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Analysis of PCB warpage
(1) The weight of the PCB itself will cause the PCB to sink and warp.
Generally reflow oven will use chain to drive PCB forward in reflow oven, that is to say, the two sides of PCB as the pivot point to support the whole PCB. if there are heavier parts on PCB or the size of PCB is too big, due to the large amount of seeds, there will be a depression in the middle, which will result in the warpage of the board.
(2) Depth and connection of V-Cut will affect the warpage of the puzzle.
Basically, V-Cut is the main culprit to damage the PCB structure. Since V-Cut cuts slots in the original large piece, V-Cut is prone to warpage.
(3) Analysis of the effects of pressing materials, structure and graphics on panel warpage
PCB is made of core board, prepreg and outer layer of copper foil. When the core board and copper foil are pressed together, they are subject to thermal warpage. 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 CTE of ordinary FR-4 substrate is (50~70)X10-6 in Z direction at Tg point;
Above TG point is (250~350)X10-6. Due to the presence of glass cloth, the X-direction CTE is generally similar to that of copper foil.
(4) Warpage caused by PCB processing
The causes of warpage in PCB processing are very complex and can be divided into thermal and mechanical stresses. Thermal stresses are mainly generated during the pressing process, while mechanical stresses are mainly generated during the stacking, transporting and baking process of the boards.The following is a brief discussion of the process sequence.
Incoming Laminates:Laminates are double sided pcb with symmetrical structure and no graphics.The CTE of copper foil and glass cloth are almost the same, so there is almost no warpage caused by different CTE in the pressing process. However,the size of the laminating press is large and there is a temperature difference between different areas of the hot plate, which causes a slight difference in the curing speed and degree of the resin in different areas during the pressing process.At the same time,the dynamic viscosity under different heating rates also has a big difference, so it will also produce local stress due to different curing techniques. Generally, this stress will be balanced after pressing, but will be gradually released during subsequent processing, resulting in warpage.
Pressing:PCB pressing is the main process to generate heat stress. The previous section analysed warpage caused by different materials or structures. Similar to the pressing of laminates,there are localised stresses caused by different curing processes.PCBs have greater thermal stresses than laminates and are more difficult to eliminate due to thicker thicknesses,more varied pattern distributions and more prepregs.The stresses in the PCB are released during subsequent drilling, moulding or baking processes, resulting in warpage of the circuit board.
Baking process for soldermask,characters,etc.:Since soldermask inks cannot be stacked on top of each other while curing,the PCB is placed on a rack and the board is baked to cure.The soldermask temperature is around 150°C, which is just above 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 under the action of strong winds in the oven.
Hot air levelling:When ordinary pcb hot air levelling,the temperature of the tin furnace is 225℃~265℃, and the time is 3S-6S.The temperature of the hot air is 280℃~300℃.After the solder levelling,the circuit board will be put into the tin furnace from room temperature, and then wash in room temperature after treatment within two minutes after leaving the furnace. The whole hot air levelling process is a sudden heating and cooling process. Due to the different materials and uneven structure of PCBs,thermal stress is inevitable during the cooling and heating process,resulting in micro-strain and overall warpage.
Storage:PCBs in the semi-finished product stage of storage are generally firmly inserted in the racks, rack tension adjustment is not appropriate,or the storage process of the board stacking will lead to mechanical warpage of the board. Especially for thin boards below 2.0mm, the impact is more serious.
In addition to the above factors, there are many other factors that affect PCB warpage.
PCB warpage
Prevent PCB Warpage
PCB warpage has a great impact on the production of printed PCBs. Warpage is also one of the important problems in PCB production. The PCB with components will be warped after soldering, and it is difficult to neatly align the component pins. the PCB cannot be installed in the chassis or sockets inside the machine, so the PCB warpage will affect the normal operation of the whole follow-up process. At this stage, printed PCBs are entering the era of surface mount and chip mount, and the process requirements for PCB warpage are getting higher and higher. Therefore, we need to find out the reasons to help warpage in the middle of the process.
