1. Standards and inspection methods for warpage

The allowable warpage and distortion for surface mounting of multilayer circuit boards are 0.75% and 1.5%. Compared with IPC-Rb-276 (1992 edition), the requirements for surface-mounted printed circuit boards have increased. At present, whether it is a double-sided circuit board or a multi-layer circuit board, the allowable warpage of various electronic assembly plants is usually 0.70-0.75%, and the warpage requirement of many SMT and BGA boards is 0.5%. Some electronics factories are pushing to increase the warpage standard to 0.3%, and the warpage test is carried out according to gb4677.5-84 or ipc-tm-650.2.4.22b.

Second, the multilayer circuit board is required to be very flat

In the automatic insertion line, if the printed multi-layer circuit board is not flat, it will cause errors, which will cause the component to be unable to be inserted into the hole and surface mount pad of the plate, and even damage the automatic insertion machine. Multi-layer circuit boards and components will bend after welding, and the component feet are not easy to cut and trim, and they cannot be installed on the chassis and sockets in the machine. Even the assembly factory is troubled by the warping of the board. Nowadays, printed boards have entered the era of surface mounting and chip mounting, and multilayer circuit board manufacturers will inevitably have stricter requirements on board warpage.

Three, to prevent plate warping during the production process

1. Straighten the sheet during electroplating

When using 0.6~0.8mm thin multilayer circuit boards for plate surface plating or drawing plating, special nip rollers should be made. After the thin plate is clamped on the automatic electroplating line, the entire clamping roller is connected to the clamping roller with a round bar, and the clamping roller and the entire coil are straightened so that the plate coil after electroplating is not deformed. If this measure is not taken, the sheet will bend after plating a copper layer of 20 or 30 microns, and it is difficult to remedy it.

2. The longitude and latitude of the prepreg

After the prepreg is laminated, the warp and weft shrinkage rates are different, and the material and repeated layers must be distinguished between the warp and weft, otherwise it will easily cause warpage after compounding, and even the pressure drying board will be difficult to correct. The reason for the warpage of the multi-layer circuit board is that the latitude and longitude of the prepreg is not clear, and the disorder is generated.