How to solve the short circuit problem of the row of foot parts during wave soldering?
In the PCBA processing wave soldering (Wave Soldering) process, it is often heard to use "drag solder pads" or "stealing solder pads" to solve the problem of solder bridge bridging tin short circuit. In fact, its English name is [solder thief pad]. Therefore, the correct Chinese name should be "Stealing Tin Solder Pads", but everyone is screaming, and now it seems to be reasonable to call it "Drag Tin" or "Steal Tin". "Dragging" is to drag tin to another dummy pad, and "stealing" is to steal excess tin to another pad.
The main purpose of the "solder thief pad" setting is to avoid the solder of the last solder joint being pulled away from the solder joint when the component foot of the circuit board soldered through the wave solder slides out of the wave solder surface. The solder resilience buffer is designed due to the rebound of tin's cohesive force, which causes a short circuit with the adjacent solder joint in front of it. This solder rebound behavior usually occurs in the "advection wave" of wave soldering and rarely occurs in the "spoiler wave ". Friends who do not understand what is "advection wave" and "spoiler wave" please refer to [What is "wave soldering"? Introduction to Wave Soldering Process Technology].
In addition, parts with a pin row (pin row) and the entire row of solder legs parallel to the direction of wave soldering are more prone to such short circuit problems of solder rebound. Regardless of whether it is the "SO (Small Outline) package" SMD parts or the solder pins of the plug-in connector, as long as the wave soldering has a chance to form this kind of welding rebound short circuit problem.
How to design "solder thief pad" to improve the wave soldering short circuit?
There are two ways to design a "solder thief pad":
Added a dummy pad. Add a dummy pad behind the last pad in the downstream direction of wave soldering. The line of this pad must be empty (no signal) or the line with the last pad is conductive, size It is as big as the last pad. Extend the last pad size (Extended pad). Another practice is to extend the size of the last pad directly back to the size of the two pads, which is to make the last pad three times longer.
The purpose of this design is because the force of solder rebound usually only reaches the previous solder joint, which is why when there is no solder pad stealing, the short circuit always occurs in the last two solder joints in the wave soldering direction. Since the short circuit cannot be avoided , Then make the short circuit rationalized, so that even if it is short-circuited, no functional problems will occur.Stealing solder pads
As for the advantages and disadvantages of these two designs, the difference is not very big. If you insist on it, "adding a fake pad" may make it easier for the production line to dispute with the quality control, because the quality control personnel always think that there is a short circuit. Defects, or all should be short-circuited or all open-circuited, some short-circuited and some open-circuited in the same batch of production, indicating that the quality is unstable, how to say, people still insist on quality control.
"Extending the last solder pad" is more unfavorable for repairs, because the area of tin eating becomes larger, and larger heat needs to be given during repairs, which is easy to damage the solder pads.
In terms of production quality alone, Shenzhen Honglijie recommends "extending the last solder pad" because the larger solder area can effectively absorb the rebound force of the solder instead of allowing the solder to rebound to the previous solder joint without control. The quality of the production line is completed, the maintenance is relatively reduced, and the quality control is also satisfactory.
Can the tray oven carrier be used instead of "solder thief pad"?
Oven tray tin foil
This is a very feasible and tryable scheme, in fact, someone really designed it this way. If the process itself needs to use a pallet carrier for wave soldering, it is really possible to design a "Solder thief tool" on the wave soldering carrier to replace the solder tin solder pads on the PCB. However, its position must be similar to or the same as the design of the solder pad on the PCB, and the size must be in accordance with the plan of "extending the last pad" instead of "adding a dummy pad". It is practically recommended to use this The piece of tin-stolen piece is longer than the solder-stitched pad on the PCB, but remember not to be too close to the solder joint in front to avoid bridging the solder joint between the tin-snapped piece and the PCB. It is troublesome when removing the PCB from the carrier.
In addition, the tin stole on this vehicle must meet the following requirements: Stolen tin must be made of tin.
Materials should be considered for reuse. Because the vehicle is to be used repeatedly, it is possible to steal the tin sheet repeatedly. You can consider using steel, and then plating nickel and then tin. The main consideration is that it must be able to eat tin repeatedly. The copper sheet is too soft, it will deform after repeated use, and lead-free wave soldering will bite copper.
Pay attention to the thickness. Too thin is easy to deform, too thick will block the flow of solder, about 1.0mm should be similar.
It must be fixed on the vehicle and cannot be moved.
Consider replacement. It is unavoidable that there will be abrasion and loss if the tin film is used for a long time. It is generally recommended to fix it with screws, which can be replaced. Of course, the screws must choose not to eat tin.
No sharp angles and sharp points. Because the tin foil will touch the PCB, sharp corners and sharp points may scratch the board.
In fact, many netizens on the Internet share their designs and examples of stealing solder pads. They can only say that there are many kinds of patterns. Even ICs such as QFP have legs that are designed to take wave soldering and add solder pads. Interested friends can find examples of others on the Internet.