I. Why the circuit board requirements are very flat on the automated insertion line. If the printed circuit board is not flat, it will result in inaccurate positioning. Components cannot be inserted into the holes of the board and the surface mounting pads, and they will even crash automatically. Inserting machine. The components mounted on the board are bent after soldering and the component legs are difficult to trim and trim. The board can not be mounted on the chassis or the socket in the machine, so the assembly plant is also very annoyed when it comes to board tilt. At present, printed boards have entered the era of surface mounting and chip mounting. Assembling plants are increasingly strict with regard to board lift requirements. II. Standards and Test Methods for Warpage According to IPC-6012 (1996 edition) “Identification and Performance Specifications for Rigid Printed Boardsâ€, the maximum allowable warpage and distortion for surface mount printed boards is 0.75%. , Other various boards allow 1.5%. This is an improvement over the IPC-RB-276 (1992 edition) requirements for surface mount printed boards. At present, the warpage degree of each electronic assembly factory license, regardless of double-sided or multi-layer, 1.6mm thickness, usually 0.70 to 0.75%, many SMT, BGA board, the requirement is 0.5%. Some electronics factories are agitating to raise the warpage standard to 0.3%. The method of testing warpage complies with GB4677.5-84 or IPC-TM-650.2.4.22B. Place the printed board on the certified platform and insert the test pin into the place where the warpage is the largest to test the diameter of the pin. Divide the length of the curved edge of the printed board to calculate the tilt of the printed board. Curvature. Three. Warpage in the manufacturing process 1. Engineering Design: Precautions for PCB Design: B. Multilayer core boards and prepregs should use the same supplier's product. C. The area of ​​the circuit pattern on the A and B sides of the outer layer should be as close as possible. If the A side is a large copper surface and the B side only takes a few lines, the printed board can easily warp after etching. If the line areas on both sides differ too much, separate grids can be added on the thin side for balance. 2. Pre-baking board before cutting: 3. The latitude and longitude of the prepreg: After the prepreg is laminated, the shrinkage in the warp direction and the weft direction is not the same. During the blanking and lamination, the warp and weft directions must be distinguished. Otherwise, after the lamination, it is very easy to cause the warpage of the finished plate, and it is difficult to correct it even if the pressured bake plate is used. The reason for the warping of the multi-layer board is that many laminations do not distinguish between the latitude and longitude of the prepreg, and they are caused by random placement. How to distinguish between latitude and longitude? Rolled prepregs are rolled in the direction of the warp, and the width is in the weft direction; for the copper foil board, the long edge is the weft, and the short edge is the warp direction. If it is uncertain, the manufacturer or supplier can be queried. 4. Post-laminating stress relief: 5 need to straighten the thin plate plating: When a 0.4-0.6mm ultrathin multilayer plate is used for plate surface plating and pattern plating, a special nip roller should be produced. After the thin plate is clamped on the flying bar of the automatic plating line, a round bar is used to clamp the whole bar. The rollers are strung together, so that all the plates on the rollers are straightened so that the plated plates will not be deformed. If this is not the case, the plate will bend after electroplating a 20-30 micron layer of copper and it is difficult to remedy. 6. Cooling of the board after hot air leveling: When the hot air is leveled, the printed board is exposed to the high temperature shock of the solder bath (approx. 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate to be naturally cooled and sent to a post-processor for cleaning. This is good for board warpage prevention. Some factories to enhance the brightness of the tin-tin surface, the board immediately after hot air leveling into cold water, removed after a few seconds in the post-treatment, this kind of hot and cold shock, on some models of the board is likely to produce warping Curved, layered or blistering. In addition, air float beds can be added to the equipment for cooling. 7. Warping board processing: In order to manage an orderly factory, the printed board will undergo a 100% flatness inspection at the final inspection. Any unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and naturally cooled under pressure. Then the pressure is relieved and the board is taken out for flatness inspection. This can save part of the board. Some boards need to be pressed two or three times before being leveled. Shanghai Huabao agent's pneumatic anti-straightening machine used by Shanghai Bell has a very good effect in remedying board warpage. If the above-mentioned warpage prevention measures are not implemented, some of the boards will not be used for bake and they will only be scrapped. Source: PCB Collection Network
A. The arrangement of the interlayer prepregs should be symmetrical, for example, a six-layer plate, and the thickness between 1 to 2 and 5 to 6 layers and the number of prepreg sheets should be the same, otherwise, the warpage will easily occur after lamination.
The purpose of pre-baking the board (150°C, time 8±2 hours) is to remove moisture from the board and fully cure the resin in the board to further eliminate the residual stress in the board. This prevents the board from warping. Help. At present, many double-sided and multi-layer boards still adhere to this step before or after baking. However, there are some exceptions to the sheet metal factories. Currently, the time requirements for the baking time of PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of printed boards produced and the customer's requirements for warpage. After cutting into a piece of dough or baking in an agglomerate, the two methods are all feasible. It is recommended to dry the plate after cutting. The inner plate should also be baked.
The multilayer board is taken out after the hot pressing and cold pressing is completed, and the flash is cut or milled, and then baked in an oven at 150 degrees Celsius for 4 hours so that the stress in the board is gradually released and the resin is completely cured. This step cannot be omitted.
How PCBs Prevent Warpage