Deformation of PCB high-frequency circuit board during processing
PCB manufacturing, PCB design and PCBA processing manufacturers explain the deformation caused by PCB high-frequency circuit board processing
The causes of PCB high-frequency circuit board deformation during processing are very complex, whICh can be divided into thermal stress and mechanical stress.
The thermal stress is mainly generated in the pressing process, and the mechanical stress is mainly generated in the process of plate stacking, handling and baking. The following is a brief discussion in the order of the process.
1. Incoming materials of CCL:
Copper clad plates are double-sided plates with symmetrical structure and no graphics. Copper foil and glass cloth CTE are almost the same, so there is almost no deformation caused by different CTE during pressing.
However, due to the large size of the CCL press and the temperature difference in different areas of the hot plate, the resin curing speed and degree in different areas during the pressing process will be slightly different, and the dynamic viscosity at different heating rates will also be greatly different, so the local stress caused by the difference in the curing process will also be generated.
Generally, this kind of stress will maintain balance after pressing, but will gradually release in the future processing to produce deformation.
The pressing process of PCB is the main process to generate thermal stress. SIMilar to the pressing process of CCL, it will also generate local stress caused by the difference in curing process. Due to the thicker thickness, diverse pattern distribution, more sEMI cured pieces and other reasons, the thermal stress of high-frequency PCB circuit board will be more and more difficult to eliminate than that of CCL.
However, the stress in the PCB high-frequency circuit board is released in the subsequent drilling, shape or barbecue processes, resulting in the deformation of the board.
3. Baking process of resistance welding and characters:
Because solder resist inks cannot be stacked with each other when curing, PCB high-frequency circuit boards will be vertically placed in the shelf to dry the boards for curing. The solder resist temperature is about 150 ℃, which just exceeds the Tg point of medium and low Tg materials. The resin above the Tg point is highly elastic, and the boards are easy to deform under the self weight or strong wind in the oven.
4. Hot air solder leveling:
The temperature of tin furnace is 225 ℃~265 ℃ and the time is 3S-6S when the hot air solder for ordinary plate is leveLED. The temperature of hot air is 280 ℃~300 ℃.
When the solder is leveled, the plate enters the tin furnace at room temperature, and the post-treatment water washing at room temperature is carried out within two minutes after it is discharged from the furnace. The whole 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 cold and hot process, resulting in micro strain and overall deformation and warping.
The storage of PCB high-frequency circuit boards in the semi-finished product stage is generally vertically inserted in the shelf. Improper adjustment of shelf tightness or stacking of boards during storage will cause mechanical deformation of the boards. Especially for the 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 PCB high-frequency circuit board.
Prevention of PCB High Frequency Circuit Board Warpage
The warpage of the circuit board has a great impact on the production of the printed circuit board, and it is also one of the important problems in the production process of the circuit board. The high-frequency circuit board with components is bent after welding, and it is difficult for the component feet to be neat.
The high-frequency circuit board can not be installed on the chassis or the socket inside the machine, so the warping of the circuit board will affect the normal operation of the whole subsequent process.
At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the requirements of the process on the warpage of the printed circuit board are getting higher and higher. So we need to find the cause of the warping of the circuit board.
1. Engineering design:
Precautions for PCB design:
A. The arrangement of interlayer prepreg shall be symmetrical. For example, for six ply boards, the thickness between 1~2 and 5~6 layers shall be consistent with the number of prepreg sheets, otherwise it is easy to warp after lamination.
B. Multilayer core board and prepreg shall be from the same supplier.
C. The area of the outer layer A and B lines should be as close as possible. If the A side is large copper and the B side only runs a few wires, this kind of printed board is easy to warp after etching. If the difference between the line areas on both sides is too large, some independent grids can be added on the sparse side for balance.
2. Drying plate before blanking:
The purpose of drying the copper clad plate (150 ° C, 8 ± 2 hours) before blanking is to remove the moisture in the plate, and at the same time, fully solidify the resin in the plate, further eliminate the residual stress in the plate, which is helpful to prevent the plate from warping.
At present, many double-sided and multilayer boards still adhere to the process of baking before or after blanking. However, there are some exceptions to this rule. At present, the drying time of PCB plants is also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of PCB produced and the customer's requirements for warpage.
The two methods are both feasible, and it is recommended to bake the board after shearing. The inner plate shall also be baked.
3. Stress relief after lamination:
Take out the Multilayer board after hot pressing and cold pressing, cut or mill off the burrs, and then place it flat in the oven at 150 ℃ for drying for 4 hours to gradually release the stress in the board and completely cure the resin. This step cannot be omitted.
4. The sheet needs to be straightened during electroplating:
When the 0.4~0.6mm ultra-thin multilayer board is used for plate surface electroplating and graphic electroplating, special clamping rollers shall be made. After the thin plate is clamped on the flying bar on the automatic electroplating line, a round bar shall be used to string the clamping rollers on the whole flying bar, so as to straighten all high-frequency circuit boards on the roller, so that the high-frequency circuit boards after electroplating will not be deformed.
Without this measure, the thin plate will be bent and difficult to repair after electroplating 20 to 30 micrometers of copper layer.
5. The warp and weft directions of the prepreg:
After laminating, the shrinkage of the prepreg in the warp and weft direction is different. The warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause warping of the finished board after lamination, and it is difficult to correct even if the pressure is applied to bake the board.
Many reasons for the warping of multilayer boards are due to the disorderly overlapping of the warp and weft directions of the prepreg during lamination.
How to distinguish longitude and latitude? The roll up direction of the rolled prepreg is the warp direction, while the width direction is the weft direction; For copper foil, the long side is in the weft direction and the short side is in the warp direction. If you are not sure, you can check with the manufacturer or supplier.
6. Cooling of high-frequency circuit board after hot air leveling:
When the PCB is leveled by hot air, it will be impacted by the high temperature of the solder bath (about 250 ℃). After being taken out, it should be put on the flat marble or steel plate for natural cooling, and then sent to the post-processing machine for cleaning. This is good for warping prevention of high-frequency circuit board.
Some factories, in order to enhance the brightness of the lead tin surface, put high-frequency circuit boards into cold water immediately after hot air leveling, and then take them out for post-treatment after a few seconds. This impact of one hot and one cold is likely to cause warping, delamination or blistering of some types of high-frequency circuit boards.
In addition, air floating bed can be installed on the equipment for cooling.
7. Treatment of warped high-frequency circuit board:
In a well managed factory, 100% flatness inspection will be conducted for printed boards during final inspection. All unqualified high-frequency circuit boards will be picked out and put into an oven, dried at 150 ℃ and under heavy pressure for 3-6 hours, and naturally cooled under heavy pressure.
Then, take out the high-frequency circuit board by pressure relief, and check the flatness. This can save some high-frequency circuit boards. Some high-frequency circuit boards need to be baked for two or three times before leveling. If the anti warping process measures mentioned above are not implemented, the baking and pressing of some high-frequency circuit boards are useless and can only be scrapped.