PCB Tech - Causes and prevention of PCB warpage

Cause analysis of PCB warpage

(1) The weight of the PCB itself will cause the PCB to sink warpage

Generally, the reflow furnace will use a chain to drive the PCB forward in the reflow furnace, that is, the two sides of the PCB are used as fulcrums to support the entire PCB. If there are heavy parts on the PCB or the size of the PCB is too large, Due to the amount of seed, it will show a depression in the middle, causing the plate to bend.

(2) The depth and connection of V-Cut will affect the warpage of the puzzle

Basically, V-Cut is the culprit that destroys the PCB structure, because V-Cut cuts grooves in the original large sheet, so warpage is prone to occur in V-Cut.

(3) Analysis of the effect of pressing materials, structure and graphics on the warpage of the panel

PCB is laminated by core board, prepreg and outer copper foil. The core board and copper foil are heated warpage when they are pressed together. The amount of warpage depends on the coefficient of thermal expansion (CTE) of the two materials;

The coefficient of thermal expansion (CTE) of copper foil is about 17X10-6;

The Z-direction CTE of ordinary FR-4 substrate at Tg point is (50~70)X10-6;

Above the TG point is (250~350) X10-6, and the X-direction CTE is generally similar to copper foil due to the presence of glass cloth.

PCB warpage

(4) Warpage caused by PCB processing

The cause of warpage in PCB processing is very complicated and can be divided into thermal stress and mechanical stress. Among them, the thermal stress is mainly generated during the pressing process, and the mechanical stress is mainly generated during the stacking, handling and baking of the plates. The following is a brief discussion in the order of the process.

Incoming copper clad laminates: copper clad laminates are all double-sided PCBs with symmetrical structure and no graphics. The CTE of copper foil and glass cloth is almost the same, so there is almost no warpage caused by the difference in CTE during the pressing process. However, the size of the copper clad laminate press is large, and there are temperature differences in different areas of the hot plate, which will cause slight differences in the curing speed and degree of the resin in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so it will also produce Local stress due to differences in curing process. Generally, this stress will maintain balance after pressing, but will gradually release in the future processing to produce warpage.

Pressing: The PCB pressing process is the main process that generates thermal stress. The warpage caused by different materials or structures is analyzed in the previous section. Similar to the pressing of copper clad laminates, local stresses caused by differences in the curing process will also occur. Due to the thicker thickness, diverse pattern distribution, and more prepregs, PCBs have more thermal stress than copper clad laminates and are more difficult to eliminate. The stress in the PCB is released during subsequent drilling, shape, or grilling processes, resulting in warpage of the board.

Baking process of solder mask, characters, etc.: Since solder mask inks cannot be stacked on each other when they are cured, PCBs will be placed on a shelf to bake the board to cure. The solder mask temperature is about 150°C, which just exceeds the Tg point and Tg point of medium and low Tg materials. The above resin is in a highly elastic state, and the board is prone to PCB warpage under its own weight or strong wind in the oven.

Hot-air solder leveling: When ordinary PCB hot-air solder leveling, the temperature of the tin furnace is 225 degree Celsius~265 degree Celsius, and the time is 3S-6S. The temperature of hot air is 280 degree Celsius~300 degree Celsius. When the solder is leveled, the board is put into the tin furnace from room temperature, and the post-treatment water washing at room temperature is carried out within two minutes after being out of the furnace. The entire hot-air solder leveling process is a sudden heating and cooling process. Due to different PCB materials and uneven structure, thermal stress will inevitably occur during the cooling and heating process, leading to microscopic strain and overall warpage area.

Storage: The storage of PCB in the semi-finished product stage is generally firmly inserted in the shelf, the shelf tightness adjustment is not appropriate, or the stacking of the boards during the storage process will cause the board to produce mechanical warpage. Especially for thin plates below 2.0mm, the impact is more serious.

In addition to the above factors, there are many factors that affect the PCB warpage.

