What are the characteristics of reflow soldering process?

Reflow soldering refers to a soft soldering process that realizes the mechanical and electrical connection between the solder ends or pins of surface assembled components and PCB pads by melting the solder paste pre printed on the PCB pads.

1. Process flow

Reflow soldering process: printing solder paste → patch → reflow soldering.

2. Process characteristics

The solder joint size is controllable. The desired solder joint size or shape requirements can be obtained through the size design of the pad and the amount of solder paste printed.

The application of solder paste generally adopts the method of steel screen printing. In order to simplify the process flow and reduce the production cost, solder paste is usually printed only once on each welding surface. This feature requires that components on each assembly surface can use a steel mesh (including steel mesh of the same thickness and stepped steel mesh) for solder paste distribution.

Reflow soldering furnace is actually a tunnel furnace with multiple temperature zones. Its main function is to heat PCBA. The components and parts laid on the bottom surface (B surface) shall meet the specified mechanical requirements, such as BGA packaging, and the requirement that the ratio of component mass to pin contact area ≤ 0.05mg/mm2, so as to prevent the top surface components from falling off when welding.

During reflow soldering, the components are completely floating on the molten solder (solder joint). If the pad size is larger than the pin size, the component layout is heavy and the pin layout is small, it is easy to shift under the surface tension of asymmetric molten solder or the forced convection hot air in the reflow soldering furnace.

Generally speaking, for components that can correct their position by themselves, the larger the overlap area of pad size and welding end or pin, the stronger the positioning function of components. We use this point to design the pads of components with positioning requirements.

The formation of weld (spot) morphology mainly depends on the wettability of molten solder and the effect of surface tension, such as 0.44mmqfp. The printed solder paste pattern is a regular cuboid.