In order to properly place SMT components, the PCB must be kept completely flat. For accurate placement, the SMT module must be released to the same height above the circuit board of all components by the SMT mounting machine.
If the PCB has warping, that is, unevenness, the machine cannot maintain a constant height when releasing the components when placing them on the circuit board – this can affect the accuracy of placement, especially for fine pitch components.
In addition, a flat PCB helps to keep the SMT components in place during reflow. If the high temperature inside the reflow furnace causes a change in the flatness of the circuit board, SMT components may slide out of position due to their floating on the molten solder, resulting in solder bridging and open circuits.
What is PCB warping?
PCB is usually made of glass fiber and other composite materials, and most PCBs are only laminated once and are very simple.
PCB warping refers to a change in the shape of the PCB, as shown in the following figure, which clearly indicates PCB warping.
What is the PCB warping standard?
According to the IPS standard, the warpage (WD) of the required PCB mounting should be less than or equal to 0.75%. That is to say, when WD is greater than 0.75%, it should be judged as a warping or defective product.
In fact, without installing components and only requiring plugins, the flatness requirement of the board is lower, and the WD standard can be less than or equal to 1.5%.
Of course, some manufacturers can pursue stricter standards in order to meet higher customer demands. Some WD standards require less than or equal to 0.5%, and even this requirement reaches less than or equal to 0.3%.
Calculation formula for PCB warpage
Warping, as the name suggests, refers to whether the PCB printed board is flat and can be perfectly inserted into the holes and surface mount pads of the board.
Warpage usually refers to the deformation of the surface of a plastic component that has not been formed according to the design shape. There are many factors that affect warping, so it is important to pay attention during the production process. After all, every small mistake can cause the entire circuit board to be scrapped. So how to calculate PCB warpage?
PCB warpage calculation formula:
Warping=Single angle warping height/(PCB diagonal length * 2) * 100%
In automatic wiring, if the PCB is not flat, it can cause inaccurate positioning and even cause the automatic insertion machine to crash. If the board bends after component welding, it is very difficult to neatly cut the component pins, ultimately resulting in the inability of the PCB circuit board to be installed in the socket of the chassis or machine, which is equivalent to scrapping one circuit board after another. If the manufacturer encounters PCB warping, it is a very painful thing.
Reasons for PCB warping
1. Reasons for PCB warping
1) The weight of the circuit board itself can cause the board to sag and deform
Generally, a reflux furnace uses a chain to drive the circuit board forward within the reflux furnace, which supports the entire board with both sides of the board as support points.
If there are heavy objects on the board or if the board size is too large, due to the quantity of the board, there will be a depression in the middle, causing the board to bend.
2) The V-cut is too deep, causing warping at the V-cuts on both sides
Basically, V-Cut is the main culprit for damaging the structure of the board, as V-Cut cuts grooves on the raw material, making it prone to warping.
The influence of material, structure, and graphics on board warping: PCB is made by pressing the core board, semi cured sheet, and outer copper foil. The core plate and copper foil will deform due to heat when pressed together. The amount of warping depends on the coefficient of thermal expansion (CTE) of both materials.
The coefficient of thermal expansion (CTE) of copper foil is approximately 17X10-6; The Z-direction CTE at Tg point of ordinary FR-4 substrate is (5070) X10-6; Above the TG point, it is (250350) X10-6. Due to the presence of glass cloth, the CTE in the X direction is generally similar to that of copper foil.
2. Warpage caused during PCB processing
The reasons for PCB warping during processing are complex and can be divided into thermal stress and mechanical stress.
Among them, thermal stress is mainly generated during the pressing process, while mechanical stress is mainly generated during the stacking, handling, and baking processes of the board.
1) PCB warping caused by incoming copper clad plate process
The copper clad plates are double-sided, structurally symmetrical, and have no patterns. The CTE of copper foil and glass cloth are almost the same, so there is almost no warping caused by different CTE during the pressing process.
However, the size of the copper clad plate press is relatively large, and the temperature difference in different areas of the hot plate can lead to slight differences in the resin curing speed and degree in different areas during the pressing process. At the same time, there are significant differences in dynamic viscosity under different heating rates, so local stress may also occur due to different curing processes.
Generally, this stress will remain balanced after pressing, but will gradually release and deform during subsequent processing.
2) PCB warping caused by PCB pressing process
The PCB pressing process is the main process that generates thermal stress. Similar to the pressing of copper clad plates, local stress may also occur due to different curing processes. Due to its thicker thickness, diverse pattern distribution, and more prepregs, thermal stress is more difficult to eliminate than copper clad plates.
The stress in the PCB board is released during subsequent drilling, forming, or grilling processes, causing the board to deform.
3) PCB warping caused by solder mask layer and silk screen baking process
Due to the inability of solder resist ink to stack with each other during the curing process, the PCB board will be placed in the rack and baked for curing.
