Printed Circuit boards (PCBs) are becoming more and more sophisticated, moving in the direction of smaller and thinner. But sometimes we encounter the problem of PCB bend (or board warping), although the slight bending can also affect the function of PCBA.

According to the “Specification and Performance Specification for Rigid Printed Boards”, the maximum allowable warping and bend SMT PCB boards is 0.75%. At present, the warping degree permitted by various electronic assembly factories, whether double-sided or multi-layered, with a thickness of 1.6 mm, is usually 0.70 to 0.75%. Many SMT and BGA boards require 0.5%. Some electronics factories are agitating to raise the standard for warping to 0.3%.

Test Method

Put the printed circuit board on the verified platform, insert the test pin to the place with the highest warping, and calculate the warping of the printed board by dividing the diameter of the test pin by the length of the curved side of the printed board Curvature.

Why does the PCB circuit board need very flat?

On the automated SMT production line, if the printed circuit board is uneven, it will cause inaccurate positioning, and components cannot be inserted into the holes and pads of the board, and will even damage the automatic insertion machine. The PCB bent will cause the mounted component bent after welding, and the component pins are difficult to cut flat and neat. The warped PCB cannot be mounted on the chassis or the socket in the machine, so it is also very troublesome for the assembly line to encounter the bent PCB. At present, printed boards have entered the era of surface mounting and chip mounting, and the requirements of assembly factories for board warping must be stricter.

How to prevent the PCB from Bending?

Judging from our 18 years of experience in the production and placement of circuit boards, we summarize some experiences and reasons about how to prevent PCB bending , and share them with you here.

Use High TG material

Generally, the TG is more than 130 degrees, the high TG is generally greater than 170 degrees, and the medium TG is greater than about 150 degrees.

Improve the TG of the substrate the heat resistance, humidity resistance, chemical resistance, and stability characteristics of the circuit board are improved. The higher the TG value, the better the temperature resistance of the plate, especially in the lead-free process, there are more high TG applications. The high TG value ensures that the PCB is not susceptible to changes and deformation of the external environment to a certain extent, that means it can prevent the PCB board from bending(warping).

TG PCB

Board thickness

Thicker boards will be stiffer and easier to keep flat. When mounting many components on the PCB boards, which need reflow soldering. When the temperature and load change, the thick substrate is not easy to be warped. If keep the thickness at 1.6mm, which can significantly reduce the probability of deformation of the board.

Evenly Distributed Copper

Printed Circuit board assembly is usually carried out in the form of panel. After panelized, if copper is also distributed in the board edge and some connected places (positions to be took off after assembling), this need maintain the overall PCB panel copper thickness and density are the same, then plate warping is not easy to occur after assembling.

Panelize way

When designing the panel, we must consider the size, not too long, and consider making the shape stable and uniform. If the panel is too long, it will collapse in the middle.

Reduce Reflow Temperature

When the force on the PCB board is uneven or excessive, the board will be warped. Where does the stress of the board from? In reflow soldering, the greatest stress comes from temperature. Temperature not only softens the circuit board, but also distorts the circuit board, as well as physical factors that cause thermal expansion of all materials. Therefore, it is regarded as the main cause of PCB board warping.

Modify design

Multilayer circuit boards will have many vias, blind holes, blind buried holes, etc. These fixed nodes limit the changes that the circuit board receives under pressure or high temperature, and also promote the occurrence of board warpage. Therefore, some design engineers will mark some slots in the place where the substrate does not have copper wiring, which can facilitate the heat dissipation of the circuit board.

Reflow carrier

The reflow carrier can reduce the PCB bending and warping, because it can hold the printed circuit board regardless of whether it will be expanded by heat. When the reflow soldering is out, the circuit board recovers the temperature and hardness. This process is limited in the reflow carrier and will not occur variety. We can also make fixtures to help fix, this requires manual pallet replacement.

Reflow carrier for pcb

Do router not by V-cut

If we make the panel by V-cut, the overall structure of the PCB board will become weaker, and board bending and board warping will occur easily. So if allowed in production, we can use the routing instead of V-cut.

Warp and weft direction of prepreg

After laminating the prepreg, the shrinkage rates in the warp and weft directions are not the same. You must distinguish between the warp and weft directions when cutting and laminating. Otherwise, it may cause warping of the finished board after lamination, and it is difficult to correct the board even if apply pressure. Many reasons for the bending of the multilayer PCB board are caused by the latitude and longitude of the prepregs being not distinguished and stacked randomly during lamination.

How to distinguish warp and weft? The rolled prepreg is rolled in the warp direction and the width direction is the weft direction. For copper foil, the long side is the zonal direction and the short side is the warp direction. If you are not sure, you can check with the manufacturer or supplier.

The semi-solidified Symmetrical

The semi-solidified should be symmetrical, for example a 6 layer PCB board, the semi-solidified sheets should be in 1-2 and 5-6 layer. Multilayer cores and prepregs should use products from the same supplier. The area of the line patterns on the A and B sides of the outer layer should be as close as possible. If the A surface is a large copper surface and the B surface is only a few lines away, this printed board will easily warp after etching. If the area of the lines on the two sides differs too much, you can add some independent grids on the rare side to balance.

Baking board before production

In order to remove the moisture that may enter during long-term storage of the board, dry the board before cutting the copper-clad board (150 degrees Celsius, time 8 ± 2 hours). And at the same time completely cure the resin in the board, further eliminating the remaining stress in the board, which prevents the board warping is helpful.

At present, many double-sided and multi-layer boards still adhere to the step of baking before or after blanking. However, there are exceptions for some plate factories. At present, the time requirements for drying boards in various PCB factories are also inconsistent, ranging from 4-10 hours. It is recommended to determine according to the grade of the printed board produced and the customer’s requirements for warpage. After cutting into panels, baking or cutting the whole block, both methods are feasible. Dry the boards after cutting and bake the inner board.

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Elva
ElvaSales Engineer
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