Tutorial: How To Avoid Wave Soldering

Why does PCB board appear tin after wave soldering?

How do you avoid it?

Once the PCB design is done, all is well?

In fact, it is not. In the process of PCB processing and production, there are often various problems, such as soldering tin after wave soldering. Of course, not every problem is a PCB design problem, but as designers, we need to make sure that our design is correct in the first place.

Wave soldering: the welding surface of the plug-in plate is directly in contact with the high temperature liquid tin to achieve the purpose of welding, the high temperature liquid tin to maintain a bevel, and by the special device to form a similar wave of liquid tin phenomenon, so it is called “wave soldering”, the main material is the solder bar.

Tin joint: Two or more solder joints are joined together with solder material, causing defects in appearance and function, rated as defective by IPC-A-610D.

Why does PCB board appear tin after wave soldering?

First of all, we need to make it clear that tin connection on PCB board is not necessarily a problem of poor PCB design, but also may be caused by poor flux activity, insufficient wettability, uneven application, preheating and uncontrolled solder temperature during wave soldering.

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

1. Whether the solder joint distance of the wave soldering device is sufficient;
2. Is the transmission direction of the plug-in reasonable?
3. 3.In the distance does not meet the requirements of the process, whether to add stolen solder pads and screen printing ink?
4. Whether the length of the plug pin is too long, etc.

How to avoid tin in PCB design?

1. Choose the appropriate components. If the board needs wave soldering, the recommended selection of device spacing (the center spacing between pins) is greater than 2.54mm, it is recommended to be at least greater than 2.0mm, otherwise the risk of tin is relatively large. Here the pads can be modified and optimized to meet the processing process and avoid tin connection.

2. Welding foot wear out not more than 2mm, otherwise it is very easy to connect tin. An empirical value, when the pin length of the board is less than or equal to 1mm, the probability of connecting tin to the tight socket will be greatly reduced.

3. The spacing between copper rings should not be less than 0.5mm, and white oil should be added between copper rings. This is why we often put a layer of silk-screen white oil on the soldering surface of the plug-in. In the design process, pay attention to avoid screen printing white oil in the open resistance area of the welding pad.

4. The green oil bridge must not be less than 2mil (except for the dense chip such as QFP type package), otherwise it is easy to lead to tin 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 that the number of pins for handling even tin will be greatly reduced. In the process of professional PCB design, design determines production, so the transmission direction and the placement of wave soldering devices are actually paid attention to.

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

7. If we must use the plug-in with close spacing, we can install a towing tin sheet on the tin position of the fixture to prevent the solder paste from forming lumps and causing the component feet to connect to the tin.