Why is the circuit board required to be very flat on the automated assembly line, if the printed board is not flat, it will cause inaccurate positioning, components can not be inserted into the holes of the board and the surface mounting pads, and even the automatic insertion machine will be damaged . The board with the components is bent after welding, and the component feet are difficult to cut flat and uniform. The board cannot be installed on the chassis or the socket inside the machine, so the assembly plant is also very upset when the board is warped. At present, printed boards have entered the period of appearance devices and chip devices, and assembly plants must have stricter and stricter requirements for board warping.
Standards and test methods for warpage
According to the US IPC-6012 (1996 edition) <<Assessment and Performance Standards for Rigid Printed Boards>>, the maximum allowable warpage and distortion of printed boards for exterior devices is 0.75%, and 1.5% for other boards. This is an improvement over the IPC-RB-276 (1992 edition) request for printed boards for external devices. At present, the warpage promised by various electronic assembly factories, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.70-0.75%. For many SMT and BGA boards, the request is 0.5%. Some electronic factories are inciting to increase the warpage specification to 0.3%, and the method of testing the warpage complies with GB4677.5-84 or IPC-TM-6126.96.36.199B. Place the printed board on the verified platform, insert the test pin into the center of the largest warpage, and divide the diameter of the test pin by the length of the curved edge of the printed board to calculate the warpage of the printed board. The curvature is gone.
Prevent board warpage during manufacturing
1. Engineering design: Matters needing attention when designing printed boards: A. The arrangement of prepregs between layers should be symmetrical, for example, for six-layer boards, the thickness between 1-2 and 5-6 layers and the number of prepregs should be different, otherwise the layers It is easy to warp after pressing.
B. Multi-layer core boards and prepregs should use the same supplier's products.
C. The area of the circuit pattern on side A and side B of the outer layer should be as close as possible. If the A side is a large copper surface, and the B side only has a few lines, this kind of printed board will easily warp after etching. If the area of the lines on the two sides is too different, some independent grids can be added on the thin side for balance.
2. Baking board before cutting: The purpose of drying the board before cutting the copper clad laminate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time make the resin in the board completely solidify, and further eliminate the remaining stress in the board. This is helpful to avoid board warping. At present, many double-sided and multi-layer boards still adhere to the step of baking before or after the blanking. However, there are some exceptions to the board factories. At present, the time rules for baking boards in various PCB factories are not different, ranging from 4 to 10 hours. It is recommended to decide based on the level of the printed board consumed and the customer's request for warpage. After cutting into a jigsaw, it is better to bake the whole block and then cut the material. Both methods are feasible. It is recommended to bake the board after cutting. The inner board should also be baked.
3. The latitude and longitude of the prepreg:
After the prepreg is laminated, the warp and weft shrinkage rates are different, and the warp and weft directions must be distinguished during blanking and lamination. Otherwise, it is easy to form the warpage of the waste board after lamination, and it is difficult to correct it even if the pressure is applied to the baking board. Many reasons for the warpage of the multilayer board are caused by the fact that the prepregs are not distinguished in the warp and weft directions during lamination.
How to distinguish the latitude and longitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the weft direction; for the copper foil board, the long side is the weft direction, and the short side is the warp direction. If you are not sure, please check with the consumer or supplier.
4. Relief of stress after lamination: Take out the multi-layer board after hot pressing and cold pressing, cut or mill off the burrs, and then put it flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and make the resin complete Curing, this step cannot be omitted.
5. Straightening is required for thin plate electroplating:
When the 0.4～0.6mm ultra-thin multilayer board is used for surface plating and pattern plating, special clamping rollers should be made. After the thin plate is clamped on the flybus on the automatic electroplating line, a round stick is used to clamp the entire flybus. The rollers are strung together to straighten all the plates on the rollers so that the plates after plating will not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the thin plate will bend and it is difficult to compensate.
6. Cooling of the board after hot air leveling:
The hot air leveling of the printed board is usually impacted by the high temperature of the solder bath (about 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning. This is very useful for preventing warpage of the board. In some factories, in order to enhance the brightness of the lead-tin surface, the boards are put into cold water immediately after the hot air is leveled, and then taken out after a few seconds for disposal after stopping. This kind of hot and cold shock may cause warping of certain types of boards. Twisted, layered or blistered. In addition, an air flotation bed can be installed on the equipment to stop cooling.
7. Disposal of warped board:
In a well-managed factory, the printed board will be 100% flatness checked during the final inspection. All unqualified boards will be picked out, put in an oven, baked at 150 degrees Celsius under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure. Then relieve the pressure to take out the board, and check the flatness, so that part of the board can be saved, and some boards need to be bake and pressed two to three times before they can be leveled. If the above-mentioned anti-warping process measures are not implemented, the partial board baking and pressing is useless and can only be scrapped.