In the SMT manufacturing process, the circuit board is easy to warp when the backflow welding, serious words will even cause components empty welding, monument and other bad, how to overcome it? The reason of PCB board warping may be different, but in the end, it can be attributed that the stress applied to the PCB board is more than the stress of the board material. When the stress on the board is uneven or the stress resistance of every place on the board is uneven, the result of PCB board warping will appear.
Where does the stress on the board come from? In fact, the biggest stress source in the reflux welding process is [temperature], the temperature will not only make the circuit board soft, but also distort the circuit board, coupled with the thermal expansion coefficient (CTE) factors and [thermal expansion and cold contraction] material characteristics, which forms the PCB board warping.
As for why some boards warp differently? Personally, I think there are several main reasons:
1.The copper surface area on the circuit board is uneven, which will worsen the warping of the PCB board.
General circuit board will design a large area of copper foil as grounding, sometimes Vcc layer will also design a large area of copper foil, when these large area of copper foil can not be evenly distributed on the same circuit board, will cause uneven heat absorption and heat dissipation speed, circuit board, of course, will be thermal expansion, if not at the same time will cause different stress and deformation, then the temperature of the plate has reached the Tg limit, the board will begin to soften, causing permanent deformation.
2.The overhole (vias) of each layer on the circuit board will limit the rise and shrinkage of the board.
Nowadays, the circuit boards are mostly multi-layer boards, and there will be perforated holes (via) like rivets between the layers, which are divided into through holes, blind holes and buried holes, where the holes will limit the effect of cold rise and contraction of the board, and will indirectly cause PCB board warping.
3.The weight of the circuit board itself causes the warping and distortion of the board.
Generally, the reflux welding furnace will use the chain to drive the circuit board forward to pass on, that is, with the two sides of the plate as the fulcrum to support the whole plate, if there are heavy parts on the plate, or the size of the plate is too large, it will present the phenomenon of the middle depression because of its own weight, resulting in warping.
4.The depth of the V-Cut and the connection strip will affect the deformation of the panel.
Basically, V-Cut is the main culprit of destroying the plate structure, because V-Cut is to cut out the V-shaped groove on the original large plate, so the V-Cut place is prone to deformation.
In view of the above-mentioned problems, special attention should be paid to the following situations in the design and production to better avoid board warping:
1.Effect of reducing temperature on plate stress
Since [temperature] is the main source of plate stress, so, as long as the return welding furnace temperature is reduced or the heating up and cooling speed of the plate in the return welding furnace is reduced, the warping of the PCB plate can be greatly reduced. But there may be other side effects, such as a solder short circuit.
2. Use High Tg plate
Tg is the glass conversion temperature, that is, the temperature of the material from glass to rubber. The lower the Tg value, the faster the plate starts to soften after entering the return welding furnace, and the time to become soft rubber will become longer, and the worse the deformation of the plate. Using a higher Tg plate can increase its ability to withstand the stress deformation, but the relative price of the material is also relatively high.
3.Increase the thickness of the circuit board
Many electronic products in order to achieve the purpose of thinner, the thickness of the board has left 1.0mm, 0.8mm, even did the 0.6mm thickness, the thickness to keep the plate in the back welding furnace deformation, really a little hard, suggest that if there is no thin requirements, board best can use the thickness of 1.6mm, can greatly reduce the risk of PCB board warping and deformation.
4.Reduce the size of circuit boards with reducing the number of panels
Since most of the back welding furnace chain to drive the circuit board forward, the larger the size because of its own weight, in the welding furnace, depression deformation, so the circuit board, the long side as a board side on the welding furnace chain, can reduce the depression deformation caused by the board weight, the board quantity is based on this reason, that means the furnace, try to use narrow edge vertical furnace direction, can achieve the lowest depression deformation.
5.Use furnace tray equipment
If the above method is difficult to do, finally is to use the furnace tray (reflux welding carrier / template) to reduce the deformation of the circuit board, the furnace tray can reduce the principle of PCB board warping because the material generally choose aluminum alloy or composite stone has high temperature resistance, so the circuit board after welding furnace heat expansion and after cooling down cold shrinkage, tray can stabilize the circuit board function, until the circuit board temperature below Tg value began to restore harden, and can also maintain the original size.
If a single layer of tray treatment can not reduce the deformation of the circuit board, you must add a layer of lid, the circuit board with two layers of tray, so as can greatly reduce the deformation of the circuit board furnace deformation. But this furnace tray is very expensive, and also have to add manual to put and recycling tray.
6.Replacing the V-Cut split board with Router instead
Since the V-Cut destroys the structural strength of the circuit panels, try not to use the V-Cut split panel, or reduce the depth of the V-Cut.
