Circuit board warpage on the production of printed circuit boards is very big, warpage is also one of the important issues in the process of circuit board production, loaded with components of the board after welding bending, component feet is difficult to neatly.
The board also can not be mounted to the chassis or the machine socket, so the board warpage will affect the normal operation of the entire post-processing process.
At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the process can be more and more demanding on the board warpage. So we have to find the cause of circuit board warpage.
Printed circuit board design should pay attention to.
A. The arrangement of inter-layer semi-curing sheet should be symmetrical, such as six-layer board, 1 to 2 and 5 to 6 layers between the thickness and the number of sheets of semi-curing sheet should be consistent, otherwise the lamination is easy to warp.
B. Multi-layer core board and semi-curing sheet should use the same supplier's products.
C. The area of the line graphics on the outer A and B sides should be as close as possible. If side A is a large copper surface, and side B only a few lines, this printed board is easy to warp after etching. If the difference between the two sides of the line area is too large, you can add some independent grid in the sparse side for balance.
Laminate baking before feeding (150 degrees Celsius, time 8 ± 2 hours) is to remove the moisture inside the board, while making the resin inside the board completely cured, to further eliminate the remaining stress in the board, which is helpful to prevent board warpage.
At present, many double-sided, multi-layer board still adhere to the baking step before or after the material. However, there are some exceptions to the board factory, the current PCB factory baking time regulations are not consistent, from 4-10 hours have, it is recommended that according to the production of the grade of the printed circuit board and customer requirements for warpage to decide.
Bake after cutting into a patchwork or bake after the whole piece of material, both methods are feasible, it is recommended to cut the material after baking the board. The inner laminate should also be baked.
Semi-curing sheet lamination after the warp and weft shrinkage rate is not the same, the material and iterative layer must be divided into the warp and weft. Otherwise, the lamination is easy to cause the finished board warpage, even if the pressure baking board is also very difficult to correct.
Many of the reasons for warpage of multi-layer board are that the warp and weft direction of the semi-cured sheet is not clearly distinguished when laminating, and it is caused by iteration.
How to distinguish between warp and weft? The direction of the rolled semi-curing sheet is warp direction, while the width direction is weft direction; for copper foil sheet, the long side is weft direction and the short side is warp direction.
After the laminate is hot pressed and cold pressed, it is removed, the raw edges are cut or milled off, and then placed flat in an oven at 150 degrees Celsius for 4 hours to allow the stress in the laminate to be gradually released and the resin to be completely cured.
After the thin board is clamped on the flybar on the automatic plating line, a round stick is used to string up all the rollers on the flybar to straighten all the boards on the rollers so that the boards will not be deformed after plating.
Without this measure, the thin board will be bent and difficult to remedy after plating 20 or 30 microns of copper layer.
Printed board hot air leveling by soldering tank (about 250 degrees Celsius) of high-temperature impact, after removal should be placed on a flat marble or steel plate natural cooling, after sending to the post-processor for cleaning. This is good for the board to prevent warping.
Some factories to enhance the brightness of the lead tin surface, the board hot air leveling immediately into the cold water, a few seconds after the removal and then post-processing, such a hot and cold impact on some models of the board is likely to produce warpage, delamination or blistering.
In addition, the equipment can be equipped with air floatation bed for cooling.
In a well-managed factory, the printed board will be checked for 100% flatness during final inspection. All unqualified boards will be picked out and put into the oven, baked at 150 degrees Celsius and under heavy pressure for 3 to 6 hours, and cooled naturally under heavy pressure.
Then remove the pressure and take out the board for flatness check, which can save some boards, and some boards need two to three times of baking and pressing to be leveled. If the above anti-warping process measures are not implemented, part of the board baking pressure is also useless, can only be scrapped.
