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.
Electroless Copper Adhesion
Interfacial Strength
Mechanical and chemical bonding between the primary copper deposit and the dielectric surface of a printed circuit board determines the stability of the conductive path. Sufficient electroless copper adhesion prevents the separation of thin metallic films from the resin during subsequent electrolytic plating or thermal processing. This property relies on the micro roughness of the hole wall created during the desmear stage of fabrication.
Without a strong bond, the copper shell may pull away from the substrate when subjected to the heat of soldering. High reliability boards require careful control of the etch rate to maximize this surface contact.
Verification Procedure
Standardized tape tests or pull strength evaluations measure the force required to detach the metal layer. Success in electroless copper adhesion signifies that the palladium catalyst and subsequent copper reduction have created a continuous anchoring network. Inspectors look for blisters or delamination after the board undergoes a thermal stress test.
Failure often occurs at the junction of the hole wall and the inner layer copper.
Chemical Influence
Control of the bath chemistry and immersion timing dictates the quality of the initial deposit. If the electroless copper adhesion is compromised by excessive stabilizer concentrations or insufficient surface activation, the entire plating structure becomes vulnerable to mechanical failure.