PCB Blog - Prevention and leveling method of PCB board warpage

People in the industry are well aware of the impact of PCB board warpage. If it makes it impossible to install SMT electronic components, or the electronic components (including integrated blocks) have poor contact with the solder joints of the printed circuit board, or when the electronic components are installed, some feet cannot be cut or will be cut to the substrate; In some parts of the substrate, the pads cannot contact the solder surface and cannot be soldered, etc.; one aspect of the cause of warpage of the printed circuit board is that the substrate (copper clad laminate) used may be warped, but during the processing of the printed circuit board, Because of thermal stress, chemical factors, and improper production process, the printed circuit board will also warp. Therefore, for the printed circuit board factory, the first is to prevent the printed circuit board from warping during the processing; the second is to have a suitable and effective treatment method for the warped PCB board.

PCB
1. Prevent the printed circuit board from warping during processing
1.1 Prevent or increase substrate warpage due to improper inventory methods
(1) Due to the moisture absorption of the CCL during the storage process, the warpage will be increased, and the moisture absorption area of the single-sided CCL is large. If the inventory environment humidity is high, the single-sided CCL will significantly increase the warpage. The moisture of the double-sided copper clad laminate 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 CCL without moisture-proof packaging, pay attention to the warehouse conditions, minimize the humidity in the warehouse and avoid the bare copper clad laminate, so as to avoid the increased warpage of the copper clad laminate in storage.
(2) Improper placement of CCL will increase warpage. Such as vertical placement or heavy objects on the CCL, poor placement, etc., will increase the warpage and deformation of the CCL.

Soldered PCBA

1.2 Avoid warpage caused by improper circuit design or improper processing technology of printed circuit board.
For example, the conductive circuit pattern of the PCB board is unbalanced or the lines on both sides of the PCB board are obviously asymmetrical, and there is a large area of copper skin on one side, which forms a large stress and causes the PCB board to warp, and the processing temperature in the PCB process is high or large heat Impact, etc. will cause the PCB board to warp. For the impact caused by the improper inventory method of the cladding board, it is better for the PCB factory to solve it. It is enough to improve the storage environment and prevent vertical placement and avoid heavy pressure. For PCB boards with a large area of copper in the circuit pattern, the copper foil is meshed to reduce stress.

1.3 Eliminate substrate stress and reduce PCB board warpage during processing
Because in the process of PCB processing, the substrate is subjected to the action of heat many times and the action of various chemical substances. For example, after the substrate is etched, it needs to be washed with water, and it needs to be dried to be heated. When the pattern is electroplated, the electroplating is hot. After printing green oil and printing logo characters, it should be dried by heating or drying with UV light. Also big and so on. These processes may warp the PCB.

1.4 During wave soldering or dip soldering, the solder temperature is too high and the operation time is too long, which will also 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 is put into use, the board (also called baked board) is first baked. Many PCB manufacturers believe that this approach is conducive to reducing the warpage of the PCB board. The function of the baking board is to fully relax the stress of the substrate, thus reducing the warpage and deformation of the substrate during the PCB manufacturing process. The method of baking the board is: the conditional PCB factory uses a large oven to bake the board. Before putting into production, a large stack of copper clad laminates is sent into the oven, and the copper clad laminates are baked for several hours to ten hours at a temperature near the glass transition temperature of the substrate. The warpage deformation of the PCB board produced by the baked copper clad laminate 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 baked, but when baking the board, there should be a heavy object to press the board, so that the substrate can be kept flat during the stress relaxation process. The temperature of the baking board should not be too high, the substrate will change color if the temperature is too high. It should not be too low, as it will take a long time to relax the stress of the substrate if the temperature is too low.

2. Warpage and leveling method of printed circuit board
2.1 Flatten the warped board in time during the PCB manufacturing process
In the PCB manufacturing process, the board with relatively large warpage is picked out and leveled with a roller leveling machine, and then put into the next process. Many PCB manufacturers believe that this approach is effective in reducing the warpage ratio of the finished PCB board.

