Once the PCB design is completed, everything will be fine? In fact, this is not the case. Various problems are often encountered during PCB processing and manufacturing, such as tin connection after wave soldering. Of course, not all problems are caused by PCB design, but as designers, we must first ensure that there are no problems in our designs.
wave soldering
Wave soldering allows the welding surface of the plug-in board to be directly contacted with high-temperature liquid tin for the purpose of welding. 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 strip.
Tin-connection
Two or more solder joints are connected together by solder, resulting in defective appearance and function, which is specified as a defect level by IPC-A-610D.
Why does the PCB board have tin connections after wave soldering?
First of all, we need to make it clear that soldering on the PCB board is not necessarily a problem of poor PCB design. It may also be caused by poor flux activity, insufficient wettability, uneven application during wave soldering, and poor preheating and solder temperature control. Well wait for the reason.
If it is a PCB design problem, we can consider 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. When the spacing does not meet the process requirements, are tin-stealing pads and screen-printing ink added?
4. Whether the length of the plug-in pins is too long, etc.
How to avoid tin connection in PCB design?
1. Choose appropriate components. If the board needs to be wave soldered, it is recommended that the selected device spacing (the center spacing between PINs) be equal to or greater than 2.54mm, and it is recommended that it be at least greater than 2.0mm, otherwise the risk of tin connection is relatively high. Here you can appropriately modify and optimize the pad to meet the processing technology while avoiding tin connections.
2. Do not extend the welding foot beyond 2mm, otherwise it will be extremely easy to connect the tin. As an empirical value, when the length of the pin out of the board is ≤1mm, the probability of tin connection of the dense-pin 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 the copper rings. This is why we often lay a layer of silk screen white oil on the soldering surface of the plug-in during design. During the design process, pay attention to avoid silk screen white oil in the solder mask area of the solder pad.
4. The green oil bridge must be no less than 2mil (except surface mount pin-dense chips such as QFP packages), otherwise it will easily lead to 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 soldering will be greatly reduced. In the professional PCB design process, design determines production, so the transmission direction, wave soldering device placement, etc. are actually particular.
6. Add a tin-stealing pad. According to the plug-in layout requirements on the board, add a tin-stealing pad at the end of the transmission direction. The size of the soldering pad can be adjusted appropriately according to the density of the board.
7. If we must use plug-ins with a close spacing, we can install a tin drag piece at the tin position of the jig to prevent the solder paste from clumping and causing the component pins to connect to the tin.
