The cause of the board warping, one aspect is that the substrate used (coppered plate) may warp, but in the printed circuit board processing process, because of thermal stress, chemical factors, and improper production process will also cause the printed circuit board warping. Therefore, for the printed circuit board factory, the first thing is to prevent the printed circuit board from warping during processing; Then there should be a suitable and effective treatment method for the PCB board that has warped.
First. prevent the printed circuit board from warping during processing
1. to prevent improper inventory caused or increased substrate warping
1.1. As the copper clad sheet will increase warping due to hygroscopicity during storage, the hygroscopicity area of the single-sided copper clad sheet is very large. If the ambient humidity of the inventory is high, the warping of the single-sided copper clad sheet will be significantly increased. The moisture of the double-sided copper clad plate can only penetrate from the end face of the product, the moisture absorption area is small, and the warpage change is slow. Therefore, for the copper clad plate without moisture-proof packaging, pay attention to the warehouse conditions, minimize the humidity of the warehouse and avoid the bare copper clad plate to avoid the increased warping of the copper clad plate in storage.
1.2. Improper placement of copper clad plate will increase warping. If there is heavy weight on the vertical or copper clad plate, the improper placement will increase the warping deformation of the copper clad plate.
2. Avoid warping due to improper design of printed circuit board circuit or improper processing process
If the PCB board conductive circuit pattern is unbalanced or the PCB board two sides of the line is obviously asymmetric, one side has a large area of copper, forming a large stress, so that the PCB board warping, in the PCB process processing temperature is high or large thermal shock will cause the PCB board warping. For the impact caused by the improper inventory method of the cladding board, the PCB factory is better to solve, improve the storage environment and eliminate vertical placement, avoid heavy pressure. For PCB boards with large areas of copper skin in the line pattern, it is best to grid the copper foil to reduce stress.
3. Eliminate substrate stress and reduce PCB warping during processing
Because in the PCB processing process, the substrate must be subjected to heat for many times and be subjected to a variety of chemical substances. For example, the substrate should be washed after etching, dried and heated, the electroplating of graphics is hot, the printing of green oil and the printing of identification characters should be heated and dried or baked with UV light, and the thermal impact of the substrate is also very large when the hot air is sprayed tin. These processes can cause the PCB to warp.
4. wave soldering or dip welding, the solder temperature is high, the operation time is long, will also increase the substrate warping. For the improvement of wave soldering process, the electronic assembly plant needs to cooperate
Since stress is the main cause of substrate warping, if the copper clad plate is baked before it is put into use (also known as the h plate), many PCB factories believe that this practice is conducive to reducing the warping of the PCB board. The role of the drying board is to relax the stress of the substrate fully, which can reduce the warping deformation of the substrate in the PCB process.
The method of h board is: conditional PCB factory is to use a large oven h board. Before putting into operation, a large stack of copper clad plates is sent into the oven, and the copper clad plates are baked for several hours to ten hours at a temperature near the glass transition temperature of the substrate. PCB board produced by copper clad plate through h plate, its warping deformation is relatively small, and the qualified rate of the product is much higher. For some small PCB factories, if there is not such a large oven, the substrate can be cut small and then baked, but there should be heavy weights to press the board when baking the board, so that the substrate can maintain a flat state during the stress relaxation process. The temperature of the drying plate should not be too high, and the substrate will change color if the temperature is too high. It should not be too low, too low temperature takes a long time to relax the stress of the substrate.
Second. the printed circuit board warping method
1. Straighten the warped board in time in the PCB process
In the PCB process, the board with a relatively large warpage is picked out and levelled with a roller flattening machine, and then put into the next process. Many PCB manufacturers believe that this practice is effective for reducing the warpage proportion of PCB finished boards.