Prevention of PCB warpage

PCB warpage has a great influence on the production of printed PCB. Warpage is also one of the important problems in the PCB production process. Warpage occurs after the PCB with components is soldered, and the component feet are difficult to be neat. The PCB cannot be installed on the chassis or the socket inside the machine, so PCB warpage will affect the normal operation of the entire subsequent process. At this stage, printed PCB has entered the era of surface mounting and chip mounting, and the process requirements for PCB warpage are getting higher and higher. So we have to find the reason for the warping of the halfway help.

1. Engineering design: Matters needing attention when designing printed boards: A. The arrangement of the prepregs between layers should be symmetrical, for example, for six-layer boards, the thickness between 1-2 and 5-6 layers and the number of prepregs should be the same, otherwise the layers It is easy to warp after pressing. B. Multilayer core board and prepreg should use the same supplier's products. C. The area of the circuit pattern on side A and side B 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, this kind of printed board is easy to warp after etching. If the area of the lines on the two sides is too different, you can add some independent grids on the thin side for balance.

2. Drying board before cutting: The purpose of drying the board before cutting the copper clad laminate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time make the resin in the board completely solidify, and further eliminate the remaining stress in the board. This is helpful to prevent the board from warping. At present, many double-sided and multi-layer boards still adhere to the step of baking before or after cutting. But there are some exceptions to the board factories. The current PCB drying time regulations are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. After cutting into a piece of panel, bake it or unload the whole block after baking. Both methods are feasible. It is recommended to bake the panel after cutting. The inner board should also be baked.

3. Warp and weft direction of the prepreg: After the prepreg is laminated, the warp and weft shrinkage rates are different, and the warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause the finished board to warp after lamination, and it is difficult to correct it even if pressure is applied to the baking board. Many reasons for the warpage of the multilayer board are that the warp and weft directions of the prepregs are not distinguished during lamination and are stacked randomly. How to distinguish the latitude and longitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the weft direction; for the copper foil board, the long side is the weft direction, and the short side is the warp direction. If you are not sure, please check with the manufacturer or supplier.

4. Relief stress after lamination: take out the multi-layer board after hot pressing and cold pressing, cut or mill off the burrs, and then put it flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and make the resin complete Curing, this step cannot be omitted.

5. The thin plate needs to be straightened when electroplating: 0.4～0.6mm ultra-thin multi-layer plates should be made of special nip rollers for surface electroplating and pattern electroplating. After the thin plate is clamped on the fly bus on the automatic electroplating line, a circle The sticks string together the nip rollers on the entire fly bus, thereby straightening all the PCBs on the rollers, so that the electroplated PCBs will not warpage. Without this measure, after plating a copper layer of 20 to 30 microns, the sheet will warpage and it is difficult to remedy it.

6. PCB cooling after hot air leveling: the printed board is impacted by the high temperature of the solder bath (about 250 degrees Celsius) during hot air leveling. After taking it out, it should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning . This is good for PCB warpage prevention. In some factories, in order to enhance the brightness of the lead-tin surface, the PCB is immediately put into cold water after the hot air is leveled, and then taken out after a few seconds for post-processing. This kind of hot and cold impact may cause warping on some types of PCB. Qu, layering or blistering. In addition, an air flotation bed can be installed on the equipment for cooling.

7. Treatment of warped PCB: In a well-managed factory, the printed board will be 100% flatness checked during the final inspection. All unqualified PCBs will be picked out, put in an oven, baked at 150 degrees Celsius under heavy pressure for 3 to 6 hours, and cooled naturally under heavy pressure. Then remove the PCB from the pressure and check the flatness, so that part of the PCB can be saved, and some PCBs need to be bake and pressed two to three times to be leveled. Shanghai Huabao's pneumatic board warping and straightening machine has been used by Shanghai Bell to remedy the warpage of the circuit board. If the above-mentioned anti-warping process measures are not implemented, part of the PCB baking and pressing is useless and can only be scrapped.