Volumetric Strain
Thermally induced dimensional movement of copper-plated blind holes along the Z-axis of high density interconnect substrates represents a structural deformation mechanism. During thermal cycling and reflow soldering, microvia expansion induces mechanical stress at the interface between the target land pad and the electrodeposited copper base. Differences in thermal expansion rates between dielectric resins and electrodeposited copper drive localized volumetric strain.
Fabricators evaluate Z-axis movement to prevent target land separation in buried and blind microvia structures.
Joint Integrity
Repeated thermal exposure during assembly reflow cycles causes dielectric resin systems to expand rapidly along the vertical axis above their glass transition temperature. Uncontrolled microvia expansion pulls the plated copper structure away from internal capture pads, creating latent micro-cracks at the base interface. Laser-drilled blind vias filled with solid copper plating experience minimal mechanical deformation compared to thin-walled, resin-filled microvias.
Chemical cleaning residue left in the laser target pit weakens copper-to-copper adhesion, accelerating interface separation under elevated thermal loads. Reliability testing subjects completed high density circuit boards to air-to-air thermal shock cycles to detect resistance shifts in microvia chains. Interconnect stress testing applies continuous high current pulses to heat test structures, measuring fractional resistance changes to detect micro-fractures before total electrical open failure occurs.
Microsection analysis under high magnification reveals structural voiding or barrel cracking caused by thermal strain.
Reliability Boundaries
Low thermal expansion resin systems reduce vertical stress on copper structures during high temperature processing. Electroplating chemistry additives control grain structure to increase copper tensile strength against cyclic Z-axis movement. Inspection criteria reject microvia structures exhibiting interface separation or resistance shifts exceeding five percent.
