PCB Technical - PCBA wave soldering technology introduction

In the early days of the electronics industry, before the PCB was developed, almost all PCBA assembly circuit boards had to go through this wave soldering process to achieve the purpose of soldering electronic parts to the circuit board. It is called "wave soldering" because it requires a full barrel of tin furnace for soldering. The tin furnace is heated to a temperature high enough to melt the tin bar and form molten tin, which sometimes looks like Just like lake water, sometimes waves can be made on it, and the circuit board slides over the surface of the lake or waves like a sampan, allowing the tin liquid to adhere between the electronic parts and the circuit board, and solder after cooling. The electronic parts are soldered to the circuit board.

With the rapid development of industrial technology, most electronic parts have become smaller and smaller, and can meet the requirements of SMT reflow soldering (such as small-sized parts and high temperature resistance), so most of the circuit boards have now abandoned this This kind of traditional wave soldering process, even if there are some parts that cannot be reduced in size, as long as the temperature resistance of the material can meet the requirements of SMT reflow soldering, the paste-in-hole process can also be used to achieve the purpose of soldering using a reflow soldering furnace. Having said that, there are still a small number of electronic parts that still cannot meet the requirements of the SMT process, so in some cases this process requires a lot of solder to be used.

PCBA wave soldering process

Now let’s briefly explain the process of wave soldering. The process of wave soldering is basically divided into four parts:

The first part is the flux addition zone ((Flux Zone)

The purpose of using flux is to improve the quality of soldering of parts, because circuit boards, electronic parts, and even tin liquid may be contaminated by the storage and use environment, resulting in oxidation that affects soldering quality, and the main function of flux It is to remove the oxide and dirt on the metal surface, and it can form a film on the metal surface to isolate the air during high temperature operation, so that the solder is not easy to oxidize. However, the wave soldering process must use molten tin as the soldering medium. Since it is tin liquid, the temperature must be higher than the melting point of the solder. The temperature of the current SAC305 lead-free solder is about 217C. The general flux cannot be kept under such a temperature for a long time, so if you want to add flux, you must apply it before the circuit board passes through the tin liquid.

Generally, there are two ways to apply flux. One is to use foaming flux. When the circuit board passes through the flux area, it will adhere to the circuit board. The disadvantage of this method is that the flux cannot be evenly applied to the circuit. The second method uses spraying, and the nozzle is set under the chain. When the circuit board passes by, spray from bottom to top. This method has a disadvantage, that is, the flux will be Passing through the gap of the circuit board, the poor ones may directly contaminate the components on the front of the circuit board, and even penetrate into the inside of some parts, forming a time bomb with unstable quality in the future, or it will remain on the top of the wave soldering machine., If there is no regular cleaning, when the flux accumulates to a certain weight, it will drip, and a large pile will directly contaminate the front of the circuit board.

The solder paste in the reflow process is also doped with flux! It's just that we generally don't notice it easily.

The second part is the Pre-Heating Zone

Just like the SMT process, the wave soldering process also needs to preheat the circuit board. This is to reduce the deformation of the circuit board and avoid moisture inside some parts. Heating too fast can easily cause popcorn and other missing.

Just like boiled white eggs, if you first heat the water to a boil and then directly put the raw eggs in and boil them, the eggs will break and the egg whites will be squeezed out. To make a perfect hard-boiled egg, you must first throw the egg into cold water and then boil it together.

The third part is the Soldering Zone

There will be a large bucket of tin bath that is heated and melted, so it is called a "tin furnace." It really just throws a lot of tin bars into the bath and heats them to melt into tin liquid, so this process requires a lot of cost. Tin material. Since it is liquid tin, various tin surfaces can be made according to the characteristics of the liquid to meet the needs of soldering.

Generally speaking, the tin tank in the tin furnace will be divided into two tanks. The first tank is called the spoiler wave, and the second tank is called the advection wave. The two tin tanks have different functions. In most cases Only advection waves are turned on:

Chip Wave

The motor is used to stir the tin liquid to form an effect similar to a fountain. Its main purpose is to weld SMD parts, because SMD parts are generally densely distributed in various areas of the circuit board, and there are large and small, high and low, because the circuit board The action is similar to the sampan sliding. 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 the tin liquid. If the tin liquid is not tossed, it cannot be exposed to these shadows. The following parts or solder joints will cause the problem of empty welding. However, because the tin liquid is always tumbling, the welding effect is sometimes not uniform, and sometimes welding bridges occur. Therefore, advection waves are generally added behind the spoiler waves.

Advection wave

It's a bit similar to a static water surface, it can effectively eliminate some of the burrs and short-circuit problems caused by the "turbulent wave" in the front. In addition, the advection wave has a very good welding effect on traditional through-hole components (long legs protruding from the circuit board). If only through-hole components are used during wave soldering, the spoiler wave can be turned off and the advection wave can be used to complete the soldering.

The fourth part is the Colling Zone

This area generally uses a cooling fan at the exit of the tin furnace to cool the circuit board that has just passed through the high-temperature tin liquid, because some soldering and repairing actions will be done next. Generally, the circuit boards that have passed the tin furnace will not use rapid cooling equipment, probably because most of them are traditional through-hole components or larger SMD parts!

A cleaning process is added to the back of some wave furnaces, because there are still some PCB circuit boards that go through the cleaning process.