Why does the PCB board appear with tin after wave soldering?

After the PCB design is completed, everything will be fine? In fact, this is not the case. In the process of PCB processing, various problems are often encountered, such as continuous tin after wave soldering. Of course, not all problems are the “pot” of PCB design, but as designers, we must first ensure that our design is free.



Wave soldering

Wave soldering is to make the soldering surface of the plug-in board directly contact the high-temperature liquid tin to achieve the purpose of soldering. The high-temperature liquid tin maintains a slope, and a special device makes the liquid tin form a wave-like phenomenon, so it is called “wave soldering”. The main material is solder bars.

Why does the PCB board appear with tin after wave soldering? How to avoid it?

Wave soldering process

Two or more solder joints are connected by solder, resulting in poor appearance and function, which is specified as a defect level by IPC-A-610D.

Why does the PCB board appear with tin after wave soldering?

First of all, we need to make it clear that the presence of tin on the PCB board is not necessarily a problem of poor PCB design. It may also be due to poor flux activity, insufficient wettability, uneven application, preheating and solder temperature during wave soldering. Good to wait for the reason.

If it is a PCB design problem, we can consider from the following aspects:

1. Whether the distance between the solder joints of the wave soldering device is sufficient;

2. Is the transmission direction of the plug-in reasonable?

3. In the case that the pitch does not meet the process requirements, is there any tin stealing pad and silk screen ink added?

4. Whether the length of the plug-in pins is too long, etc.

How to avoid even tin in PCB design?

1. Choose the right components. If the board needs wave soldering, the recommended device spacing (center spacing between PINs) is greater than 2.54mm, and it is recommended to be greater than 2.0mm, otherwise the risk of tin connection is relatively high. Here you can appropriately modify the optimized pad to meet the processing technology while avoiding tin connection.

2. Do not penetrate the soldering foot beyond 2mm, otherwise it is extremely easy to connect tin. An empirical value, when the length of the lead out of the board is ≤1mm, the chance of connecting the tin of the dense-pin socket will be greatly reduced.

3. The distance between the copper rings should not be less than 0.5mm, and white oil should be added between the copper rings. This is why we often put a layer of silkscreen white oil on the welding surface of the plug-in when designing. During the design process, when the pad is opened in the solder mask area, pay attention to avoid the white oil on the silk screen.

4. The green oil bridge must be no less than 2mil (except for surface mount pin-intensive chips such as QFP packages), otherwise it is easy to cause tin connection between the pads during processing.

5. The length direction of the components is consistent with the transmission direction of the board in the track, so the number of pins for handling the tin connection will be greatly reduced. In the professional PCB design process, the design determines the production, so the transmission direction and the placement of wave soldering devices are actually exquisite.

6. Add tin stealing pads, add tin stealing pads at the end of the transmission direction according to the layout requirements of the plug-in on the board. The size of the tin stealing pad can be adjusted appropriately according to the density of the board.

7. If you must use a denser pitch plug-in, we can install a solder drag piece on the upper tin position of the fixture to prevent the solder paste from forming and causing the component feet to connect to the tin.