The resistance welding temperature is around 150 ℃, which exceeds the Tg value of the copper clad plate. The PCB is prone to softening and cannot withstand high temperatures. The manufacturer must evenly heat both sides of the substrate while keeping the processing time as short as possible to reduce substrate warping.
4) PCB warping caused by PCB cooling and heating process
The tin furnace temperature is 225 ℃ -265 ℃, and the leveling time of ordinary hot air solder for plate is 3s-6s. The hot air temperature is 280 ℃ -300 ℃.
After the solder is leveled, the board is placed in the tin furnace at room temperature and subjected to room temperature post-treatment and water washing within two minutes after discharge. The entire process of hot air soldering and leveling is a sudden heating and cooling process.
Due to the different materials and uneven structure of circuit boards, thermal stress inevitably occurs during the cooling and heating processes, leading to micro strain and overall deformation and warping areas.
5) PCB warping caused by improper storage
The storage of PCB boards in the semi-finished stage is generally firmly inserted on the shelves, and the tightness of the shelves is not adjusted properly, or the stacking of boards during storage can cause mechanical deformation of the boards.
Especially for thin plates below 2.0mm, the impact is more severe.
Reasons for engineering design
- The copper surface area on the circuit board is uneven, with more on one side and less on the other. In areas with sparse lines, the surface tension will be weaker than in dense areas, and excessive temperature will cause the board to warp.
- Due to special dielectric or impedance relationships, the laminated structure may be asymmetric, leading to board warping.
- The hollow position of the board itself is too large and numerous, and it is prone to warping when the temperature is too high.
- There are too many panels on the board, and the spacing between the panels is hollow, especially rectangular panels, which are prone to warping.
How to improve PCB warping?
1. Precautions during PCB design
1) Lay copper inside the board to increase the tension of the layout.
When the plate length is over 80mm and there is no copper, and the plate thickness is less than 1.0mm, it will cause the plate to warp.
If the board cannot be pressed together and cannot be thickened, use a heavy pressure rocker.
2) Copper coating on hollowed out areas, coupled with craftsmanship
When there are too many hollow positions on the board and the board is too large, it is easy to bend after reflow soldering.
In the manufacturing process of PCB, all dielectric layers are evenly distributed. However, it is the uneven distribution of the copper layer that causes PCB warping. To prevent warping, design engineers must balance the copper pattern on each layer of the circuit board with the circuit area. Design engineers must also balance component layout, assembly distribution, and heat distribution to reduce warping.
Copper coating on hollowed out areas, coupled with craftsmanship
For example, if the outermost top surface has a large copper surface and the outermost bottom surface has only a few copper traces, then the PCB will have a greater tendency to warp after etching. The design engineer must ensure that the line pattern areas of the two outer layers match as much as possible. If there is a significant difference in copper area on both sides, it is best for the designer to add a separate copper grid on the thin side to balance the two.
Suggestion: Plating copper in the hollow area to reduce board warping; In addition, if the interior of the board does not affect its function, copper should also be laid; The final suggestion is to lay copper while processing the craft.
3) Core board and PP board are of the same brand
The core board and PP board of the multi-layer board must be of the same brand, otherwise the board will warp.
For example, the PP sheet material of the 6-layer board is asymmetric: the PP sheet material of the 2-3 core board is thin, and the PP sheet material of the 4-5 core board is thick, which causes it to warp and be pressed out. Therefore, the core board and PP sheet must be of the same brand to ensure consistent thickness and symmetry of the multi-layer PP sheet.
4) The arrangement of the layer prepreg remains symmetrical
The manufacturer must ensure that the arrangement of interlayer prepregs remains symmetrical. For example, for a six layer board, it is best to have the same thickness between 1-2 layers and 5-6 layers, including the number of prepreg sheets. This will prevent warping after lamination.
2. Precautions during PCB processing
1) PCB before cutting
The purpose of baking the PCB before cutting the copper clad board (150 ℃, time 8 ± 2 hours) is to remove moisture from the board and fully cure the resin in the board, further eliminating residual stress in the circuit board. This is useful in preventing the circuit board from warping.
At present, many double-sided and multi-layer boards still adhere to the baking steps before or after cutting. However, some board factories also have exceptions. At present, the drying time of PCB factories is also inconsistent, ranging from 4 to 10 hours. It is recommended to decide based on the grade of printed boards produced and the customer’s requirements for warping.
Both methods are feasible when the whole piece is baked and cut into a puzzle or cut into pieces before baking. Suggest cutting and baking the board, and the inner board should also be baked.
2) Longitude and latitude of prepreg
After the pre impregnated material is bonded, the shrinkage rate in the warp and weft directions is different. It is necessary to distinguish between the warp and weft directions during material dropping and bonding, otherwise it is easy to cause the finished board to warp after bonding, and even if pressure is applied to the baking board, it is difficult to correct.
