Wave soldering is to make the soldering surface of the circuit board directly contact with high-temperature liquid tin to achieve the purpose of soldering. The high-temperature liquid tin maintains an inclined surface, and a special device causes the liquid tin to form a wave-like phenomenon, so it is called "wave soldering". The main material is solder bars.
Wave soldering uses a full barrel of tin furnace during soldering. The tin furnace is heated to a temperature sufficient to melt the tin bars and form molten tin. We can think of this furnace as a pool of "lake water." It is called "smooth wave" when the lake is calm and there is no wave, and it is called "turbulent wave" when the water of the lake is stirred and the waves surge. The circuit board is like a sampan boat gliding across the surface of the lake, allowing the tin liquid to adhere between the feet of the electronic components and the circuit board. After the tin liquid is cooled, the electronic components are soldered to the circuit board.
The following will briefly explain the "wave soldering" process. The wave soldering process is basically divided into four parts:
1. Flux Zone
The purpose of using flux is to improve the soldering quality, because circuit boards, electronic components, and even tin liquid has the chance to get some pollution, which causes oxidation and affects the soldering quality. The main function of flux is to remove oxides and dirt on the surface of the metal, and it can also form a film on the surface of the metal to isolate the air during high temperature operations, so that the solder is not easily oxidized.
There are two general methods of applying flux. One uses foamed flux, which sticks to the circuit board when it passes through the flux zone.
The second method of applying flux is spraying. The nozzle is placed under the chain and sprayed up from the bottom when the circuit board passes through the flux zone.
2. Pre-Heating Zone
In the wave soldering process, the circuit board needs to be pre-heated before actual soldering. This is to reduce the deformation of the circuit board and avoid the internal moisture of some components, otherwise it will be heated directly from room temperature to a temperature above 217°C, which will easily cause popcorn or delamination.
The wave soldering machine is basically pre-heated by thermal radiation. The most commonly used wave soldering preheating methods are forced hot air convection, electric heating plate convection, electric heating rod heating and infrared heating. Among these methods, forced hot air convection is generally considered to be the most effective heat transfer method for wave soldering machines in most processes.
3. Soldering Zone
There is a large barrel of tin that has been heated and melted in the soldering zone, which is called a tin furnace. It throws a lot of tin bars into the tank and then heats and melts it into a tin solution, so this process requires a lot of tin material. Generally speaking, the tin tank in the tin furnace will be divided into two tanks. The first tank is called turbulent wave and the second bath is called smooth wave. These two tanks have different functions. In most cases only smooth waves will be turned on:
Turbulent wave The motor is used to stir the tin liquid to form a fountain-like effect. Its main purpose is to solder SMD components, because SMD components are generally densely distributed in various areas of the circuit board, and there are also large and small, high and low, because the action of the circuit board is similar to sampan gliding. If there is a large object under the sampan, the so-called "shadow effect" will be formed behind the large object when sliding. The same is true for tin liquid. If there is no tumbling tin liquid, it will not be able to contact these shadows. The following components or soldering points will cause the problem of missing soldering. However, because the solder is always rolling, the soldering effect is sometimes not uniform, and sometimes welding bridges occur, so the smooth wave is generally added after the turbulent wave.
Smooth wave Similar to the static water surface, it can effectively eliminate some of the burrs generated by the previous turbulent wave and the short circuit of the welding bridge. In addition, the smooth wave has a very good soldering effect on the traditional through-hole components (long legs extend out of the circuit board). If only the through-hole components are used in wave soldering, the turbulent wave can be turned off, and the soldering can be completed with the smooth wave.
4. Cooling Zone
In this area, a cooling fan is generally used at the exit of the tin furnace, which is responsible for cooling the circuit board that has just passed the high-temperature tin liquid.
Why do we need to have a tilt angle during wave soldering?
The wave soldering track has a certain inclination angle with the tin surface. The general inclination angle is set at about 3~7°. The reason why it is slightly inclined is to facilitate the removal of tin when the solder joint is separated from the tin surface, and this inclination angle is also called desoldering angle. When passing through the tin, the PCB board and the liquid molten tin surface need an angle. The smaller the tin removal angle, the larger the solder joint will be, and vice versa.
If the track and the tin surface are not inclined during wave soldering and there is no desoldering angle, the solder joint will be too large, and a large number of short bridges will appear. When wave soldering is given a certain inclination angle, it can promote excess molten tin liquid on the solder joints flows into the wave soldering furnace along the desoldering angle to achieve the purpose of controlling the amount of tin of the solder joint.
