Latent Assembly Defect Screening Standards for High Density Interconnects
Screening latent microvia and fine pitch HDI assembly defects combines thermal shock cycling with continuous surface insulation resistance measurement.
Coffer Manson Strain
Plastic Deformation
Thermal fatigue analysis of microelectronic solder joints utilizes an empirical model to relate the plastic strain range to the number of cycles to failure. Board assemblers analyze the coffer manson strain to estimate the mechanical endurance of lead-free solder connections.
Fatigue Calculation
Mathematical modeling of thermomechanical fatigue divides the total strain range into elastic and plastic components to isolate the primary driver of crack initiation. The coffer manson strain coefficient represents the plastic strain portion that dominates low-cycle fatigue regimes in surface mount components. Calculation procedures involve thermal shock testing between temperature extremes to extract the exponent that characterizes the specific solder alloy.
Thermal Cycling
Acceleration factors derived from solder joint failure rates allow test laboratories to compress months of operational temperature swings into days of test chamber thermal cycling. The coffer manson strain formulation converts laboratory cycles to expected field life for a given printed circuit board assembly. Poor alignment of thermal expansion coefficients between the chip carrier and the epoxy board shortens the fatigue life by amplifying the strain in the corner joint.
Physical failure analysis subsequently verifies whether the fracture surface shows the characteristic striations of low-cycle plastic fatigue or if mechanical overload caused the early separation.