PCBA Tech - PCBA processing dip soldering and wave soldering?

1. Immersion Soldering

The PCBA processing dip soldering process is the earliest simple method that is aimed at the simpler mass soldering method (Mass Soldering), and some small factories or experimental methods are still in use. The installed board is installed horizontally in the frame to directly contact the molten tin surface, so that it can be fully welded at the same time. The continuous process of flux coating, preheating, dip soldering and cleaning can be manually or automatically conveyed, depending on the situation, but most of them are dip soldering for PTH socket soldering.

2. Wave Soldering

The PCBA processing wave soldering process is to use the molten liquid tin driven by the motor to raise the single wave or double wave upwards to force the liquid tin into the hole against the board that is conveyed from the diagonal upwards. Or locate the empty feet of the SMD components by dispensing glue, and fill them with solder to form solder joints, which is called "wave soldering". This "volume soldering" method has been practiced for many years, even if the current board with a mix of insertion and placement is still available. The key points are now organized as follows:

1. Flux

In the wave soldering connection, the liquid flux is applied to the board surface. There are about three methods: foam type, wave immersion type and spray type, namely:

1.1 Foam Flux:

The air blown by the "low pressure air compressor" is passed through a porous natural stone or plastic product and a special filter (with a pore size of about 50~60µm) to form many fine bubbles, and then blow into the flux reservoir. In the pool, a lot of flux bubbles can be poured upwards. When the PCBA assembly board passes through the upper gap, the bottom surface of the board can be evenly coated with a thin layer.

And before it leaves, the excess droplets must be blown off with cold air in an oblique direction of about 50~60 degree Celsius to prevent troubles to subsequent preheating and welding. And it can force the flux to gush upwards from the hole top and hole ring of each PTH to complete the cleaning action.

1.2 Spray Fluxing:

It is often used for no-clean low solids (Low Solid; solid content is about 1-3%) flux, but it is not suitable for the earlier rosin (Rosin) type flux with higher solids. Since clogging occurs more frequently, nitrogen gas should be used to assist the ejection, which can not only prevent fire and reduce the trouble of flux oxidation. The spraying principle also has several different methods, such as the use of stainless steel mesh drum (Rotating Drum) to bring the liquid film from the liquid, and then blow up the nitrogen from the cylinder to form a mist, and then continue to blow the nitrogen upwards. Coating.

1.3 Wave Flux:

Directly use the helper and the nozzle to lift up the liquid, and under the control of the slit, a long wave crest can be obtained, and the coating can be carried out when the bottom of the PCBA assembly board passes through. This method may present an excessive amount of liquid, and the subsequent air knife (Air Knife) blowing action should be more thorough.

2. Warm up

Generally, the preheating before wave soldering is enough if the upper plate surface is heated to 65-121 degree Celsius, and the heating rate is about 2 degree Celsius/S～40 degree Celsius/S. When the preheating is insufficient, the activity of the flux may not reach the extreme, and it is difficult for the solderability to reach the optimal level. Moreover, when the volatile matter has not rushed out, when the viscosity of the flux on the surface to be soldered is still low, it will lead to the loss of solder joints (Dewetting) and solder tips (Solder Icicles).

3. Wave soldering process control

3.1 Tin temperature management:

At present, the alloy composition of the solder in the tin bath is still mostly Sn 63/Pb37 and Sn 60/Pb40, so the operating temperature should be controlled at 260°±5°C. However, the overall weight of the plates and parts to be welded must still be considered. The temperature can be increased to 280°C for large ones, and 230°C for small plates or products that are too sensitive to heat, which are expedient. And it must be matched with the conveying speed and preheating. The ideal way is to change the conveying speed, and the tin temperature should be unchanged, because the tin temperature will affect the fluidity of the molten tin.

