PCB News - Prevention of warpage and deformation of PCB long board/long board

Prevention of warpage and deformation of PCB long board/long board

PCB long board/Long board warping has a great influence on the production of printed circuit boards. Warpage is also one of the important problems in the production process of PCB long board/long board. Install components The PCB long board/long board board is bent after welding, and the component feet are difficult to be neat. The PCB long board/long board board cannot be installed on the chassis or the socket in the machine, so the warpage of the circuit board will affect the normal operation of the entire subsequent process. At this stage Printed circuit boards have entered the era of surface mounting and chip mounting, and the process requirements for circuit board warping are getting higher and higher. So we have to find the reason for the warping of the halfway help.
1. Engineering design: Points to note when designing long PCB/long strips: A. The arrangement of the prepregs between layers should be symmetrical, such as a six-layer circuit board, the thickness between 1-2 and 5-6 layers, and the number of prepregs. It should be consistent, otherwise it is easy to warp after lamination. B. Multilayer circuit board core board and prepreg 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 wires, this kind of printed circuit board is easy to warp after etching. If the area of the lines on the two sides is too different, you can add some independent grids on the thin side for balance.
2. Drying the 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 prevent the board from 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 exceptions for some board factories. The current time regulations for the drying of PCB long boards/long boards are also inconsistent, ranging from 4 to 10 hours. It is recommended to follow the grade of the printed board produced and the customer’s requirements for warpage. Decided. Bake after cutting into a jigsaw or blanking after the whole block is baked. Both methods are feasible. It is recommended to bake the board after cutting. The inner board should also be baked.

3. The warp and weft direction 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 circuit board are that the warp and weft directions of the prepregs are not distinguished when the PCB is laminated. 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, you can check with the manufacturer or supplier.
4. Relief stress after lamination: take out the multi-layer circuit 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. The thin plate needs to be straightened when electroplating: 0.4～0.6mm ultra-thin multi-layer circuit boards should be made of special nip rollers for surface electroplating and pattern electroplating. After the thin plate is clamped on the fly bus on the automatic electroplating line, one The round rods string the nip rollers on the entire fly bus, thereby straightening all the PCB circuit boards on the rollers, so that the electroplated circuit boards 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 PCB circuit board after hot air leveling: The printed board is subjected to the high temperature impact of the solder bath (about 250 degrees Celsius) when the printed board is leveled by hot air. After taking it out, it should be placed on a flat marble or steel plate for natural cooling, and then sent to the post-processing machine. For cleaning. This is very good for preventing warpage of PCB long board/long board board. In some circuit board factories, in order to enhance the brightness of the lead-tin surface, the circuit boards are immediately put into cold water after the hot air is leveled, and then taken out after a few seconds for post-processing. This hot and cold impact will affect certain types of PCBs. The long board/long board is likely to warp, delamination or blisters. In addition, an air flotation bed can be installed on the equipment for cooling.
7. Treatment of warped PCB long board/long board: In a well-managed factory, the printed board will be checked for 100% flatness 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 and take out the PCB long board/long board board, and check the flatness, so that part of the board can be saved. Some PCB long board/long board boards need to be bake and pressed two to three times before they can be flattened. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.