PCB manufacturers tell you how to prevent PCB board warpage?
One aspect of the cause of circuit board warpage is that the substrate (copper clad laminate) used may warp, but in the process of printed circuit board processing, thermal stress, chemical factors, and improper production processes can also cause printed circuits The board warped. Therefore, for the printed circuit board factory, the first thing is to prevent the printed circuit board from warping during the processing; and then there must be a suitable and effective treatment method for the warped PCB board.
Prevent printed circuit board from warping during processing
1. Prevent or increase substrate warpage due to improper inventory method
(1)Since the copper clad laminate is in the storage process, because moisture absorption will increase the warpage, the moisture absorption area of the single-sided copper clad laminate is large. If the inventory environment humidity is high, the single-sided copper clad laminate will significantly increase the warpage. The moisture of double-sided copper clad laminate can only penetrate from the end surface of the product, the moisture absorption area is small, and the warpage changes slowly. Therefore, for the copper clad laminates without moisture-proof packaging, pay attention to the warehouse conditions, try to reduce the humidity in the warehouse and avoid the exposed copper clad laminates, so as to avoid the copper clad laminates in storage from increasing warping.
(2)Improper placement of copper clad laminates will increase warpage. Such as vertical placement or heavy objects on the copper clad laminate, poor placement, etc. will increase the warpage of the copper clad laminate.
2. Avoid warping caused by improper printed circuit board circuit design or improper processing technology
For example, the conductive circuit pattern of the PCB board is not balanced or the circuit on both sides of the PCB board is obviously asymmetrical, and there is a large area of copper on one side, which forms a large stress, which causes the PCB board to warp, and the processing temperature is high or hot in the PCB process. Impact, etc. will cause the PCB board to warp. For the impact caused by improper storage method of the laminated board, the PCB factory is better to solve it, and it is enough to improve the storage environment and eliminate vertical placement and avoid heavy pressure. For PCB boards with a large area of copper in the circuit pattern, it is best to mesh the copper foil to reduce stress.
3. Eliminate substrate stress and reduce PCB board warpage during processing
In the process of PCB processing, the substrate has to be subjected to heat and many chemical substances. For example, after the substrate is etched, it needs to be washed with water, dried and heated. Electroplating is hot during pattern electroplating. After printing green oil and marking characters, it must be heated or dried with UV light. Thermal shock to the substrate when hot air is sprayed. It's also big, etc. These processes may cause the PCB board to warp.
4. When wave soldering or dip soldering, the solder temperature is too high and the operation time is too long, which will increase the warpage of the substrate. For the improvement of the wave soldering process, the electronic assembly factory needs to cooperate.
Since stress is the main cause of substrate warpage, if the copper clad laminate (also called h board) is baked before the copper clad laminate is put into use, many PCB manufacturers believe that this approach will help reduce the warpage of the PCB board. The function of the baking sheet is to fully relax the stress of the substrate, thereby reducing the warpage of the substrate during the PCB manufacturing process.
The method of h board is: conditional PCB factories use large oven h board. Put a large stack of copper clad laminates into the oven before production, and bake the copper clad laminates for several to ten hours at a temperature near the glass transition temperature of the substrate. The warpage deformation of the PCB board produced by the copper clad laminate of the h board 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 into small pieces and then bake, but there should be a heavy object to press the plate when baking the plate, so that the substrate can remain flat during the stress relaxation process. The temperature of the baking plate should not be too high, because the substrate will change color if the temperature is too high. It should not be too low, and it takes a long time for the temperature to be too low to relax the substrate stress.
Printed circuit board warpage leveling method.
1. Flatten the warped board in time during the PCB manufacturing process
In the PCB manufacturing process, pick out the board with relatively large warpage and level it with a roller leveler, and then put it into the next process. Many PCB manufacturers believe that this approach is effective in reducing the warpage ratio of PCB finished boards.
