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.
Residual Tensile Strain
Internal Strain
Latent mechanical forces remain locked within the metallic deposits of a printed circuit board after fabrication and assembly processes are complete. The presence of residual tensile strain occurs when electroplating conditions, cooling cycles, or mechanical bending leave the copper layers in a state of continuous stretching. This internal force reduces the remaining capacity of the copper to withstand additional mechanical or thermal loads before failure.
If this internal force is too high, it leads to premature cracking of circuit traces and plated through-holes under normal operating conditions.
Strain Origin
Electrodeposition processes generate high levels of internal force when organic additives are not properly controlled or when the current density is uneven. During cooling after the reflow soldering process, the difference in thermal contraction between the copper plating and the substrate locks in additional tension. This tension remains stored within the copper grain structure, where it acts as a continuous load on the atomic bonds of the metal.
The level of strain can be measured using x-ray diffraction or by monitoring the curvature of electroplated test strips after they are stripped from their substrates.
Relief Process
Thermal annealing is used to reduce this internal force by heating the boards to a moderate temperature for several hours. This baking step allows the metal grains to reorganize and relieves the locked-in strain without damaging the organic board materials. The annealing process is typically performed before the final assembly stages to ensure structural stability.