Defect Propagation
A structural degradation process occurs when tiny cavities within a solder joint grow and coalesce during thermal or mechanical loading. This microvoid expansion reduces the effective contact area and weakens the mechanical strength of the connection. The growth of these voids is driven by strain and elevated temperatures.
It can eventually lead to the propagation of cracks and complete electrical failure.
Thermal Cycling
Repeated temperature changes generate local shear stresses due to mismatched expansion rates between materials. These stresses cause plastic deformation in the solder, promoting the growth of existing voids. Higher temperatures also accelerate diffusion, allowing atoms to move and vacant spaces to merge.
Analysis Procedure
Solder joints are inspected using high resolution x-ray systems or cross sectional microscopy to track the size and distribution of the cavities. Initial void levels are measured after reflow, and then reassessed after environmental stress testing. The total area of the voids must not exceed the limits specified in industry reliability standards.
Comparing these images before and after testing reveals the rate of defect growth.
Process Control
Minimizing the initial void density during assembly requires optimized stencil printing and reflow profiles. Using solder pastes with low outgassing characteristics reduces the number of trapped gas bubbles. Proper temperature control during operation also slows down the growth of these defects.
