What is Wave Soldering Process?
Wave soldering processing is a method to assemble electronics components onto printed circuit boards. The board passes through a pot of molten solder via an inclined conveyor belt, in the oven, the pump produces standing waves of molten solder. When the PCB board is in contact with the wave, the parts will be soldered to the board, creating the mechanically and electrically reliable joints. The wave soldering process is primarily used for through-hole components assembly, but can also be used for surface mounting somewhere.
How to Go with Wave Soldering Process?
Wave Soldering Machine
The wave soldering machine consists of a heated solder pot, keeping it at the temperature required for the soldering. Inside the tank, a wave of solder is placed and the printed circuit board passes over it so that the bottom surface of the board just contacts the solder wave.
Note to adjust the height of the wave so that it does not flow through the top surface of the board, as this can cause the solder to enter unwanted locations.
The boards are usually fixed on a salver on the conveyor, which is able to withstand the temperatures and it will not be soldered.
Wave soldering machines used in electronics factories are typically bimodal or electromagnetic pump wave soldering machines, the brands include SEHO (DE), Soltect (US), ERSA (DE), Jingtuo(CN), Suneast (CN), and so on. In PS Electronics, the machines of Soltect and Jingtuo are under employing.
Flux
In order to ensure that the area to be soldered is clean and free from oxidation, etc., flux is required. There are two types of fluxes, corrosive and non-corrosive. Non-corrosive fluxes require pre-cleaning and are used when low acidity is required. Corrosive fluxes are fast, require little pre-cleaning, but have high acidity.
The flux is applied to one side of the board to be soldered, that is, the bottom side. Careful control of the amount of flux is required, too little flux, there is a high risk of poor joints, too much flux, and flux might remain on the board.
There are two main methods of applying flux:
1. Spray flux. A fine mist of flux is sprayed onto the underside of the board that is to be soldered. Some systems may even use a compressed air jet to remove the excess flux.
2. Foam flux. The electronic printed circuit board is passed over a cascading head of flux foam. This is generated using a tank of flux into which a plastic cylinder with tiny holes is immersed. The plastic cylinder is covered with a metal chimney and the air is forced through the cylinder. This causes flux foam to rise up the chimney.
Pre-heating
The wave soldering process exposes PCB boards and devices to relatively high heat levels, which is much higher than the heat experienced during hand soldering. This thermal shock might result in a significant increase in the level of failure. To overcome this problem, the boards should be preheated to stably stabilize it to the desired temperature to minimize thermal shock.
The preheating zone typically uses hot air heaters that blow hot air onto the plates as they pass through the wave soldering machine. In some cases, especially if the board is dense, an infrared heater can also be used. This ensures that all boards are heated evenly and there are no shaded areas.
Preheating temperature is usually set to 90-110 degrees:
- Single-sided wave soldering/mixing assembly 90~100
- Double-sided wave soldering/mixing assembly 100~110
- Double-sided reflow soldering 100~110
- Multilayered PCB wave soldering/mixing assembly 115~125
- Multilayered PCB reflow soldering 115~125
Solder Types
The ever most popular combination is Sn63Pb37 (63/37) with a tin content of 63% and a lead content of 37%. But lead pollutes the living environment of human beings while it is a toxic metal that is harmful to the human body and highly destructive to the natural environment so that lead-free solder is introduced, in which, Cu6Sn5 and Ni3Sn4 are the common choices.
Inside Oven Temperature
Take the 63/37 tin bar as an example. In general, the tin bath temperature should be adjusted to 245 to 255 degrees. Try not to exceed 260 degrees because the new tin bath temperature will exceed 260 degrees. Will accelerate the production of its oxides.
Solder Wave Height
The height of the solder wave is a key parameter to be evaluated when setting up the wave soldering process. The contact time between the solder wave and the components being welded is usually set to 2 to 4 seconds. The contact time is controlled by two parameters on the machine, namely the conveyor speed and the wave height. Changing either of these two parameters will result in a change in contact time.
The wave height is usually controlled by increasing or decreasing the pump speed on the machine. If a more detailed record is required, the tempered glass plate can be used to evaluate and check for changes, and if there is a fixture, the contact time, height and speed can be recorded digitally.
Cooling Rate
It is important to allow the PCB to cool at a reasonable rate after soldering. If cool down quickly, the PCB may warp and the solder joints may get damaged.
On the other hand, if the PCB board is cooled too slowly, the PCB will become brittle and some components may be damaged by high-temperature. The PCB should be cooled by water-cooling or air-cooling to reduce damage to the PCB board.
Inspection
After soldering, the assembled boards must get through visual inspection and AOI machine to prevent any issue flowing to the customers.
Although the wave soldering process is almost replaced by the reflow soldering, the wave soldering machine is still necessary for each PCB assembly plant, this assembly technology is still indispensable.
