What are the characteristics of reflow welding process?

Reflow flow welding refers to a welding process that realizes mechanical and electrical connections between solder ends or pins of surface assembly components and PCB solder pads by melting solder paste pre-printed on PCB solder pads.
1. Process flow
Process flow of reflow soldering: printing solder paste → mounter → reflow soldering.

2. Process characteristics
The solder joint size is controllable. The desired size or shape of the solder joint can be obtained from the size design of the pad and the amount of paste printed.
Welding paste is generally applied by steel screen printing. In order to simplify the process flow and reduce the production cost, usually only one welding paste is printed for each welding surface. This feature requires that the components on each assembly face be able to distribute solder paste using a single mesh (including a mesh of the same thickness and a stepped mesh).

The reflow furnace is actually a multi-temperature tunnel furnace whose main function is to heat PCBA. Components arranged on the bottom surface (side B) should meet the fixed mechanical requirements, such as BGA package, component mass and pin contact area ratio ≤0.05mg/mm2, in order to prevent the top surface components from falling off when welding.

In reflow soldering, the component is completely floating on molten solder (solder joint). If the pad size is larger than the pin size, the component layout is heavier, and the pin layout is smaller, it is prone to displacement due to asymmetric molten solder surface tension or forced convective hot air blowing in the reflow furnace.

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

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