1、 Why do circuit boards require a very flat surface

　　The circuit board prototype manufacturer tells you that if the printed circuit board is not flat on the automated insertion line, it will cause inaccurate positioning, prevent components from being inserted into the holes and surface mounting pads of the board, and even damage the automatic insertion machine. The board where the components are installed is bent after welding, making it difficult to cut the component legs flat and neat. The board cannot be installed on the chassis or socket inside the machine, so it is also very troublesome for the assembly plant to encounter board warping. At present, printed boards have entered the era of surface installation and chip installation, and assembly plants are bound to have increasingly strict requirements for board warping.

　　2、 Standards and testing methods for warpage

　　According to the American IPC-6012 (1996 edition)<>, the allowable large warpage and distortion for surface mounted printed boards is 0.75%, while other types of boards are allowed to have 1.5%. This increases the requirement for surface mounted printed boards compared to IPC-RB-276 (1992 edition). At present, the allowable warpage for electronic assembly plants, whether double-sided or multi-layer, with a thickness of 1.6mm, is usually 0.70-0.75%. For many SMT and BGA boards, the requirement is 0.5%. Some electronic factories are advocating to increase the standard for warpage to 0.3%, and the testing method for warpage should follow GB4677.5-84 or IPC-TM-650.24.22B. The PCB sampling factory teaches you: place the printed board on a verified platform, insert the test needle into a location with a higher degree of warping, divide the diameter of the test needle by the length of the curved edge of the printed board, and the warping degree of the printed board can be calculated.