1, the damage of PCB line plate deformation
On the automatic surface mount line, if the printed circuit board is not smooth, it will cause incorrect positioning. The components cannot be inserted or attached to the hole and surface pad of the board, or even crash the automatic cartridge loader. Circuit board components after welding, bending element, foot hard cut neatly. The board can not be installed in the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to be warped. The current surface mount technology is developing towards high accuracy, high speed and intelligence, which brings higher flatness requirement for PCB boards made of various components.
The PCB board is made up of copper foil, resin and glass cloth. The physical and chemical properties of all materials are different. After pressing together, there will be residual thermal stress and deformation. At the same time during the processing of PCB, after high temperature, mechanical cutting, wet processing and other processes, will also have an important impact on plate deformation, the cause of PCB plate deformation is complicated, how to reduce or eliminate the deformation caused by different material properties or processing, has become one of the PCB manufacturers face the most complex the problem.
2, analysis of the cause of deformation
The deformation of PCB plate needs to be studied from several aspects, such as material, structure, graphic distribution, machining process and so on. This paper will analyze and explain various reasons for deformation and improvement methods.
The area of the copper paving surface on the circuit board is uneven, which will deteriorate the bending of the plate and the warping of the plate.
General circuit board design will have a large area of copper foil as a ground for, sometimes Vcc layer can make the design a large area of copper foil, copper foil can not when these large areas were evenly divided at the same time on the circuit board, it will cause heat and cooling speed is not uniform of course, the circuit board will be thermal expansion and contraction, if the expansion cannot at the same time will result in different stress and deformation, the board at this time if the temperature has reached the limit of Tg value, the board will begin to soften, causing permanent deformation.
Today's PCB is mostly multi-storey, and there will be a connection point between ribbed pin and layer (vias). The connection points can also be divided into through holes, blind holes and buried holes. The location of connecting points will limit the effect of plate expansion and contraction, and will also cause plate bending and plate warping indirectly.
General reflow oven will drive circuit board on the reflow oven in advance using the chain, which is on both sides of the board when the fulcrum to hold up the whole board, if the board above the heavy parts, or the board size is too large, because it will be a while showing itself due to depression in the middle. By bending.
3, improve the Countermeasures
How can it prevent plate bending and plate warping from the back furnace of the plate?
The effect of reducing temperature on the stress of the plate
Since "temperature" is the main source of plate stress, as long as reducing the temperature of reflow furnace or slowing down the heating and cooling speed of plate in reflow furnace, it will greatly reduce the occurrence of plate bending and plate warping. But there may be other side effects is.
High Tg plate
Tg is the glass transition temperature, that is material made of glass transition into a rubbery temperature, the lower the Tg value of material, said the board into the reflow oven after softened faster, but also into the soft rubbery time becomes longer, the deformation of the board will be more serious. The ability to withstand stress and deformation can be increased by a higher Tg plate, but the price of the relative material is also higher.
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, and even make the thickness of the 0.6mm, so the thickness to keep the board after the reflow oven is not deformed, really a bit difficult, if there is no suggestion of thin plate, the best can use the thickness of 1.6mm, can greatly reduce the bending and deformation of the risk.
Reduce the size of the circuit board and reduce the number of Collages
Since most of the reflow oven at the chain is used to drive the circuit board forward, circuit board size is large because of its own weight, sag deformation in the reflow oven, so try to make the long edge of the circuit board as the board edge on the chain of reflow furnace, can reduce the sag deformation circuit the board itself caused by the weight, to reduce the number of pieces is also based on this reason, it is said that when the furnace, as far as possible with narrow vertical furnace, can reach the minimum amount of depression.