Problems and Countermeasures of wave soldering (4)

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Problems and Countermeasures of wave soldering (4)

Causes of non wetting and anti wetting:

1. The basic metal body shall not be welded

2. Insufficient flux activity or deterioration failure.

3. The oil or grease on the surface makes the flux and solder not contact with the surface to be welded.

4. Improper control of wave soldering time or temperature.

Non wetting and anti wetting solutions:

1. Improve the weldability of the metal to be welded.

2. Use flux with strong activity.

3. Reasonably adjust the temperature and time of flux.

4. Thoroughly remove the pollutants on the surface of the welded metal.

5. Keep the solder purity in the solder groove.

Contour of solder joint (overlay / shrinkage)

The reasons for the formation of the contour of the welding spot are as follows:

If the solder is overlaid too much, the solder will pile up too much on the solder joint to form a convex surface, and the outline of the pin line of the element can not be seen.

The solder is too few and shriveled, and the seat is seriously insufficient, so the connected wire cannot be completely sealed, so that part of it is exposed.

When the circular section leads are soldered on the PCB, if the contact angle is less than 15 °, the measurement error of tensile strength will be large. And the average value of tensile strength is much lower than the bad brazing state. If the contact angle is greater than 45 °, the average tensile strength is also lower than the maximum.

The best range of contact angle is 15 ° < ⊙ < 45 °

It is required that the wetting height of the solder joint to the extended lead h ≥ D Fig. 3

Contour shaping solution for solder joints:

1. Improve the weldability of the welded metal surface

2. Graphics and wiring of tangent actual PCB.

3. Reasonably adjust the solder temperature, clamping speed and clamping angle.

4. Reasonably adjust the preheating temperature.

Dark or granular solder joints

Causes of dark or granular solder joints:

1. Excessive accumulation of metal essence in solder makes solder joints dark gray or white.

2. Decrease of metal content in solder

3. The solder joint is darkened by chemical corrosion.

4. Anti oxidation oil will cause particles and unevenness in the solder joint.

5. Overheating during welding.

Dark or granular solder solutions:

1. Replace the bath tin.

2. Clean other impurities in the tin furnace with pure tin.

3. Reduce the welding temperature.

Solder joint bridging or short circuit

Solder joint bridging or short circuit explanation: too much solder makes adjacent lines or piles on the same conductor, which are called bridging and short circuit respectively.

PCB design is unreasonable, and pad spacing is too narrow. Meet DFM design requirements.

The pins of plug-in components are irregular or skewed. Before welding, the pins have been close to each other or touched. The pins of plug-in components shall be shaped according to the hole diameter and assembly requirements of the printed board. If the short plug-in welding process is adopted, the original pins shall be 0.8-3mm exposed to the welding surface of the printed board, and the components shall be upright during plug-in.

Too low temperature or too fast speed of the conveyor belt make the viscosity of the molten solder too high. The temperature of tin wave is 250 ± 5 ℃, and the welding time is 3-5s. When the temperature is a little low, the conveyor belt should be slowed down.

Flux activity is poor. Change the flux.

Causes of bridge and short circuit:

1. temperature

2. The distance between adjacent wires or pads is too short.

3. The surface of base metal is not clean.

4. The solder purity is not enough.

5. Flux activity and preheating temperature.

6. The thermal capacity of the board surface is too large due to the unreasonable design of PCB electrical installation.

7. Solder depth of PCB.

8. Height of component pin extending out of PCB

9. PCB clamping speed.

10. PCB clamping angle.

Bridging and short circuit solutions:

1. Adjust the temperature properly.

2. Change conductor or pad spacing design.

3. Clean the metal welding surface.

4. Change tin or add pure tin to improve the purity of solder.

5. Replace the flux with strong activity.

6. Change the design of PCB electrical installation to uniform the heat capacity of the board surface.

7. Adjust the wave crest depth.

8. The thickness of component pin is 1.5-3mm higher than that of PCB

9. Adjust the clamping speed and angle Pinch angle 3-7 degrees