2, preheating system
In the substrate after the coating flux, the excess solvent, the evaporation flux to increase viscosity. This substrate should be preheated prior to welding. If the viscosity is too low, the exclusive agent of the flux will be molten tin prematurely, resulting in poor surface wetting. Dry flux also enhance the surface activity, and speed up the welding process. In the preheating stage, the substrate and components are heated to 100-105 ° C, the substrate and the molten tin contacts, reducing the thermal shock, and reduce the possibility of substrate warpage.
Through the wave soldering, preheating, there are a number of reasons:
1) Raising the temperature of the welding surface. Therefore requires less temperate energy from the crest, and so contribute to the response of the flux surface and faster welding.
2) warm-up reduces the thermal shock of the peak components, when the components exposed to sudden temperature gradient may be weakened or become not run.
3) warm-up to accelerate the speed of evaporation of volatile substances from the PCB. These volatile substances from the flux, but may also be from an earlier operation, storage conditions and handling. The emergence of volatiles on the crest may be caused by solder splash and solder balls on the PCB.
Control of the preheating temperature gradient, the preheating temperature and warm-up time is the key to achieve a good welded joints. Ensure that the flux at the appropriate time to properly stimulate and maintain to leave the crest until the PCB. Preheat the PCB must be brought to a high enough temperature to provide the activity of the flux being used. Most of the flux supplier push kept the temperature rise rate and the maximum and minimum top and bottom preheat temperature.
For any flux, insufficient warm-up time and temperature will cause more welding residues, and perhaps lack of activity, resulting in poor wettability. Preheat may also lead to the welding gas release caused by the solder ball, does not provide sufficient warm-up before the crest to the evaporation of water, the liquid solvent to reach the crest is often caused by solder splash. This situation is particularly evident in the low VOC water-based flux. When the preheating temperature is too high or warm-up time is too long, resulting in the flux may have been lost before you reach the crest. The flux in the crest on the main role is to reduce the surface tension of the solder, improve wetting properties. If the flux of the active surface points premature volatilization may be made surface bridging or icicles. The best preheating temperature on the crest to leave a sufficient flux to help solder peeling from the metal surface of the PCB exit crest.
Wave soldering preheat length determined by the yield and speed of the conveyor belt, the higher the yield, the faster the speed of the conveyor belt. You need a longer warm-up area for the board to achieve the desired infiltration temperature. In addition, due to the large heat capacity of double-sided and multilayer boards, so they need to be higher than single-panel preheat temperature.
Wave soldering machine is basically thermal radiation to warm up, the most commonly used method of wave soldering preheat forced hot air convection, the convection of electric hot plate, electric heating rods and infrared heating.
3, wave soldering system
Wave soldering welding mechanism is molten liquid solder, with the role of power pumps in the solder bath surface to form a specific shape of the solder wave, and plug the components of the PCB is set with the conveyor, after a particular point of view as well as certain immersion depth through the solder wave solder welding process.
The surface of the solder wave is a uniform layer of oxide scale covering almost along the entire length of the solder wave direction remain static in the process of wave soldering, PCB move at the same rate.
When the PCB into the peak before the end of the substrate and the pin is heated, not left the instant of the peak end of a small amount of solder wetting force, the role of adhesion in pad and contraction for the center to lead to a minimum state, this time between the solder and pad wetting force days the solder cohesion between the two pads. Will form a full, round the entire solder joint, leaving the excess solder tail crest, due to gravity, down to the tin pot.
3.1 lead-free solder oxidation
Compared with SN-PB alloy solder, lead-free solder for high SN content in the high-temperature welding more easily oxidized, thus forming oxides of tin furnace surface residue affecting the weld quality, but also wasteful. Typical the dross structure 90% of the available metal in the center outside contains 10% of the oxides. Produce dross reasons: (1) the original quality of the solder.
(2) welding temperature.
(3) peak height.
(4) crest of the disturbance degree.