Solder spattering during wave soldering.
What are the likely causes of the common problem of solder spatter formation during wave soldering, and how can I prevent them?
Mitch Holtzer from Alpha Assembly Solutions took the floor, saying that there are two most likely causes of the problem: the temperature of the crucible causes a rapid phase change from liquid to vapor, which can cause spattering. These causes can be eliminated by reducing the amount of flux sprayed onto the plate or by extending the preheating step to remove the solvent contained in the flux.
Moreover, if the pallets are too cool as they pass through the wave, this can also cause splashing. Reducing the conveyor speed or extending the preheat again (or both) may help.
However, if you overdo these measures, it may lead to the formation of an 'icycle' or the filling of the PCB holes may be insufficient '. The phenomenon of 'icycling' is presented in the adjacent box.
Tony Lentz of FCT has a very similar opinion on this: 'Solder spatter during wave soldering is often caused by excessive flux and/or inadequate heating. Volatile flux materials, typically solvents, evaporate "explosively" on contact with the hot wave of solder, causing spattering. If too much flux is used, preheating will not be able to remove all volatile materials. If the preheat temperatures are too low or the board residence time is too short at this stage, some volatiles will still remain on the PCB. We recommend that both the flux rate and preheat settings be in accordance with the specifications of the flux manufacturers. To sum up: reduce the amount of flux applied or increase the heating time or temperature '.
But too much flux or a suboptimal preheat step is not the only possible cause of the spatter problem. Leo Lambert of EPTAC Corporation points out another point: 'Most wave soldering equipment today has an inclined conveyor that is pre-set by the manufacturer and you should make sure you don't change the settings. Also, check the amount of solder in the crucible. If the solder level is too low, the solder that is pushed from the back of the wave has a long way to travel before it returns to the crucible, causing it to flow unevenly, which can create solder splashes. […]
For soldering pallets, make sure the holes are smooth and at the correct angle to allow the solder to flow smoothly into the area to be soldered. Also, do not insert PCBs too deep into the wave. '
John Norton from Vitronics Soltec, as a representative of the wave maker, looks at this issue from another angle: 'First of all, it has not been determined whether the splashes occur on the upper or lower side of the plate. Regardless, in most cases, it is due to some type of turbulence in the wave region. If the solder is splashing on the top side, I would look at the wave as it goes from standby to run or from off to run. […] Also, make sure that when the solder flow returns to the crucible, there is no excessive spattering upwards at this point.
If splashes are observed on the underside of the wafer, I also believe that the cause is flux residue that has suddenly been heated […] and sticks to the mask. You should also check the solder level in the crucible and make sure you are on the right track.
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