PCB Tech - Why is the PCB board deformed and how to prevent it 2

Why is the PCB board deformed and how to prevent it 2

2.2 Deformation caused during PCB processing

The causes of deformation during PCB board processing are very complex and can be divided into two types of stress: 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 laminate: The copper clad laminates are all double-sided, with symmetrical structure and no graphics. The CTE of copper foil and glass cloth is almost the same, so there is almost no deformation 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 kind of stress will maintain balance after pressing, but will gradually release and deform during future processing.

Pressing: The PCB pressing process is the main process that generates thermal stress, and the deformation due to different materials or structures is shown in the analysis in the previous section. Similar to the pressing of copper clad laminates, local stresses caused by differences in the curing process will also occur. PCB boards have more thermal stress than copper clad laminates due to thicker thickness, diverse pattern distribution, and more prepregs. The stress in the PCB board is released during subsequent drilling, shape, or grilling processes, causing the board to deform.

Baking process of solder mask, characters, etc.: As solder mask inks cannot be stacked on each other when they are cured, PCB boards will be placed in a rack for curing. The solder mask temperature is about 150°C, which just exceeds the Tg point of medium and low Tg materials, Tg The resin above the point is highly elastic, and the plate is easily deformed under the action of its own weight or the strong wind of the oven.

Hot-air solder leveling: The temperature of the tin furnace is 225 degree Celsius~265 degree Celsius, and the time is 3S-6S when the ordinary board hot-air solder leveling. The hot air temperature 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 will be 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 the different materials of the circuit board and the uneven structure, thermal stress will inevitably appear during the cooling and heating process, leading to microscopic strain and overall deformation and warping area.

Storage: The storage of PCB boards at the stage of semi-finished products 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 mechanical deformation of the boards. 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 deformation of the PCB board.

3. Prevention of PCB board warpage and deformation

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

1. Engineering design: Attention should be paid to the printed board design: A. The arrangement of the interlayer prepregs should be symmetrical, such as 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. Multi-layer 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 side is a large copper surface, and the B side only has a few lines, this kind of printed board will easily 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 the 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 the blanking. However, there are exceptions for some plate factories. The current PCB drying time regulations are also inconsistent, ranging from 4-10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Bake after cutting into a jigsaw or blanking after the whole block is baked. Both methods are feasible. It is recommended to bake the board after cutting. The inner board should also be baked.

3. The 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 during blanking and lamination. Otherwise, it is easy to cause the finished board to warp after lamination, and it is difficult to correct it even if the pressure is applied to the baking board. Many reasons for the warpage of the multilayer board are that the prepregs are not distinguished in the warp and weft directions during lamination, and they are stacked randomly. How to distinguish the warp and weft directions? The rolled up direction of the prepreg is the warp direction, while the width direction is the weft direction; for copper foil boards, the long side is the weft direction and the short side is the warp direction. Supplier inquiries.

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 rod strung the nip rollers on the entire fly bus to straighten all the plates on the rollers so that the plates after plating would not be deformed. Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will bend and it is difficult to remedy it.

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

7. Treatment of warped boards: In a well-managed factory, the printed boards will be checked for 100% flatness during the final inspection. All unqualified boards will be picked out, put in an oven, baked at 150 degrees Celsius under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure. Then relieve the pressure to take out the board, and check the flatness, so that part of the board can be saved, and some boards need to be bake and pressed two to three times before they can be leveled. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.

4, PCB board warpage change standard

For PCB warpage standards, please refer to IPC-A-600G No. 2.11 Flatness Standard: For surface-mounted components (such as SMT mounting) printed boards, the distortion and bowing standards are not more than 0.75%, and other types of boards are not more than 1.5%. The test method refers to IPC-TM-6502.4.22.