1.Engineering design: Printed board design should pay attention to: A. Prepreg layer arrangement should be symmetrical, such as six-layer board, thickness between 1-2 layers and 5-6 layers, the number of layers of prepreg should be the same, otherwise the layers are easy to warp after pressing. b. Multi-layer core boards and prepreg should be used with the same supplier's products. c. Outer layer of the A-side and the B-side of the circuit pattern area should be as close as possible. If side A has a large copper surface and side B has only a few lines, the board will warp easily after etching. If there is a big difference in the area of the lines on both sides, you can add some independent grids on the smaller side to balance it out.
2.Drying the board before cutting:The purpose of drying the board before cutting the laminate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time to make the board in the resin completely cured, and further eliminate the residual stress in the board. This helps to prevent the board from warping. Currently, many double-sided. Multilayer boards also insist on a baking step before or after cutting. However, there are some exceptions.Current PCB drying time requirements are inconsistent, ranging from 4 to 10 hours. It is recommended that this be determined by the grade of the board being produced and the customer's requirements for warpage. Cut the boards into pieces and bake them or bake them and unload them. Both methods are possible. It is recommended to bake the panels after cutting. The inner panels should also be baked.
3.Longitudinal and latitudinal direction of prepreg: After lamination of prepreg,the shrinkage rate of longitudinal and latitudinal direction is different, so it is necessary to distinguish between longitudinal and latitudinal direction when unloading and laminating.Otherwise,it will easily cause warpage of the finished board after lamination, and it will be difficult to correct it even if pressure is applied to the baking board. The reason for warpage of multi-layer boards is that the prepregs are not differentiated between longitudinal and latitudinal directions during lamination and are stacked randomly. How to distinguish the latitude and longitude? For rolled prepregs, the rolling direction is the warp direction and the width direction is the latitude direction; for copper foil panels, the long side is the latitude direction and the short side is the warp direction. If you are not sure.
4.Stress release after lamination:Take out the hot and cold pressed multilayer boards,cut off the burrs or grind them off,and then lay them flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stresses in the board layers. This step should not be omitted to make the board and the resin completely cured.
5.Thin plate plating needs to be straightened: 0.4 ~ 0.6mm ultra-thin multi-layer plate should be used for surface plating and pattern plating with special folder rollers.Thin plate clamped in the automatic plating line on the flyer line, a round bar will be the entire flyer line on the folder roller strung together, so that all the PCB on the roller straightened,so that the plated PCB will not warpage.If there is no such measure,after plating 20 to 30 micron copper layer, the board will warp and it is hard to be remedied.
6.Hot air levelling after PCB cooling: hot air levelling when the board is affected by the high temperature of the solder bath (about 250 degrees Celsius). After removal, the PCB should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processor for cleaning. This helps to prevent PCB warpage. In some factories, in order to improve the brightness of the lead-silver surface, the PCB is put into cold water immediately after the hot air blowing, and then taken out for post-processing after a few seconds. This hot/cold shock may cause some types of PCB warpage. This may cause some types of PCBs to warp, delaminate, or blister. In addition, an air-float bed can be installed on the equipment for cooling.
7.Handling of PCB warpage:In a well-managed factory, the printed boards are checked for 100% flatness at the final inspection.All unqualified PCBs will be picked out and put into the oven at 150 degrees Celsius and baked under heavy pressure for 3-6 hours, and then cooled down naturally under heavy pressure. Then take out the PCB from the pressure and check the flatness,this can save part of the PCB, some PCB need to bake and flatten two to three times to be flat. Shanghai Huabao Pneumatic Board Levelling Machine has been used by Shanghai Bell to repair PCB warpage. If you do not implement the above measures to prevent warpage, some of the PCB baking and pressing is useless and can only be scrapped.