一、Why is the PCB thick copper plate required to be very smooth?
On the automatic insertion line, if the printed circuit board is not flat, it will cause inaccurate positioning, components cannot be inserted into the holes of the board and surface mount pads, and even the automatic insertion machine will be damaged. The board on which the components are installed is bent after welding, and it is difficult to cut the component feet neatly. If this is the case, the PCB board cannot be installed on the chassis or the socket in the machine, so it is also very troublesome for the assembly factory to encounter board warping. At present, printed circuit boards have entered the era of surface mounting and chip mounting, and assembly plants must have stricter requirements for board warping.
二、the standard and test method of warpage
According to (Qualification and Performance Specifications for Rigid Printed Boards), the maximum allowable warpage and twist for surface mount printed boards is 0.75%, and 1.5% for other various boards. At present, the warpage allowed by each electronic assembly factory, regardless of double-sided circuit board or multi-layer circuit board, PCB thick copper plate, 1.6mm thickness, is usually 0.70~0.75%, and many SMT and BGA boards require 0.5% . Some electronics factories are advocating to raise the warpage standard to 0.3%. Put the printed board on the certified platform, insert the test pin to the place where the warpage is the largest, 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. curvature.
三、Anti-board warpage during the manufacturing process of PCB thick copper plate
1. Engineering design:
Matters needing attention when designing printed circuit boards:
A. The arrangement of prepregs between layers should be symmetrical. For example, for a six-layer PCB, the thickness between layers 1~2 and 5~6 should be consistent with the number of prepregs, otherwise it will warp easily after lamination.
B. Multi-layer circuit board core board and prepreg should use products from the same supplier.
C. The area of the circuit graphics on the outer A side and the B side 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 printed board is easy to warp after etching. If the line area of the two sides is too different, some independent grids can be added on the thin side for balance.
2. Baking plate before blanking:
The purpose of baking the PCB thick copper board before blanking (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 cured, and further eliminate the remaining stress in the board, which is important for preventing board warping helpful. At present, many double-sided and multi-layer PCB boards still insist on the step of drying the board before or after blanking. However, there are exceptions for some board factories. At present, the board drying time regulations of various PCB factories are 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. Cutting into jigsaw panels and then drying, or the whole piece of large material drying and then blanking, both methods are feasible, it is recommended to dry the panels after cutting. The inner board should also be baked.
3. Warp and weft direction of prepreg:
The warp and weft shrinkage of the prepreg is not the same after lamination, and the warp and weft must be distinguished when blanking and laminating. Otherwise, it is easy to cause warping of the finished board after lamination, and it is difficult to correct even if the pressure is applied to the board. Many of the reasons for the warping of multi-layer boards are that the warp and weft directions of the prepregs are not distinguished during lamination and are caused by random stacking.
How to distinguish warp and weft directions? The rolled up prepreg direction is the warp direction, while 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. Stress relief after lamination of PCB thick copper plate:
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 the oven at 150 degrees Celsius for 4 hours, so that the stress in the board can be gradually released and the resin can be completely cured. This step cannot be omitted. .
5. The sheet needs to be straightened during electroplating:
When the 0.4~0.6mm ultra-thin multi-layer board is used for plate surface plating and graphic plating, special clamping spokes should be made. After clamping the thin plate on the flying bar on the automatic electroplating line, use a round stick to clamp the entire flying bar. Kun is strung together to straighten and roll all the boards, so that the boards after electroplating will not be deformed. Without this measure, after electroplating a copper layer of twenty or thirty microns, the sheet will be warped and it is difficult to remedy.
6. Cooling of the board after hot air leveling:
The PCB thick copper plate is subjected to the high temperature impact of the solder bath (about 250 degrees Celsius) during hot air leveling. 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 preventing warping of the board. In some factories, in order to enhance the brightness of the lead-tin surface, the board is put into cold water immediately after the hot air is leveled, and it is taken out after a few seconds for post-processing. Curvy, layered or bubbly. In addition, an air flotation bed can be installed on the equipment for cooling.
7. Treatment of warped boards:
In a well-managed PCB factory, printed boards will be checked for 100% flatness during final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius under heavy pressure for 3 to 6 hours, and cooled naturally under heavy pressure. Then release the pressure and take out the board, and check the flatness. This can save some boards, and some boards need to be baked for two to three times to be leveled. If the above-mentioned anti-warping process measures are not implemented, some boards are useless for baking and pressing, and can only be scrapped.