2.2 Warpage and leveling method of PCB finished board
For PCB boards that have been completed and the warpage is obviously out of tolerance and cannot be leveled by a roller leveling machine, some PCB factories put it into a small press (or similar fixture) to press the warped PCB board. After a few hours to ten hours for cold-pressing and leveling, the effect of this method is not very obvious from practical application. One is that the leveling effect is not great, and the other is that the flattened board is easy to bounce back (that is, to recover warpage). There are also PCB manufacturer that heat the small press to a certain temperature, and then hot-press and level the warped PCB board. The effect is better than cold pressing, but if the pressure is too high, the wire will be deformed; There will be defects such as discoloration of rosin, discoloration of the base, and so on. And whether it is cold-pressed or hot-pressed, it takes a long time (several hours to more than ten hours) to see the effect, and the rate of warpage and rebound of the leveled PCB board is also high.

2.3 Hot pressing and leveling method of bow-shaped mold for warped PCB board
According to the mechanical properties of polymer materials and years of work practice, this paper recommends the hot-pressing leveling method for bow-shaped molds. According to the area of the PCB board to be leveled, make several very simple bow-shaped molds. Here are two leveling operation methods:

(1) Clamp the warped PCB board into the bow-shaped mold and put it into the oven to bake the leveling method:
Place the warped surface of the warped PCB board against the curved surface of the mold bow, adjust the clamp screw, so that the PCB board is slightly deformed in the opposite direction of its warpage, and then put the mold with the PCB board into the oven that has been heated to a certain temperature. Bake for a while. Under the heating condition, the stress of the substrate gradually relaxes, so that the deformed PCB board returns to a flat state. But the baking temperature should not be too high to avoid discoloration of rosin or substrate yellowing. However, the temperature should not be too low. It takes a long time to completely relax the stress at a lower temperature. Usually, the glass transition temperature of the substrate can be 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 segments can be rearranged and oriented, so that the substrate stress can be fully relaxed. Due to some leveling effect is obvious. The advantage of using a bow-shaped mold for leveling is that the investment is very small. Every PCB factory in the oven has it. The leveling operation is very simple. If the number of warped boards is relatively large, it is enough to make several more bow-shaped molds. The mold is attached, and the baking time is relatively short (about tens of minutes), so the leveling work efficiency is relatively high.

(2) First soften the PCB board and then clamp it into the bow-shaped mold for pressing and leveling:
For PCB boards with relatively small warpage deformation, the PCB board to be leveled can be placed in an oven that has been heated to a certain temperature (the temperature setting can refer to the glass transition temperature of the substrate and after the substrate is baked in the oven for a certain period of time), observe its softening condition to determine. Usually the baking temperature of glass fiber cloth substrate is higher, the baking temperature of 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 Some; the baking temperature should not be too high for the PCB board that has been sprayed with rosin.) Bake for a certain period of time, then take out several to a dozen sheets, clip them into the bow-shaped mold, and adjust the pressure screw to make the PCB board slightly The warp is deformed in the opposite direction. After the board is cooled and shaped, the mold can be removed and the flattened PCB board can be taken out. Some users do not know much about the glass transition temperature of the substrate. The reference baking temperature is recommended here. The baking temperature of the paper substrate is 110 degree Celsius~130 degree Celsius, and the FR-4 is 130 degree Celsius~150 degree Celsius. When leveling, make several small tests on the selected baking temperature and baking time 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 PCB board is less warped and rebounded after leveling. The PCB board that has been leveled by the bow-shaped mold has a low warpage rebound rate; it can basically remain flat even after wave soldering; it has little effect on the appearance and color of the PCB board. Warpage of the PCB board is a great headache for the PCB factory. 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 suitable, the warped PCB board can be leveled and the problem of delivery time can be solved.