Profiling Convective Heat Transfer across Variable Weight Copper Stackups
Profiling variable weight copper stackups requires extended soak dwell and high gas velocity to equalize thermal delta across light pads and heavy ground planes.
Thermal Sag
Deformation Defect
Downward physical deflection of a printed circuit board under high heat environments represents a major risk during the reflow or wave soldering process. The occurrence of thermal sag occurs when the temperature of the board rises past the glass transition temperature of the polymer resin, causing the substrate to lose its structural stiffness. This bowing leads to uneven solder joints, component misalignment, and potential bridges as the board travels on the oven conveyor.
Material Susceptibility
Substrate thickness and density affect the severity of board bending under thermal stress. Thin multi-layer boards are particularly vulnerable to thermal sag because they lack the necessary mechanical thickness to support their own weight and the weight of mounted components when heated. Solder joints formed on a warped surface suffer from residual tensile stresses that can cause joint failure when the board cools down and tries to return to its flat state, causing trace fracturing or component lifting at the board edges.
Mitigation Strategy
Conveyor design and support fixtures prevent board warping by providing middle support rails or composite carrier pallets. Deploying these tooling options ensures the assembly remains perfectly flat while passing through the heated zones of the reflow oven. Assemblers adjust the support configuration based on board dimensions to eliminate any deflection that would result in assembly defects or trace fractures.