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.
Microvia Interfacial Separation
Defect Mechanism
Cracking along the boundary between plated copper structures in printed circuit boards represents a major reliability failure in high density designs. This physical detachment, known as microvia interfacial separation, occurs at the junction of the microvia fill and the target pad.
Manufacturing Cause
During chemical deposition of copper, any residue left on the target pad prevents the electroplated copper from forming a strong bond. This contamination often arises from improper desmear processes or flash copper oxidation during the manufacturing sequence. If the cleaning step fails to scour the surface, the weak copper joint cannot withstand the forces generated during subsequent reflow cycles.
Testing through cross sectioning or highly accelerated thermal shock is typically used to expose these weak boundaries.
Thermal Stress
Thermal expansion differences between the epoxy board material and the copper plating drive the physical failure of these joints. As the board heats up during soldering or operation, the resin expands rapidly along the vertical axis. This movement pulls the copper microvia away from the target pad, causing an open circuit that may be intermittent.
Such failures are extremely difficult to diagnose because they often heal themselves when the board cools back down to room temperature. Implementing strict acid cleaning and plasma treatment of target pads before plating eliminates the microvia separation hazard.