Microvia Target Pad Interfacial Separation under Reflow Thermal Stress
Microvia target pad separation stems from z-axis thermal expansion strain exceeding electroless copper interfacial bond strength during assembly reflow.
Thermal Strain Mismatch
Expansion Differential
Discrepancy in the rate of dimensional change between different materials as they are heated or cooled creates internal mechanical tension. The thermal strain mismatch in a printed circuit board primarily occurs between the copper conductors and the surrounding resin system. Because copper expands at a different rate than the glass reinforced epoxy, the structure undergoes considerable stress during reflow or operation.
This effect is most pronounced along the z axis of the board where the resin expansion is least constrained.
Mechanical Stress
Accumulation of force at the interface of the via barrel and the internal pads can lead to fatigue and eventual fracture. The thermal strain mismatch drives the barrel of a plated through hole to stretch, while the surrounding laminate expands even further. Over multiple thermal cycles, this movement can cause the copper to work harden and crack.
Components with large footprints also experience this tension at the solder joints when the board and the package expand at different rates.
Material Selection
Designers mitigate the risk of failure by choosing laminates with a low coefficient of thermal expansion that more closely matches the copper. Managing thermal strain mismatch is a primary concern for high reliability applications where the board must survive extreme temperature swings without losing electrical continuity.