How to prevent circuit board warping

1、 Why is the circuit board required to be very flat

On the automatic insertion line, if the printed board is not flat, it will cause inaccurate positioning, components can not be inserted into the holes and surface mounting pads of the board, and even damage the automatic insertion machine. The board installed with components is bent after welding, and the component feet are difficult to cut flat and neat. The board can not be installed on the chassis or the socket in the machine, so it is also very troublesome for the assembly factory to encounter the board warping. At present, printed boards have entered the era of surface installation and chip installation, and assembly plants must have more and more strict requirements for board warping.

2、 Standard and test method for warpage

According to the American ipc-6012 (1996 edition) < < 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 improves the requirements for surface mounted printed boards compared with ipc-rb-276 (1992 edition). At present, the allowable warpage of each electronic assembly factory, whether double-sided or multi-layer, is 1.6mm thick, usually 0.70 ~ 0.75%. For many SMT and BGA boards, it is required to be 0.5%. Some electronic factories are advocating to raise the standard of warpage to 0.3%. The method of testing warpage shall comply with gb4677.5-84 or ipc-tm-650.2.4.22b. Put the printed board on the verified platform, insert the test needle into the place with the largest warpage, and divide the diameter of the test needle by the length of the curved edge of the printed board to calculate the warpage of the printed board.

3、 Anti warping plate during manufacturing

1. Engineering design: precautions in PCB Design:

A. The arrangement of semi cured sheets between layers shall be symmetrical. For example, the thickness between layers 1 ~ 2 and 5 ~ 6 of six layers shall be consistent with the number of semi cured sheets, otherwise it is easy to warp after lamination.

B. The products of the same supplier shall be used for multilayer core board and semi cured sheet.

C. The area of line pattern on surface a and surface B of the outer layer shall be as close as possible. If surface a is a large copper surface and surface B only takes a few wires, the printed board is easy to warp after etching. If the line area difference between the two sides is too large, some independent grids can be added on the sparse side for balance.

2. Drying plate before blanking:

The purpose of drying the copper clad laminate before blanking (150 ° C, time 8 ± 2 hours) is to remove the moisture in the plate, completely solidify the resin in the plate and further eliminate the residual stress in the plate, which is helpful to prevent plate warpage. At present, many double-sided and multilayer boards still adhere to the step of drying before or after blanking. However, there are exceptions in some plate factories. At present, the drying time regulations 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. Both methods are feasible. It is recommended to dry the board after cutting. The inner plate shall also be dried.

3. Longitude and latitude of semi cured sheet:

The warp and weft shrinkage of semi cured sheet after lamination are different, so the warp and weft must be distinguished during blanking and lamination. Otherwise, it is easy to cause the warpage of the finished plate after lamination, and it is difficult to correct even if the pressure is applied to dry the plate. Many reasons for the warpage of multilayer boards are caused by the unclear longitude and latitude of semi cured sheets during lamination.

How to distinguish between longitude and latitude? The rolling direction of the rolled semi cured sheet is the warp direction, and the width direction is the weft direction; For copper foil, the long side is in the weft direction, and the short side is in the warp direction. If you are not sure, you can check with the manufacturer or supplier.

4. Stress relief after lamination:

After hot pressing and cold pressing, take out the multilayer board, cut or mill off the burr, and then place it flat in the oven at 150 ℃ for 4 hours, so as to gradually release the stress in the board and completely cure the resin. This step can not be omitted.

5. The sheet needs to be straightened during electroplating:

When 0.4 ~ 0.6mm ultra-thin multilayer board is used for plate surface electroplating and pattern electroplating, special pinch rollers shall be made. After clamping thin plates on the flying bar on the automatic electroplating line, use a round rod to string the pinch rollers on the whole flying bar, so as to straighten all the plates on the roller, so that the plated plates will not deform. Without this measure, the thin plate will bend after electroplating a copper layer of 20 or 30 microns, and it is difficult to remedy.

6. Cooling of plate after hot air leveling:

When the printed board is leveled by hot air, it is impacted by the high temperature of the solder bath (about 250 ℃), and then it shall be placed on the flat marble or steel plate for natural cooling, and sent to the post processor for cleaning. This is good for the anti warping of the board. In order to enhance the brightness of the lead tin surface, some factories put the plates into cold water immediately after hot air leveling, and take them out for post-treatment after a few seconds. This one heat and one cold impact is likely to produce warpage, delamination or blistering on some types of plates. In addition, an air floating bed can be installed on the equipment for cooling.

7. Treatment of warping plate:

In a well managed factory, 100% flatness inspection will be carried out during the final inspection of printed boards. All unqualified boards will be picked out, put into the oven, dried at 150 ℃ and under heavy pressure for 3 ~ 6 hours, and cooled naturally under heavy pressure. Then take out the board after pressure relief and check the flatness. In this way, some boards can be saved. Some boards need to be dried and pressed twice or three times to be leveled. The pneumatic plate warping and straightening machine represented by Shanghai Huabao has been used by Shanghai Bell to remedy the warpage of circuit board. If the above anti warping process measures are not implemented, some boards are useless and can only be scrapped.