Thermal Reliability
Thermal mechanical strain occurs in surface mount joints when differences in the coefficient of thermal expansion between the component body and the printed circuit board induce stress during temperature cycling. Sac305 solder fatigue describes the progressive structural degradation and micro-cracking of this tin-silver-copper alloy as it sustains repeated elastic and plastic deformation. This process proceeds through grain coarsening and the formation of intermetallic layers at the interface between the solder and the copper pad.
Cyclic Degradation
Shear strain accumulates within the solder volume during each heating and cooling transition in the operating environment. High concentrations of strain appear at the corners of the component package where the distance from the neutral point reaches a maximum. Constant exposure to fluctuating thermal loads drives the crack propagation along the boundaries of the solder grains.
Initial cracks propagate slowly across the joint area until the reduction in effective contact surface area leads to a catastrophic loss of electrical conductivity. Mechanical failure follows this degradation when the remaining ligaments of the joint material cannot carry the required current or maintain the physical connection of the package to the substrate.
Boundary Conditions
Evaluation of performance under stress relies upon calibrated cycling profiles that simulate the end application of the assembly. Manufacturers perform cross-sectional analysis on samples taken from reliability test vehicles to quantify the extent of phase coarsening and the depth of surface cracks. These assessments identify the susceptibility of the joint architecture to premature failure before the design reaches mass production.
Correct material application and reflow control limit the sensitivity of the assembly to these thermal environmental stresses.
