SMT is also known as surface mount technology. In the manufacturing process, solder paste is heated and melted under a kind of heating environment, so that PCB solder pad can be reliably combined with surface mount components by solder paste alloy. We call this process reflow soldering. When the circuit board passes Reflow (Reflow soldering), it is easy for the plate to bend and become warped. In serious cases, it may even lead to empty soldering and erecting of components.
In the automatic insert line, if the printing plate is not smooth, it will cause inaccurate positioning, and the components cannot be inserted into the hole and surface of the plate, and even break the automatic insert machine. The welded plates with the components are bent, and the component feet are difficult to be cut in order. The boards can’t fit into the case or the socket, so it’s equally annoying for an assembly plant to get stuck on the board. At present, the PCB has entered the era of surface mounting and chip mounting, and the assembly plant must be more and stricter.
According to IPC – 6012, specification for the identification and performance of rigid PCB, the maximum permissible warp and twist for surface mounting of PCB is 0.75%, and other boards allow 1.5%. This is an improvement over IPC – RB – 276. Currently, the warping degree approved by each electronic assembly plant, whether double-sided or multilayer, is 1.6mm thick, usually 0.70~0.75%, and many SMT, BGA boards require 0.5%. Some electronics factories are agitating to increase the standard of warpage to 0.3% and test the warpage using gb4677.5-84 or ipc-tm-618.104.22.168b. Put the printing plate on the platform to be checked, and insert the test pin into the place with the maximum warp.
The reason for the formation of each plate bending and plate warping may be different, but it should be attributed to the stress applied on the plate is greater than the stress that can be borne by the plate material. The following are the four causes of plate bending and plate warping.
1. The area of the copper surface on the circuit board is not uniform, which will deteriorate the bending and warping of the board
General circuit boards will be designed with a large area of copper foil for grounding, sometimes Vcc layer design has a large area of copper foil, when these large areas of copper foil can not evenly distributed in the same circuit boards, will cause uneven heat and cooling speed, circuit boards, of course, also can heat bilges cold shrink, if increases and cannot at the same time can cause different stress and deformation, the temperature of the board at this time if the Tg has reached the upper limit value, the board will start to soften, causing permanent deformation.
2. The weight of the board itself will cause the sag deformation of the board
Generally, the backweld furnace will use the chain to drive the circuit board forward in the backweld furnace, that is, the whole board will be supported by the fulcrum on both sides of the board.
3. The depth and connection of v-cut will affect the deformation of the panel
Basically, v-cut is the culprit of damaging the structure of the board, because v-cut is to Cut the v-groove on the original large sheet of board, so the place where v-cut is prone to deformation.
4. The connection points (vias) of each layer on the circuit board will limit the expansion and contraction of the board
Nowadays, the board is mostly multi-layered, and there will be rivet-like connection points (via) between the layers. The connection points are divided into through-hole, blind hole and buried hole. Where there are connection points, the effect of expansion and contraction of the board will be limited, and also the bending and warping of the board will be indirectly caused.