Thermal Cycle
Manufacturing stages that involve heating a populated circuit board or carrier to melt solder and create permanent electrical joints. During substrate reflow, the entire assembly passes through a furnace with multiple temperature zones that are carefully controlled to follow a specific profile. This process transforms the solder paste from a suspension of metal particles into a solid alloy that anchors the components in place.
Surface Wetting
Tension pulls the molten solder into the desired shape around the pads and component leads. A successful substrate reflow depends on the flux removing oxides from the metal surfaces so the liquid metal can flow and bond effectively. If the temperature is too low, the result is a cold joint that may fail under mechanical stress later in the product life.
Different Expansion
Materials in the assembly expand at varying rates, which can cause the board to twist during heating.
Cooling Management
Rates of temperature reduction must be managed to prevent the formation of brittle intermetallic layers or internal voids in the solder. Thermocouples are attached to test boards to verify that every part of the substrate reflow environment stays within the specified tolerances. High-quality thermal management ensures that the mechanical integrity of the connections is maintained through subsequent assembly and testing.
