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.
Target Pad Detachment
Failure Mode
Separation of a copper terminal pad from the laminate substrate of a printed circuit board indicates a severe degradation of structural integrity. This specific failure mode, referred to as target pad detachment, occurs when the adhesive bond between the copper and resin layers fails.
Physical Stress
Mechanical forces generated during thermal cycles or assembly handling put immense stress on the copper pad boundary. Since the pad acts as the anchor for vias or component leads, any pulling force is transmitted straight to the laminate. If the resin has been weakened by excessive heating or multiple rework cycles, the pad can lift off the surface during solder solidification.
This defect makes subsequent soldering impossible and requires scrapping the entire assembly.
Prevention Method
Moisture absorption by the board laminate is a common precursor to adhesive bond failure at the metal interface. When the board is heated during assembly, trapped moisture vaporizes and expands, forcing the copper away from the resin. Baking boards prior to soldering removes this moisture and helps prevent the structural damage.
In addition, specifying high glass transition temperature materials increases the board resistance to pad lift during lead free assembly. Careful control of solder iron temperature during rework operations also minimizes localized thermal stress that can cause the detachment.