Meaning
Resistance of the boundary layer between a solder joint and a copper pad to localized deformation is a primary indicator of the mechanical strength of an assembly. Engineers measure interfacial microhardness to detect the presence of brittle phases that could lead to sudden fractures. This measurement is restricted to the thin region where the two materials have diffused into each other.
Phase Formation
Growth of intermetallic compounds during the soldering process creates a layer that is typically harder than the surrounding bulk materials. High interfacial microhardness often correlates with a thick layer of Cu6Sn5 or Cu3Sn which can become a point of failure under shock. Controlling the heat during reflow prevents these hard phases from becoming too thick and compromising the joint.
Testing Geometry
Precise placement of a diamond indenter is required to get a reading from a layer that is only a few microns wide. Measuring interfacial microhardness involves cross sectioning the joint and polishing the surface to a mirror finish before applying a very small load. The resulting indentation is measured under a microscope to calculate the Vickers or Knoop hardness value.
Material Performance
Variation in the chemical composition of the surface finish can alter the stiffness of the bond. Monitoring interfacial microhardness allows manufacturers to adjust their plating processes to ensure the joint is tough enough to withstand thermal cycling. A stable hardness value across a batch indicates a consistent and reliable manufacturing process.
