Thermal Stress
An environmental test procedure subjects printed circuit board assemblies to multiple consecutive passes through a solder reflow oven to evaluate their long-term thermal and mechanical reliability. Sequential reflow cycling simulates the thermal stresses experienced by double-sided boards during assembly, rework and subsequent component attachment processes. This repeated heating and cooling drives thermal expansion mismatches between the copper vias and the surrounding resin matrix, which can lead to material degradation or circuit failure.
It provides a standard qualification method for verifying the durability of via structures in high-reliability applications.
Failure Mechanisms
During each pass through the reflow oven, the temperature of the assembly rises above the glass transition temperature of the laminate, which causes the material to expand rapidly in the vertical direction. This expansion exerts high tensile stress on the copper plating in through-holes and microvias, which can cause micro-cracks at the knee or the base of the via transition. If the adhesion between the resin and the glass reinforcement is weak, these thermal cycles can also cause delamination or blistering between layers, which degrades the electrical insulation of the board.
Inspectors evaluate these failures by performing cross-sectional analysis and measuring the change in electrical resistance of trace chains after each reflow cycle.
Design Optimization
Analyzing the behavior of different board designs under this repeated thermal stress helps engineers select laminates with lower coefficients of thermal expansion and higher glass transition temperatures. This material selection is particularly important for thick multilayer boards that undergo complex assembly flows with multiple soldering and rework steps. The test results allow fabricators to optimize the lamination parameters and plated copper thickness to ensure the via structure survive the assembly process.
