Quantifying Inner Layer Interconnect Strain during Solder Reflow
Matching dielectric CTE above Tg to copper ductility prevents inner layer interconnect fracture during high temperature reflow thermal profile cycles.
Non-Functional Pad Retention
Board Adhesion
The physical bond strength that holds unused copper rings to the laminate surface prevents them from lifting during the thermal and mechanical actions of drilling and lamination. Known as non-functional pad retention, this characteristic is important for multilayer boards where certain layers do not have active electrical traces connected to a particular through-hole. These pads are kept on the laminate to provide mechanical support to the hole wall, even though they serve no electrical purpose.
Ensuring adequate bond strength prevents these unanchored pads from detaching and causing defects during drilling or thermal stress. When pads remain firmly attached, they act as anchor points that reinforce the plated copper barrel against the expansion of the board.
Hole Stabilization
Manufacturing practices often involve optimizing the lamination process and using treated copper foils to maximize the adhesion of the metallic structure to the resin. When resin flows around the non-functional pads during lamination, it locks them into the dielectric stack. If the adhesive bond is weak, the rotating force of the drill bit can tear the copper pad away from the hole wall, causing micro-voids or resin damage.
To mitigate this risk, board fabricators must carefully control the resin cure profile and drill parameters such as feed rate and spindle speed.
Structural Assessment
Evaluation of pad adhesion is achieved through microsection analysis and thermal shock testing. Technicians examine cross-sections under a microscope to check for any separation between the unused copper pads and the surrounding laminate after soldering simulation. If any pad has lifted, the board fails the workmanship standard because the gap can collect moisture or plating chemicals.
These liquid residues could cause long-term reliability issues such as conductive anodic filament growth during the service life of the circuit.