Microvia Stress Mechanics under Reflow Temperature Profiles
Substrate Z-axis thermal expansion above Tg drives low-cycle fatigue and target pad separation in HDI microvias during SAC305 lead-free reflow profiles.
Copper Grain Boundary Cracking
Material Failure
Intergranular fracture occurs in the copper foils and plating of printed circuit boards under elevated thermal or mechanical load. The occurrence of copper grain boundary cracking is associated with the separation of individual metal grains along their shared interfaces rather than through the grains themselves. This structural separation breaks the electrical continuity of circuit traces or plated through-hole barrels, causing intermittent signals or total board failure during operation.
Such defects are identified by cross-sectional analysis and high-magnification scanning electron microscopy.
Failure Mechanism
Stress concentrations accumulate at the microscopic boundaries of the metallic structure when the assembly undergoes rapid temperature changes or mechanical bending. High temperatures during lead-free soldering cause the surrounding laminate material to expand at a much higher rate than the copper plating. This expansion mismatch exerts a severe tensile force on the thin copper walls of plated through-holes, pulling the grains apart where their cohesive strength is lowest.
The resulting cracks propagate rapidly along these boundary paths until they breach the entire thickness of the metal layer. Continuous thermal cycling testing is performed on representative board samples to verify that the plating can survive hundreds of transitions without initiating cracks.
Mitigation Method
Preventing this defect requires optimizing the electroplating process to produce a fine, equiaxed grain structure that resists boundary sliding. The addition of specific organic additives to the plating bath ensures uniform copper deposition with minimal internal stress. This processing step increases the ductility of the copper layer, allowing it to withstand the thermal expansion of the laminate during reflow without cracking.