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.
Low-Cycle Copper Fatigue
Thermal Strain Analysis
Plastic deformation of barrel copper occurs when plated through holes experience repetitive expansion and contraction cycles. Low-cycle copper fatigue arises from the mismatch in coefficients of thermal expansion between the copper plating and the laminate substrate during thermal excursions. Repeated excursions beyond the elastic limit of the ductile material create localized dislocations within the metal grain structure.
These dislocations form persistent slip bands that eventually transition into micro-cracks along the vertical wall of the via. Such degradation progresses through every subsequent thermal cycle until the barrel wall separates completely from the internal pad structure.
Mechanical Stress Limit
Axial loads generated by substrate warping exert significant force on the plated copper during soldering operations. High peak temperatures during reflow cycles force the laminate to expand rapidly while the copper wall resists this expansion. Internal stress concentrations peak at the corners of the barrel and the transition zones where the drill hole intersects inner layers.
Ductility of the copper determines how many cycles the via survives before electrical continuity fails. Thicker plating increases the mechanical rigidity of the barrel but simultaneously raises the force applied to the interface during thermal expansion. Proper control of the aspect ratio ensures the total displacement remains within the strain capacity of the copper.
Inspection Boundary
Metallographic cross-sectioning verifies the condition of the barrel wall after thermal shock testing. Micro-cracks appear at the vertical edges of the copper plating when the grain structure reaches its limit of plastic work. Scanning electron microscopy reveals the extent of grain boundary separation in samples that passed functional electrical testing prior to environmental exposure.
Failure occurs when the length of these cracks compromises the mechanical integrity of the connection. Resistance measurements provide a secondary indicator but physical examination remains the standard for diagnosing the progress of structural failure.