanical deformation occurs when an applied force acts parallel to the face of a material, causing the internal layers to slide past one another. In microelectronic solder joints, shear strain is defined as the displacement of the joint divided by its original height, representing the angle of deformation under load. This parameter is generated by the differences in thermal expansion between the silicon chip and the underlying circuit board during temperature changes.
The resulting mechanical stress accumulates in the solder joints, eventually leading to fatigue cracks and electrical failures.
Strain Analysis
Solder joints experience varying levels of deformation depending on their distance from the center of the component package. The furthest joints from the neutral point experience the highest levels of shear strain and are the first to fail under thermal cycling. Engineers use finite element analysis to calculate these stress distributions during the design phase of the assembly.
Material Response
Lead-free solder alloys exhibit viscoplastic behavior, meaning they deform permanently over time when subjected to continuous strain. This deformation rate increases at elevated temperatures, which accelerates the creep damage in the joint. Selecting alloys with higher resistance to deformation helps mitigate the effects of this stress.
Testing Methods
Mechanical test systems measure the shear strength of individual solder joints by applying a lateral force until failure occurs. These tests provide the force and displacement data needed to calculate the maximum strain the joint can withstand. This information is used to verify that the assembly meets the required industrial durability standards.
