Copper Delamination
Expansion forces originating from mismatched coefficients of thermal expansion between metallic traces and dielectric substrates generate severe interlayer shear during thermal cycling. Printed circuit board fabrication exposes laminated assemblies to extreme reflow temperatures where volatile compounds expand rapidly within trapped voids. Thermal mechanical stress damages plated through holes by fracturing copper barrels when axial expansion exceeds the elastic limit of the electrodeposited metal.
Subsequent electrical testing uncovers intermittent open circuits originating from fatigue cracks grown inside barrel walls during previous thermal excursions.
Substrate Distortion
Differential heating rates across thick copper planes create steep thermal gradients that warp composite panels beyond acceptable flatness tolerances during wave soldering. Glass transition temperatures dictate the threshold where epoxy resin matrices soften and lose rigidity under mechanical loads imposed by heavy components. Solder joint fatigue accelerates when constrained expansion forces concentrate at package corners during accelerated life testing cycles.
High density interconnect structures require strict resin content controls to minimize Z-axis dimensional changes that rupture stacked microvias.
Boundary Fracture
Residual mechanical energy stored within rigid encapsulants during molding operations discharges through silicon die surfaces when cooling down from cure temperatures. Package on package assemblies experience joint separation at the interface between microbumps and copper pads when localized strain exceeds intermetallic compound bond strength. Accelerated thermal shock screening exposes latent defects by cycling assemblies between extreme temperatures to precipitate brittle failures before final deployment.
Final qualification requires destructive cross section analysis to verify that package structures withstand maximum anticipated operational strain amplitudes without intermetallic degradation.
