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Component tin-connection problem in wave soldering

Then what are the components tin-connection? Please see the following picture.

We can see that the pins of the main chip in the red circle are connected together. This is an abnormal phenomenon. Therefore, the components tin-connection are two or more solder joints connected by solder, which causes poor appearance and function, and is defined as a defect level by IPC-A-610D.

Then how are the components tin-connection formed?

As far as we know, the components tin-connection is a kind of short circuit. It may be that the preheating temperature is not enough to cause the component to fail to meet the temperature requirements. During the soldering process, the component absorbs a large amount of heat, which leads to poor tin drag and the formation of tin. It is also possible that the temperature of the tin furnace is low, or the soldering speed is too fast. The specific reasons are divided into the following types:

1. Flux is almost the main cause of components tin-connection.

A. The flux activity is not enough.

B. The wettability of the flux is not enough.

C. The amount of flux applied is too small.

D. Uneven coating of flux.

E. The circuit board cannot be coated with flux.

As we all know, the activity is strong enough to remove the oxide layer on the surface of the copper pin in a very short time, but it should be ensured that the steam during the soldering process will not cause subsequent corrosion of the pin (green). Strengthen the rosin or resin content of the flux. Guaranteed good carrier performance under high temperature soldering.

2. The circuit board is not tinned regionally.

This problem is also the key to components tin-connection.

Therefore, when the soldering point of the circuit board is not sticky, we need to check whether there is copper skin around the soldering point, if there is any, scrape the solder resist on it and re-solder. If there is no copper skin around, it is necessary to heat the soldering point with a soldering iron, and then put the enameled wire head coated with solder in advance up to be soldered. Generally, the soldering should be heated before connecting, first dip the tin on the wire, which is much better.

3. Some pads or solder feet are seriously oxidized.

If the pads of the PCB board or the pins of the components have been seriously oxidized, it is easy to cause false soldering or components tin-connection. In fact, the component pins are oxidized, you can first use a small tin furnace to dip this batch of components first, and then solder. Secondly, choose ferrochrome with a slightly higher temperature. If the pad is oxidized, it must be cleaned or sanded once with sandpaper to remove the oxidation. After soldering, the inspection position is added to ensure that the soldering position is not fake.

4. The wiring of the circuit board is unreasonable (the distribution of meta-parts is unreasonable).

5. The walking direction is wrong.

6. Insufficient tin content, or copper exceeding the standard; [Excess impurities cause the melting point (liquid line) of the tin liquid to rise]

7. The foaming tube is blocked and the foaming is not uniform, resulting in uneven coating of the flux on the circuit board.

8. The setting of the air knife is unreasonable (the flux is not evenly blown).

9. The walking speed and preheating coordination are not good.

Therefore, I suggest that before re-welding, we need to use KIC to test whether the component temperature and the board surface temperature meet the soldering requirements.

Next, when the components tin-connection occurs, what improvement measures should we take? Please see the following 5 effective methods:

1. Design according to PCB design specifications. The long axes of the two tip Chips are perpendicular to the welding direction, and the long axes of SOT and SOP should be parallel to the welding direction. Widen the pad of the second pin of the SOP (design a tin steal pad)

2. The pins of the inserted components should be shaped according to the hole distance of the printed board and the assembly requirements. For example, if the short insertion sub-welding process is used, the component pins of the soldering surface are exposed from the surface of the printed board by 0.8 ~ 3mm, and the component body is required during insertion correct.

3. Set the preheating temperature according to the PCB size, whether it is a multi-layer board, how many components, there are mounted components, etc.

4. The temperature of tin wave is 250 ± 5 ℃, and the soldering time is 3 ~ 5s. When the temperature is slightly lower, the conveyor speed should be slowed down.

5. Replace the flux. The main component of flux is rosin. Rosin will be decomposed by tin at about 260 degrees Celsius, so the tin bath temperature should not be too high. Flux is a chemical substance that promotes welding. In soldering, it is an indispensable auxiliary material, and its role is extremely important. If components tin-connection appear in wave soldering, it means that we urgently need to find a new flux to change this bad phenomenon.

In addition, I think that during wave soldering, the factory must operate according to the following precautions, so that the components tin-connection phenomenon can be better avoided. During the soldering process, the solder temperature should generally be controlled within the range of 250 ℃ ± 5 ℃, whether the temperature is suitable for directly affecting the welding quality; the inclination angle of the welding fixture into the wave mouth should be adjusted to about 6℃.

The wire speed of soldering should be controlled within 1 ~ 1.6 n / min; the peak height of the tin surface of the solder bath is about 10 mm, and the peak top is generally controlled at 1/2 ~ 213 of the thickness of the circuit board. Excessive assembly will cause molten solder to flow to the circuit the surface of the board forms a "bridge". After wave soldering, the circuit board must be properly cooled by strong wind. After cooling the circuit board, the component leads are cut off.

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