1. Why is the PCB multilayer circuit board required to be very flat:
In the automated assembly line, if the PCB multi-layer board is not flat, it will cause inaccurate positioning, components cannot be inserted into the holes and surface mount pads of the board, and even the automatic plug-in assembly machine will be damaged. The board with the components is bent after welding, and it is difficult to cut the component feet neatly. The board cannot be installed on the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to encounter the board warping. At present, the printed circuit board has entered the era of surface mounting and chip mounting, and the requirements of assembly plants for PCB board warping must be more and more stringent.
2. Standards and test methods of warpage:
According to the US IPC-6012 (Edition 1996) “Identification and Performance Specification for Rigid Printed Boards”, the maximum allowable warpage and distortion for surface-mounted printed boards is 0.75%, and 1.5% for other boards. This has improved the requirements for surface mount printed boards than IPC-RB-276 (1992 edition). At present, the warpage permitted by various electronic assembly plants, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.7~0.75%. For many SMT and BGA boards, the requirement is 0.5%. Some electronic factories are urging to increase the standard of warpage to 0.3%. The method of testing warpage is in accordance with GB4677.5-84 or IPC-TM650.2.4.22B. Put the printed board on the verified platform, insert the test pin to the place where the degree of warpage is the greatest, 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 over.
3. Preventing PCB multi-layer board warping during the manufacturing process:
1. Engineering design: Matters needing attention when designing printed boards:
A. The arrangement of interlayer prepregs should be symmetrical, for example: six-layer board, the thickness between 1~2 and 5~6 layers and the number of prepregs should be the same, otherwise it is easy to warp after lamination.
B. Multilayer core board and prepreg should use the same supplier’s products.
C. The circuit pattern area of the outer layer A side and B side should be as close as possible. If the A side is a large copper surface, and the B side runs 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 to balance it.
2. Baking board before cutting:
The purpose of baking 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, which is helpful to prevent the board from warping. of. At present, many double-sided and multi-layer boards still insist on the step of baking before or after cutting. However, there are exceptions for some plate factories. The current PCB drying time regulations of various PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer’s requirements 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 cause the finished board to warp 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 that the warp and weft directions of the prepregs are not distinguished during lamination, and they are stacked randomly.
How to distinguish the latitude and longitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the latitude direction. For copper foil boards, the long side is the latitude direction and the short side is the warp direction. If you are not sure, you can check with the manufacturer or supplier.
4. In addition to stress after depressurization:
After the multi-layer board is hot-pressed and cold-pressed, it is taken out, cut or milled off the burrs, and then placed in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and fully cure the resin. This step cannot be omitted.
5. The thin plate needs to be straightened during electroplating:
When the 0.4~0.6mm ultra-thin multilayer board is used for surface electroplating and pattern electroplating, special clamping rollers should be made. After the thin plate is clamped on the fly bus on the automatic electroplating line, a round stick is used to clamp the entire fly bus. The rollers are strung together to straighten the plate on the roller so that the plate after plating will not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will bend and it is difficult to remedy it.
6. Cooling of the board after hot air leveling:
When the printed board is leveled with hot air, it is impacted by the high temperature of the welding trough (about 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processing machine for cleaning. This is very good for the board to prevent warping. In some factories, in order to enhance the brightness of the lead surface, the boards are put into cold water immediately after the hot air is leveled, and then taken out after a few seconds for post-processing. This kind of hot and cold impact may cause warping on certain types of boards. Song, layered
Or blistering. In addition, an air flotation bed can be installed on the equipment for cooling.
7. Treatment 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 the oven, baked at 150 degrees Celsius under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure. Then remove the pressure and 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. Shanghai Huabao’s pneumatic board warping and straightening machine has been used by Shanghai Bell to have a very good effect in remedying the warpage of the circuit board. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.
