Why is Reflow soldering called “reflow”

- Apr 03, 2018 -

The reflow soldering machine which is pre - distributed to the solder plate of the printed board, and realizes the soldering between the soldering end of the surface assembly and the mechanical and electrical connection between the pin and the plate of the printed board. Reflow soldering is to weld components to PCB boards, and reflow soldering is for surface mounted devices. Reflow welding is based on the effect of hot air on the solder joints. The physical reaction of the colloidal flux is SMD under a certain high temperature air flow; it is called "reflow soldering" because the gas is circulating in the welding machine to produce high temperature to reach the welding purpose.


A. when PCB enters the heating area, the solvent and gas in the solder paste evaporates. At the same time, the soldering flux in the solder paste wetted the weld plate, the end of the components and the pins. The solder paste softened and collapsed and covered the plate. The solder plate, the component pin and the oxygen were isolated.


B.When PCB enters the thermal insulation area, the PCB and components are fully preheated, so as to prevent PCB from suddenly entering the high temperature zone of soldering and damage PCB and components.

C. when PCB enters the soldering area, the solder paste reaches the melting state when the temperature rises rapidly, and the liquid solder joints the solder joints of the PCB's pads, components end and pin wetting, diffusion, flow or reflux.

D.PCB enters the cooling zone to solidify the solder joints, and the reflow soldering is completed.

 Introduction of 2. reflow welding process

The process of reflow soldering is surface mounted. The process is complex. It can be divided into two parts: one side mounting and two side mounting. 

A, single side mounting: pre coated solder paste, patch (manual mounting and automatic mounting), reflow soldering, inspection and electrical test.

B, double-sided mounting: A surface pre coated tin paste and patch (divided into manual and machine automatic mounting) - reflow welding, B surface pre coated tin paste and patch (divided into manual and machine automatic mounting), reflow welding, inspection and electrical testing.

The simplest process of reflow welding is "Screen printing paste - Mounter - reflow soldering", its core is the accuracy of screen printing, the Chip mounter is determined by the machine's PPM rate, reflow soldering is to control the temperature rise and the highest temperature and drop temperature profile.