What is the Process Flow of PCB Pressing Board?
Thermal stress is mainly generated in the press process, mechanical stress is mainly generated in the board stacking, handling, baking process. The following is a brief discussion of the process order.
A. Incoming Copper-clad Laminates:
Copper-clad laminates are double-sided,symmetrical structure, no graphics, copper foil and glass cloth CTE are very similar, so there is almost no deformation caused by different CTE in the pressing process.
However, the large size of the copper cladding press and the temperature difference between different areas of the hot plate will lead to slight differences in the curing speed and degree of the resin in different areas during the pressing process, while the dynamic viscosity under different heating rates also has a large difference, so there will also be local stresses due to differences in the curing process.
Generally, such stresses will be maintained in balance after pressing, but will be gradually released to produce deformation in later processing.
PCB pressing process is the main process of generating thermal stresses, similar to the copper-clad board pressing, will also produce local stresses due to differences in the curing process, PCB board due to the thickness of the thicker, graphic distribution of a variety of reasons, more semi-cured sheet, its thermal stresses will be more than the copper-clad board is more difficult to eliminate.
The PCB board in the presence of stress, in the subsequent drilling, shape or grilling process release, resulting in deformation of the board.
C. Soldermask, characters and other baking processes:
Soldermask ink curing can not be stacked on top of each other, so the PCB board will be placed vertically in the shelf baking board curing, soldermask temperature of 150 ℃ or so, just over the Tg point of the low Tg material, Tg point above the resin for the high elasticity of the state, the board is prone to deformation under the action of the self-weight or the strong wind of the drying oven.
D. Hot Air Solder Lleveling:
The temperature of the oven is 225℃~265℃ and the time is 3S-6S. The temperature of the hot air is 280℃~300℃.
Solder leveling board from room temperature into the tin furnace, out of the two minutes after the room temperature and post-treatment washing. The entire hot air solder leveling process for the sudden heat and cold process.
Due to the circuit board material is different, the structure is not uniform, in the hot and cold process is bound to thermal stress, resulting in micro-strain and overall deformation warping.
PCB boards in the semi-finished stage of storage are generally inserted vertically in the rack, the rack is not suitable for adjusting the tightness, or the storage process of stacking boards, etc. will make the board mechanical deformation. Especially for the 2.0mm below the thin board more serious impact.
In addition to the above factors, there are many other factors that affect the PCB board deformation.
PCB warping deformation prevention
Circuit board warping on the production of printed circuit boards is very great, warping is also one of the important issues in the production process of circuit boards, loaded with components of the board after welding occurs after bending, component feet is difficult to neatly.
The board also can not be mounted on the chassis or socket in the machine, so the circuit board warpage will affect the normal operation of the entire post-process.
At this stage, printed circuit boards have entered the era of surface mount and chip mount, the process of circuit board warpage can be said to be more and more demanding. So we need to find the cause of circuit board warpage.
A. Engineering Design:
Precautions Should Be Taken When Designing Printed Boards:
a, The arrangement of semi-cured sheets between layers should be symmetrical, for example, for six-layer boards, the thicknesses of layers 1 to 2 and 5 to 6 and the number of semi-cured sheets should be the same, otherwise the laminated boards are prone to warpage.
b, Multilayer core and semi-cured sheets should be from the same supplier.
c, The area of the line art on the outer A-side and B-side should be as close as possible. If the A-side is a large copper surface and the B-side has only a few lines, the board will warp easily after etching. If the difference between the two sides of the line area is too large, you can add some independent grid in the sparse side to balance.
B. Bake The Board Before Discharging:
Copper-clad laminate baking board before discharging (150 degrees Celsius, time 8 ± 2 hours) is to remove the moisture inside the board, while the resin inside the board is completely cured, and further eliminate the remaining stress in the board, which is helpful in preventing the board from warping.
At present, many double-sided, multi-layer boards still adhere to the step of baking the board before or after the material. However, there are some exceptions to the board factory, the current PCB factory baking time regulations are not consistent, from 4-10 hours have, it is recommended that according to the grade of the production of printed circuit boards and the customer's requirements for the degree of warpage to decide.
Cut into a patchwork after baking or the whole piece of material baked after the material, both methods are feasible, it is recommended to cut the material after baking board. The inner layer board should also be baked.
C. half-cured Sheet of The Warp and Weft Direction:
Semi-cured sheet lamination after the warp and weft shrinkage is not the same, the material and iteration must be clear when the warp and weft direction. Otherwise, after lamination, it is easy to cause warping of the finished board, even if the pressure baking board is also difficult to correct.
Multi-layer board warping reasons, many is the lamination of semi-cured sheet of the warp and weft direction is not clear, chaotic iteration and caused.
How to distinguish between warp and weft? Rolls of semi-cured sheets are rolled up in the warp direction, while the width direction is in 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.
D. Stress Relief After Llamination:
Multi-layer boards are taken out after hot and cold pressing, the raw edges are cut or milled off, and then placed flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stresses in the boards and make the resin fully cured, a step which should not be omitted.
E. Thin Plates Need to Be Straightened for Plating:
0.4 ~ 0.6mm ultra-thin multilayer plate for surface plating and graphic plating should make a special pinch roller, in the automatic plating line on the flybar clamped on the thin plate, with a round stick to the entire flybar on the pinch roller string, thus straightening the roller on all the board, so that the board after plating will not be deformed.
Without this measure, after plating twenty or thirty microns of copper layer, the plate will be bent, and it is difficult to remedy.
F. hot Air Leveling After the Board Cooling:
Printed board hot air leveling by the solder tank (about 250 degrees Celsius) of high-temperature impact, after taking out the marble or steel plate should be placed on a flat natural cooling, sent to the post-processing machine for cleaning. This is very good for the board to prevent warping.
Some factories in order to enhance the brightness of the lead tin surface, the plate hot air leveling immediately into the cold water, a few seconds after taking out and then post-treatment, this hot and cold shock, for some types of boards are likely to produce warping, delamination or blistering.
In addition, the equipment can be equipped with an air-float bed for cooling.
G. Bowed Board Processing:
In a well-managed plant, boards are inspected for 100% flatness at final inspection. Any boards that do not pass the inspection are picked out and placed in an oven at 150 degrees Celsius and under heavy pressure for 3 to 6 hours and cooled naturally under heavy pressure.
Then remove the pressure to take out the board, and then make the flatness check, which can save some of the boards, some boards need to make two to three times of the baking pressure in order to level. If the above process measures involved in the warpage prevention is not implemented, part of the board baking pressure is useless, can only be scrapped.
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