Problems and Countermeasures of wave soldering (3)
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Problems and Countermeasures of wave soldering (3)
Solder ball (bead)
If the preheating temperature of PCB is too low or the preheating time is too short, the solvent and moisture in the flux will not volatilize, which will cause solder spatter during welding. Increase the preheating temperature or extend the preheating time.
Causes of spatter ball (bead):
1. PCB is damped during manufacturing or storage.
2. The environmental humidity is high, the moisture condenses on the multi slit PCB, and no moisture inspection measures are taken in the plant.
3. The coating and flux are insoluble, and the flux is not properly selected.
4. The flux is omitted or the coating amount is not in conformity with the area, and the flux absorbs moisture and water.
5. The resistance welding layer is not good, with solder residue.
6. Poor processing of base plate and rough hole wall lead to the accumulation of tank fluid, which was not analyzed in PCB design.
7. Improper preheating temperature.
8. Silver plated parts are dense.
9. Improper selection of solder crest shape.
Spatter solution:
1. Change PCB storage conditions to reduce moisture.
2. Select appropriate flux.
3. Spray flux evenly and increase preheating temperature.
4. Change the PCB design scheme and analyze the thermal uniformity.
5. Open flat wave shaping PCB solder joint.
Air hole (bubble / pinhole)
If the impurity of solder exceeds the standard and the Al content is too high, the solder joint will be empty. Replace the solder.
The surface oxide, residue and pollution of solder are serious. Debris shall be removed at the end of each day's work.
The climbing angle of PCB is small, which is not conducive to flux exhaust. PCB climbing angle is 3-7 °
The peak height is too low, which is not conducive to exhaust. The peak height is generally controlled at 2 / 3 of the PCB thickness.
Cause of formation of air hole (bubble or pinhole):
1. Excessive flux or insufficient volume before welding.
2. The base plate is damped.
3. The gap between hole position and lead wire is large and the exhaust of base plate is not smooth.
4. Poor hole metal.
During wave soldering, the heat capacity of the heated matrix is very large. Although the soldering is over, it has not been cooled yet. Due to the thermal inertia, the temperature is still rising. At this time, the outside of the solder joint begins to solidify, while the internal temperature of the solder joint decreases slowly, and the residual gas continues to expand. Extruding the outer surface will spray out the solidified solder to form the inner shape and pores of the solder joint.
Pore (bubble or pinhole) solution:
1. Increase preheating temperature and give full play to flux.
2. Reduce the substrate pre storage time.
3. Design welding pad correctly to ensure smooth exhaust
4. Prevent oxidation and pollution of pad metal.
Poor wetting < extreme case of poor weldability due to serious surface contamination. Non wetting and semi wetting coexist on the same surface at the same time
The adhesion of metal electrode at the end of chip element is poor or single-layer electrode is used, which results in decapping phenomenon under the welding temperature. The three-layer end structure is used in the wave soldering of surface mount components, which can withstand two or more 260 ℃ wave soldering temperature shocks.
PCB design is unreasonable, shadow effect in wave soldering results in solder missing. Meet DFM design requirements
The PCB warps, making the PCB warping position in poor contact with the wave crest. PCB warpage less than 0.8-1.0%
The two sides of the conveyor belt are not parallel, so that the PCB and the wave crest contact are not parallel. Adjust the level.
The wave crest is not smooth, and the height of both sides of the wave crest is not parallel. Especially, if the tin wave nozzle of the electromagnetic pump wave soldering machine is blocked by oxide, the wave crest will appear serration, which is easy to cause missing welding and lack of welding. Clean the tin wave nozzle.
Poor flux activity results in poor wetting. Change the flux.
PCB preheating temperature is too high, so that the flux carbonization, loss of activity, resulting in poor wetting. Set the proper preheating temperature.
Non wetting and anti wetting
Non wetting: continuous solder film is produced on the basic metal surface after wave soldering. On the non wetting surface, the solder does not completely contact with the base metal, but the bare base metal surface can be seen by cotton thread.
Anti wetting: wave soldering solder first moistens the base metal surface and then retracts with insufficient wetting to leave a layer of very wave solder on the base metal surface. At the same time, there are some separated solder balls intermittently. The big solder ball has a large contact angle at the contact point of the base metal.
Anti wetting is similar to the phenomenon of semi wetting on the surface of non wetting base metal
When the metal impurity concentration in the solder groove reaches a certain value, semi wetting will also occur.