3.2 Wave surface contact:

Since the bottom surface of the PCBA assembly board travels and touches the upwelling tin wave, until it completely passes through the contact with the molten tin gushing surface, the time course of its mutual contact must be controlled between 3-6 seconds. The length of this welding time depends on the "contact length" composed of the conveying speed (Conveyor Speed) and the waveform and immersion depth; too short a time period will not fully exert the solderability, and too long a time period will affect the plate or Sensitive parts cause injury. If the wave solder connection is directly installed in general air, thin oxides will continue to form on the surface of the tin wave. Due to the flow and the PCBA assembly board (PWA), the PCBA assembly board (PWA) will continue to float away, so the whole will not accumulate. Too much oxide. However, if the entire system, especially the wave soldering section, is enveloped in a nitrogen environment, the occurrence of oxidation reactions can be greatly reduced, and of course the solderability has been significantly improved.

The transmission surface of the PCBA assembly board must present an elevation angle of 4º~12º. This will greatly improve the welding action at the back of the part body where the blocked "leeward wave" is not strong. Generally, the current wave soldering machine is equipped with dual auxiliary and dual waves that can be individually controlled (the tin pool has single and double waves). The front wave is called "turbulent wave (turbulent wave)". "It is formed by forcing a strong stream of tin to pass through multiple rows of rounded holes of various diameters, which can be directly punched to the surface of the walking bottom plate, which is very beneficial for welding through-hole pins or mounting tail pins.

3.3 Details of contact:

If we further discuss the details of its instant contact welding, we can further describe it in the following:

(1) At the initial stage of contact between the board surface and the turbulence wave, the flux immediately volatilizes and disperses, which in turn causes the metal surface to be soldered to also begin to wet (Wetting). A low-frequency oscillating device can also be added to this wave to strengthen and match the rubbing action of the surface to be soldered to receive the flux. This will greatly help the tin filling and tinning of the foot of the mounted parts, and can reduce the "skipping" phenomenon at the leeward slope. Of course, the overall solderability will be better under the different effects of the strong first and gentle second wave of the double wave.

(2) When the board surface enters the "Heat Transfer Region" (Heat Transfer Region) at the center of the tin wave, driven by a large amount of thermal energy, the Spreading action at the moment of Wetting also starts quickly.

(3) After that is the "Break Away" at the exit of the tin wave. At this time, various solder joints have been formed, and various undesirable shortcomings have also appeared one after another. If the PCBA assembly board can be quickly and smoothly separated from the tin wave, everything will be fine. The dragging that is hard to separate will of course become the main cause of bad solder bridges (Solder Bridge) or solder tips (Solder icicles) or even solder balls (Solder Ball). Although the speed of separation directly depends on the conveying speed, when the conveyor belt is deliberately raised 4º~12º, it can also be separated more simply and conveniently with the help of the cooperation of gravity. As for the board surface defects caused by the muddy water, of course, there is still a chance to be repaired by the hot air coming soon. At this time, cold air cannot be used to avoid the adverse effects of thermal shock (Thermal Shock) caused by excessive temperature fluctuations of the assembly.

3.4 Cooperation in nitrogen environment:

Under the weak vitality of no-clean flux (only containing 1% Carboxylic Acid), it is necessary to demand better solderability. Isn't it the reason for a fish rolling pin to blow fire? However, avoiding the environmental pressure of solvent cleaning is not a violation, of course, we have to find another way to find a solution. Therefore, if the tin pool area of the PCB wave soldering line can be modified into a nitrogen environment to reduce the adverse reactions of oxidation, it will naturally greatly help soldering. As a result of many previous tests, the residual oxygen content of the solder bath area in a nitrogen environment is the best with a residual oxygen content of less than 100ppm, but the additional increase in cost is self-evident. In order to save money, most of the general practical specifications set the residual oxygen rate range around 500ppm to 1000ppm. A well-designed "nitrogen furnace" is equipped with isolation and sealing facilities for the import and export of the parts to be welded and the gas filling device, which can automatically reduce the unnecessary consumption of nitrogen. These nitrogen furnace wave soldering lines have the following benefits:

(1) Improve welding yield.

(2) Reduce the amount of flux.

(3) Improve the appearance and shape of solder joints.

(4) Reduce the adhesion of flux residue and make it easier to remove.

(5) Reduce the chance of unit maintenance and increase output efficiency.

(6) The occurrence of Dross on the surface of the tin pool is greatly reduced, the amount of soldering tin is saved, and the processing cost is reduced.