Selective Solder Nozzle Geometry and Thermal Preheat Window Boundaries
Optimal selective soldering requires balancing nozzle orifice dimensions with precise top-side preheat windows to achieve Class 3 vertical barrel fill.
Thermal Shock Ramp Rate
Temperature Gradient
Speed at which an assembly transitions between thermal extremes during reliability testing defines the severity of the stress applied to the solder joints. The thermal shock ramp rate is measured in degrees per second and is much faster than standard thermal cycling. This parameter is used to identify weaknesses in the laminate or the interlaminar bonds.
Mechanical Stress
Rapid changes in temperature cause materials with different coefficients of thermal expansion to expand or contract at different speeds. A high thermal shock ramp rate induces heavy mechanical strain on the interface between the copper barrel and the resin of the PCB. This stress can lead to corner cracks in the plating or fractures in the solder fillets themselves.
Testing usually involves moving the board rapidly between a hot chamber and a cold liquid bath or chamber.
Failure Discovery
Results from these tests help engineers determine the expected lifespan of a product in harsh environments. If the thermal shock ramp rate exceeds the material capability, the board will fail through delamination or via fatigue. Most automotive and aerospace specifications define the exact slope of the temperature change.
Successful completion of the test requires the board to maintain electrical continuity and structural integrity throughout the cycles.