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 when blanking and laminating. 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 prepregs are not clearly distinguished in the warp and weft directions during lamination, and they are stacked randomly.
How to distinguish the warp and weft directions? The roll-up direction of the prepreg is the warp direction, and the width direction is the weft direction; for copper foil boards, the long side is the weft direction and the short side is the warp direction. If you are not sure, you can ask the manufacturer or Supplier inquiries.
4.) Relieve stress after lamination of PCB thick copper plates:
The multi-layer PCB board is taken out after hot pressing and cold pressing, cut or milled off the burrs, and then placed flat 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.
5.) It needs to be straightened during plating:
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 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 sheet will bend and it is difficult to remedy it.
6.) Cooling of the board after hot air leveling:
PCB thick copper plates are subjected to the high-temperature impact of the solder bath (about 250 degrees Celsius) during hot air leveling. After being taken out, they 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 good for preventing the 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 post-processing. This kind of hot and cold impact may cause warping on certain types of boards. Twisted, layered, or blistered. In addition, an air flotation bed can be installed on the equipment for cooling.
7.) Treatment of warped board:
In an orderly managed PCB 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 to 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, some of the boards will be useless and can only be scrapped.
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