2. PCB finished board warping leveling method
For PCBs that have been completed, the warpage is obviously out of tolerance and cannot be leveled with a roller leveling machine. Some PCB factories put it in a small press (or similar fixture) to press the warped PCB. Live for a few hours to more than ten hours for cold pressing and leveling. Observed from practical applications, the effect of this practice is not very obvious. One is that the leveling effect is not great, and the other is that the leveled board is easy to rebound (that is, the warpage is restored).
There are also PCB factories that heat the small press to a certain temperature, and then hot-press and level the warped PCB. The effect is better than cold pressing, but if the pressure is too large, the wire will be deformed; if the temperature is too high Defects such as discoloration of pine perfume to base discoloration will occur. Moreover, it takes a long time (several hours to ten hours) to see the effect whether it is cold pressing leveling or hot pressing leveling, and the proportion of warpage rebound of the leveled PCB board is also high. Is there a better leveling method?
3. Hot pressing and leveling method for warped PCB board arch mold According to the mechanical properties of polymer materials and years of work practice, this article recommends the arc-shaped mold hot-pressing and leveling method. According to the area of the PCB board to be leveled, a number of very simple bow molds are made. Here are two leveling operation methods.
(1) Clamp the warped PCB board into a bow-shaped mold and put it into an oven to bake and level:
Place the warped PCB board's warped surface against the mold bow curve, adjust the clamp screw to slightly deform the PCB board in the opposite direction of its warpage, and then put the mold with the PCB board in an oven heated to a certain temperature for baking Bake for a while. Under heated conditions, the substrate stress gradually relaxes, so that the deformed PCB board is restored to a flat state. However, the baking temperature should not be too high to avoid discoloration of rosin perfume or yellowing of substrate. But the temperature should not be too low, it takes a long time to fully relax the stress at a lower temperature.
Generally, the glass transition temperature of the substrate can be used as the reference temperature for baking. The glass transition temperature is the phase transition point of the resin. At this temperature, the polymer chain segments can be rearranged and oriented to fully relax the substrate stress. Because the leveling effect is obvious. The advantage of flattening with bow-shaped molds is that there is very little investment. Every PCB factory has ovens. The flattening operation is very simple. If the number of warped boards is relatively large, you can make a few more bow-shaped molds. You can put them in the oven at one time. Several molds, and the drying time is relatively short (about tens of minutes), so the leveling efficiency is relatively high.
(2) The PCB board is first baked and then clamped into the arch mold to press and flatten:
For the PCB board with relatively small warpage deformation, you can first put the PCB board to be leveled into an oven that has been heated to a certain temperature (the temperature setting can refer to the glass transition temperature of the substrate and the substrate is baked in the oven for a certain period of time. , Observe its softening condition to determine. Generally, 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 the thick plate can be slightly higher, and the baking temperature of the thin plate can be slightly lower Some; for those already sprayed with loose perfume.
The baking temperature of the PCB board should not be too high. ) Bake for a certain period of time, then take out a few to a dozen sheets, clamp them into the arch mold, adjust the pressure screw to make the PCB board slightly deform in the opposite direction of its warpage, and after the board is cooled and shaped, the mold can be removed , Take out the leveled PCB board.
Some users do not know the glass transition temperature of the substrate very well. The recommended baking reference temperature is 110℃~130℃ for the paper substrate, and 130℃~150℃ for FR-4. When leveling, do several small tests on the selected baking temperature and baking time to determine the baking temperature and baking time for leveling. The baking time is longer, the substrate is baked thoroughly, the leveling effect is better, and the PCB board warpage rebound is less after leveling.
The PCB board that has been leveled by the bow-shaped mold has a low warpage rebound rate; even after wave soldering, it can still remain basically flat; it has little effect on the appearance and color of the PCB board.
PCB board warping is an extremely headache for PCB factories. It not only reduces the yield, but also affects the delivery time. If the bow-shaped mold is used for thermal leveling, and the leveling process is reasonable and appropriate, the warped PCB board can be leveled and the problem of delivery time can be solved.