2. PCB finished board warping and leveling method
For the PCB board that has been completed, the warpage degree is obviously out of tolerance, and the roller flattening machine can not be levelled, some PCB factories put it into a small press (or similar fixture), and press the warped PCB board for a few hours to ten hours for cold pressing and leveling, from the practical application observation, the effect of this practice is not very obvious. One is that the effect of leveling is not large, and the other is that the board after leveling is easy to rebound (that is, restore warping).
Some PCB factories will heat the small press to a certain temperature, and then the warped PCB board for hot pressing, the effect is better than cold pressing, but if the pressure is too large, the wire deformation; If the temperature is too high, it will produce defects such as rosin discoloration to base discoloration. And whether it is cold pressing or hot pressing, it takes a long time (several hours to more than ten hours) to see the effect, and the proportion of warping rebound of the PCB board after leveling is also higher. Is there a better way to flatten?
3. Hot pressing and flattening method of warped PCB bow die
According to the mechanical properties of polymer materials and many years of work practice, this paper recommends the hot pressing and flattening method of bow die. According to the area of the PCB board to be levelled, a number of very simple bow molds are made, and two leveling operation methods are pushed here.
3.1. Clamp the warped PCB board into the bow mold and put it into the oven for baking and leveling:
The warping of the PCB board faces the curved surface of the die bow, adjust the fixture screws, so that the PCB board slightly deforms in the opposite direction of its warping, and then put the mold with the PCB board into the oven that has been heated to a certain temperature to bake for some time. Under the heating condition, the stress of the substrate gradually relaxes, so that the deformed PCB board returns to a flat state. However, the baking temperature should not be too high, so as not to discolor the rosin or turn the substrate yellow. However, the temperature should not be too low, and it takes a long time to completely relax the stress at a lower temperature.
The glass transition temperature of the substrate is usually used as the reference temperature for baking, and the glass transition temperature is the phase transition point of the resin. At this temperature, the polymer chain segment can be rearranged and the stress of the substrate can be fully relaxed. Because some leveling effect is very obvious. The advantage of leveling with the bow mold is that the investment is very little, the oven PCB factory has, the leveling operation is very simple, if the number of warped plates is more, more pay the bow mold can be done, you can put several pay the mold in the oven at a time, and the drying time is relatively short (tens of minutes or so), so the leveling work efficiency is relatively high.
3.2. The PCB board is first baked and then clamped into the bow mold for middle pressing and leveling method:
For PCB boards with relatively small warping deformation, the PCB board to be levelled can be first placed in the oven that has been heated to a certain temperature (the temperature setting can be determined by referring to the glass transition temperature of the substrate and the softening of the substrate after baking for a certain time in the oven. Usually, the baking temperature of the glass fiber cloth substrate is higher, and the baking temperature of the paper substrate can be lower; The baking temperature of thick plate can be slightly higher, and the baking temperature of thin plate can be slightly lower; The baking temperature of the PCB board that has been sprayed with rosin water should not be too high. Bake for a certain time, and then take out a few to a dozen pieces, clip into the bow mold, adjust the pressure screw, so that the PCB board slightly deformation in the opposite direction of its warping, after the board is cooled, you can unplug the mold, take out the flattened PCB board.
Some users do not know much about the glass transition temperature of the substrate. The recommended baking reference temperature is 110 ° C ~130 ° C for the paper substrate, and 130 ° C ~150 ° C for FR-4. During the whole day, the selected baking temperature and baking time are tested several times to determine the leveling baking temperature and baking time. The baking time is longer, the substrate is thoroughly baked, the leveling effect is better, and the warping rebound of the PCB board is also less after leveling. The PCB board that has been levelled by the bow mold has low warpage rebound rate; Even after wave soldering can still basically maintain a flat state; The effect on the appearance of PCB board color is also very small.
PCB board warping is a very headache for PCB factories, which not only reduces the yield, but also affects the delivery time. If the hot leveling of the bow mold is used, and the leveling process is reasonable and appropriate, the warped PCB board can be leveled and the delivery date can be solved.
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