Dimensional Distortion
Plastic deformation occurs when materials undergo continuous stress under high temperatures over extended periods. Multi-axis creep strain identifies the cumulative movement of solder joints or dielectric structures when loads act simultaneously across several planes. It represents a permanent change in shape that does not recover when the load is removed.
Solder Behavior
Lead free alloys are particularly susceptible to time dependent deformation during the thermal cycles of field operation. Multi-axis creep strain in a solder ball is driven by the mismatch in the coefficients of thermal expansion between the component and the board. Shear forces and tensile forces combine to accelerate the movement of the metallic grains.
Thermal Loading
Predicting the long term reliability of high power electronics requires an understanding of how strain accumulates under constant pressure. The multi-axis creep strain is measured using specialized sensors or digital image correlation during accelerated life testing. Failure occurs when the distorted material creates a bridge between conductors or when the cross section of a joint thins to the point of rupture.
Designers mitigate this effect by selecting materials with higher creep resistance or by adding underfill to distribute the load across a larger surface area. Calculation of the strain rate involves the secondary creep stage where the deformation is nearly linear with time. This value provides a limit for the operational life of the assembly in automotive or industrial environments.
Reliability depends on these rates.
