In the process of hot air reflow welding, the solder paste needs to go through the following stages, the solvent volatilization; Flux removes oxides from the weldment surface; Solder paste melting, reactivity and solder paste cooling, condensation.
(1) Preheating zone
Intention: To make PCB and components preheat, achieve balance, remove moisture and solvent in the solder paste together, to prevent the solder paste collapse and solder splash. Make sure the temperature is slow and the solvent is volatile. Mild, the thermal impact on components as small as possible, too fast temperature rise will cause damage to components, such as cracking of multi-layer ceramic capacitors. This will also cause solder spatter, forming solder balls and solder deficient solder spots throughout the non-welded area of the PCB.
(2) Thermal insulation area
Intent: To ensure that the solder is thoroughly dry before reaching the reflow temperature, together with the effect of flux activation, to eradicate metal oxides in components, pads, and welding powder. The time is about 60~120 seconds, depending on the nature of the solder.
(3) reflow welding area
Intent: The solder in the solder paste makes the gold powder begin to melt and become active again, replacing the liquid flux to wet the pad and components. This wetting effect leads to further expansion of the solder, and the wetting time for most solder is 60 to 90 seconds. The reflow welding temperature should be higher than the melting point temperature of the solder paste, and generally 20 degrees beyond the melting point temperature can ensure the quality of the reflow welding. Sometimes the region is divided into two zones, namely the melt zone and the re-flow zone.
(4) Cooling zone
The solder condenses with the decrease of temperature, so that the components and the solder paste form a good electric touch, and the cooling speed is the same as the preheating speed.
What are the factors that affect the welding function in SMT patch processing?
1. Process elements:
Pre-welding treatment, treatment type, method, thickness, layer number. Whether it is heated, cut or otherwise processed between the time of processing and welding.
2. Design of welding process:
Welding area: refers to the size, clearance, solder joint clearance guide (wiring) : shape, thermal conductivity, heat capacity of the welded material: refers to the welding direction, orientation, pressure, bonding state, etc.
3. Welding conditions:
Refers to the welding temperature and time, preheating conditions, heating, cooling speed the way of welding heating, the way of the carrier of the heat source (wavelength, heat conduction speed, etc.)
4. Welding materials:
Flux: Composition, concentration, activity, melting point, boiling point, etc
Solder: Composition, structure, impurity content, melting point, etc
Base material: base material composition, structure, thermal conductivity, etc
Solder paste viscosity, specific gravity, thixotropic function of the substrate material, type, cladding metal, etc
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