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Method for preventing warpage of PCB printed board

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Published by: Keyou August 09,2019

1. Why the circuit board requirements are very flat

In the automated instrumentation line, if the printed board is not flat, it will cause misalignment, the components can not be inserted into the hole of the board and the surface mount pad, and even the automatic inserter will be damaged. The board on which the components are mounted is bent after soldering, and the component legs are difficult to trim. The board can't be installed in the chassis or the socket inside the machine, so it is very troublesome for the assembly factory to encounter the board. At present, printed boards have entered the era of surface mounting and chip mounting, and the assembly plant's requirements for board warping must be stricter.

    2. Standards and test methods for warpage

According to US IPC-6012 (1996 edition) <<Rigid Printed Board Identification and Performance Specification>>, the maximum warpage and distortion allowed for surface mount printed boards is 0.75%, and other various boards allow 1.5%. This is an increase in the requirement for surface mount printed boards than IPC-RB-276 (1992 version). At present, the warpage degree approved by each electronic assembly factory, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.70-0.75%, and many SMT, BGA boards, the requirement is 0.5%. Some electronics factories are encouraging the standard of warpage to 0.3%, and the method of testing warpage is in accordance with GB4677.5-84 or IPC-TM-650.2.4.22B. Place the printed board on the calibrated platform, insert the test pin into the place with the highest warpage, and test the diameter of the needle. Divided by the length of the printed board, you can calculate the warp of the printed board. The curvature is gone.

    Third, the anti-plate warping during the manufacturing process

 

    1. Engineering design: Note on the design of printed boards:

A. The arrangement of the interlayer prepreg should be symmetrical, for example, a six-layer board, and the thickness between the layers 1 to 2 and 5 to 6 and the number of sheets of the prepreg should be the same, otherwise it is easy to warp after lamination.

B. Multilayer core sheets and prepregs should use the same supplier's products.

C. The area of ​​the line pattern of 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 is only a few lines, the printed board is easily warped after etching. If the line areas on both sides are too different, add a separate grid on the thin side to balance.

 

    2, before baking the baking sheet:

The copper plate is pre-baked (150 degrees Celsius, 8±2 hours). The purpose is to remove the moisture in the plate and completely cure the resin in the plate to further eliminate the residual stress in the plate. Help. At present, many double-sided, multi-layer boards still adhere to the step of before or after baking. However, there are some exceptions for sheet metal. At present, the time regulations for baking sheets of PCB factories are also inconsistent, from 4-10 hours. It is recommended to decide according to the grade of the printed board produced by the customer and the customer's requirements for warpage. After cutting into a jigsaw and drying it, the whole piece of material is baked and then unloaded. Both methods are feasible. It is recommended to dry the plate after cutting. The inner panel should also be baked.

 

             PCB printed board

 

    3. The warp and weft of the prepreg:

The warp and weft shrinkage rates of the prepreg are different after lamination, and the warp and weft directions must be separated when cutting and laminating. Otherwise, it is easy to cause warpage of the finished board after lamination, even if it is pressed, it is difficult to correct. The reason for the warpage of the multi-layer board is that the latitude and longitude of the prepreg is not separated when it is laminated, and it is caused by the disorder.

How to distinguish the warp and latitude? The rolled prepreg is wound in the warp direction and the width direction is the weft direction; for the copper foil sheet, the long side is in the weft direction and the short side is the warp direction. If it is not certain, it can be inquired to the manufacturer or supplier.

 

    4. After stress removal after lamination:

The multi-layer board is taken out after hot pressing and cold pressing, sheared or milled off the burrs, and then baked in an oven at 150 ° C for 4 hours to gradually release the stress in the sheet and completely cure the resin. This step cannot be omitted.

 

    5, the plate needs to be straightened when plating:

When the 0.4-0.6mm ultra-thin multi-layer board is used for plate surface plating and pattern plating, special nip rolls should be made. After the thin plate is clamped on the flying plate of the automatic plating line, the clip on the whole flying bar is used with a round stick. The rollers are strung together to straighten all the plates on the roll so that the plate after plating does not deform. Without this measure, after plating a copper layer of twenty or thirty microns, the sheet would bend and be difficult to remedy.

 

 

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