In the early days of the electronics industry, before SMT (surface mount technology) was fully developed, almost all circuit board assemblies had to go through wave soldering process to solder electronic components to the circuit board.
What is wave soldering
The reason why it is called “wave soldering” is that it uses a tin furnace during soldering. The tin furnace will be heated to a temperature sufficient to melt the tin bar and form a molten tin liquid. We can regard this tin liquid as a pool of “lake water”. When it is calm and there is no wave, which is called “advection wave”. When the lake water is stirred to make waves move, it is called “turbulent wave”.
The boat glides over the calm or slightly wavy lake surface, allowing the tin liquid to adhere between the feet of the electronic parts and the circuit board. After passing through the tin liquid, it cools quickly, and the solder will weld the electronic parts to the circuit board.
Why wave soldering has not been replaced by SMT totally
With the rapid development of industrial technology, most electronic parts are getting smaller, which can meet the requirements of SMT reflow soldering (such as small size parts and reflow high temperature resistance), so most of the PCB manufacturers have abandoned the wave soldering process, even for 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 (PIH) paste-in-hole process can also be used to achieve soldering by using a reflow furnace.
Having said that, there are still a small number of electronic components that still cannot meet the requirements of the SMT process, so in some cases, this process that consumes a lot of solder.
The process of wave soldering
The purpose of using flux is to improve the quality of parts soldering, because PCB, electronic parts, and even tin liquid may be polluted in the storage and use environment, which will cause oxidation and affect the soldering quality. The main function of “flux” is to remove oxides and dirt on the metal surface, and it can also form a thin 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 solder. The current temperature of SAC305 lead-free solder is about 217°C , the general flux cannot be kept under such high 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 board, resulting in poor soldering of parts that are not coated with flux.
The second method of applying flux is spraying. The nozzle is set at the bottom of the chain, and when the circuit board passes by, it is sprayed from bottom to top. This method also has a disadvantage, that is, the flux is easier to pass through the gap of the circuit board. Sometimes, the flux may directly contaminate the parts on the front of the circuit board, or even penetrate into the inside of some parts that are sensitive to flux, leading to a threat in the future. Or it will remain on the top of the wave soldering machine.
If the machine is not cleaned regularly, when the flux accumulates to a certain amount, it will drip, and a large lump will directly contaminate the front of the circuit board.If the flux is directly dropped on the circuit board without treatment, it is likely to cause quality problems such as circuit board corrosion or micro-short circuit.
Just like the SMT production line, the wave soldering process also needs to preheat the circuit board before the actual soldering. This is to reduce the deformation of the circuit board and avoid the internal moisture of some parts. Otherwise, it will be directly heated from room temperature to a temperature above 217°C, which will It is easy to cause delamination.
A lot of tin bars are thrown into the tank and then heated and melted into tin liquid, so this process requires a lot of 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 tin.
Generally speaking, the tin bath in the tin furnace will be divided into two slots. The first slot is called chip wave, and the second slot is called wave. These two tin slots have different functions. In most cases Only advective waves are turned on
Use a tool to stir the tin liquid to form a fountain-like effect. Its main purpose is to solder 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 action of the circuit board is similar to the sliding of a sampan. Imagine that 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 cannot touch these parts or solder joints under shadows, then causing the problem of empty welding. However, because the tin liquid is always tumbling, sometimes the welding is not equal enough, and even welding bridging or sharpening may occur, so the wave is generally added after the turbulent wave.
It is somewhat similar to a still water surface, but in fact it is a non-stop flowing tin liquid, but the flow is very smooth, which can effectively eliminate some burrs and welding bridging and short circuit problems caused by turbulent waves. In addition, the advection wave has a very good welding effect on traditional through-hole components (long legs protruding from the circuit board). If there are only through-hole components during wave soldering, you can also consider turning off the spoiler wave and only use advection to complete the welding.
A single advection wave also helps to reduce the problem of short-circuit bridging of multi-pin plug-ins, because the flux will be evaporated in the turbulence wave of the double-wave process, and when it comes to the advection wave, there is less deoxidation and soldering support of the flux. If the wettability of the solder becomes.
This area generally uses a cooling fan at the exit of the tin furnace, which is responsible for cooling the circuit board that has just passed the high-temperature tin liquid, because there will be some soldering and repairing actions to be done immediately. Generally, the circuit boards passing through the tin furnace do not use rapid cooling equipment, probably because most of them are traditional through-hole components or larger SMD parts.
Some wave furnaces will add an additional cleaning process at the end, because some circuit boards will still go through the cleaning process.
Why is it necessary to have an inclination angle during wave soldering
The track of “wave soldering” has a certain inclination angle with the tin surface. Generally, the inclination angle is set at about 3~7°. The reason for the slight inclination is to facilitate the removal of tin when the solder joint is separated from the tin surface. And this inclination angle is also called “de-tinning angle”. When passing tin, an angle is required when the PCB board and the liquid molten tin surface are separated. If the de-tinning angle is smaller, the solder joint will be larger, and vice versa.
If the track and the tin surface are not inclined during “wave soldering” and there is no desoldering angle, the solder joints will be too large, and a large number of solder joints will easily appear. A certain desoldering angle during wave soldering can promote excess solder joints. The molten tin liquid flows into the wave soldering furnace along the de-tinning angle by gravity to achieve the purpose of controlling the amount of tin in the solder joint.
What is selective wave soldering
Because not all parts of circuit boards require wave soldering , there are often hundreds of parts on a board, but less than 5 parts require wave soldering, so the selective wave soldering was born
There are two types of selective wave soldering:
The first type of selective wave soldering is to use a wave soldering furnace carrier to cover the parts that do not need wave soldering, and the process still follows the original wave soldering tin furnace. The second selective wave soldering is to use a small tin furnace. The small nozzle, and then move the nozzle to align with the parts that need to be welded, which can save the solder.
Even with many drawbacks comparing with SMT, wave soldering still has been used in the PCB Assembly factory for its unique characteristics, and cannot be easily replaced by other manufacturing process.
There are still a small number of electronic components that still cannot meet the requirements of the SMT process, so in some cases, this process that consumes a lot of solder.