Many reasons for the warping of multi-layer boards are due to the lack of distinction between the warp and weft directions of the prepreg during lamination, resulting in arbitrary stacking.
How to distinguish longitude and latitude? The rolling direction of the rolled prepreg is in the warp direction and the width direction is in the weft direction; For copper foil, the long side is in the latitudinal direction and the short side is in the meridional direction. If you are unsure, you can check with the manufacturer or supplier.
3) Stress relief after fitting
After hot pressing and cold pressing, the multi-layer board is removed, cut or milled to remove burrs, and then placed flat in a 150 ℃ oven for 4 hours to gradually release the stress inside the board and allow the resin to fully cure. This step cannot be omitted.
4) Thin plates need to be straightened during electroplating
When 0.4~0.6mm ultra-thin multi-layer plates are used for surface electroplating and pattern electroplating, special clamping rollers should be made. After clamping the thin plate onto the clamping roller on the automatic electroplating line, use a round rod to clamp the entire clamping roller. String the rollers together and straighten all the plates on the rollers to ensure that the plated plates do not deform.
Without this measure, after electroplating a 20 to 30um copper layer, the plate will bend, making it difficult to remedy.
5) Cooling of the board after hot air leveling
PCB leveling with hot air will be affected by the high temperature of the soldering bath (about 250 ℃). After removal, it should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning, which helps to prevent the circuit board from warping.
Some factories, in order to improve the brightness of the lead tin surface, immediately place the board in cold water after hot air leveling, and take it out for post-processing after a few seconds. This cold and hot shock may cause some types of circuit boards to warp. Twisted, layered, or bubbling.
PCB warping repair method
1. PCB warping repair in PCB process
In the PCB process, boards with large warpage are picked out and leveled by a roller leveling machine before entering the next process. Many PCB manufacturers believe that this approach is effective in reducing the warpage rate of finished PCB boards.
2. PCB finished board warping repair
For the finished product, the warping is significantly out of tolerance and cannot be leveled with a roller leveling machine. Some PCB factories place them in small presses (or similar fixtures) to suppress warped PCB boards, and leave them for several to ten hours for cold pressing and leveling. From practical applications, the effect of this approach is not very obvious. Firstly, the leveling effect is not significant, and secondly, the flattened board is prone to rebound (i.e., warping recovery).
Some PCB factories will heat the small press to a certain temperature and then perform hot pressing on the flattened PCB board, which will achieve better results than cold pressing. However, excessive pressure can cause wire deformation; If the temperature is too high, it will cause defects such as rosin discoloration and discoloration. Moreover, whether it is cold press leveling or hot press leveling, it takes a long time (from a few hours to more than ten hours) to see the effect, and the warping and rebound ratio of the flattened PCB board is also high.
The hot stamping square pressing method for bow shaped molds is recommended here. Based on the area of the PCB to be flattened, use a simple bow shaped mold (see Figure 1). Here, two types of leveling operations are proposed.
1) Clip the warped PCB board into the arched mold and place it in the oven for baking and leveling. Method:
The bent PCB board is facing the curved surface of the mold. Adjust the fixture screws to deform and warp the PCB board in the opposite direction. Then, place the mold with the PCB board in an oven and heat it to a certain temperature for baking. Bake for a while. Under heating conditions, the substrate stress gradually relaxes, and the deformed PCB board returns to a flat state. But the baking temperature should not be too high to avoid discoloration of rosin or yellowing of the substrate. But the temperature should not be too low, as complete relaxation of stress at lower temperatures requires a longer time.
The glass transition temperature of the substrate can generally be used as the reference temperature for baking. The glass transition temperature is the phase transition point of the resin, at which the polymer chain segments can be rearranged to fully relax the substrate stress.
Because the leveling effect is very obvious, the advantage of using a bow shaped mold to flatten is that the investment is very small. Ovens can be obtained from PCB factories. The leveling operation is very simple. If there are a large number of rocker plates, making a few arched molds is enough. You can put them in the oven once. There are few molds and the drying time is relatively short (about tens of minutes), so the leveling work efficiency is relatively high.
2) Soften the PCB board and clamp it into an arched mold for flattening:
For PCB boards with relatively small warping deformation, The PCB board that needs to be leveled can be placed in an oven that has been heated to a certain temperature (i.e. the temperature setting can be determined by referring to the glass transition temperature of the substrate. The substrate is baked in the oven for a certain time and the softening condition is observed to determine. Generally, the baking temperature of glass fiber cloth substrate is higher, while the baking temperature of paper substrate can be lower; the baking temperature of thick plate can be slightly higher, and the baking temperature of thin plate can be slightly lower.
PCB boards sprayed with rosin should not have a high baking temperature. Bake for a certain period of time, then take a few to ten sheets, clip them into the arch mold, adjust the pressure screw, and make the PCB board slightly warp and deform in the opposite direction. After the board is cooled and shaped, the mold can be removed and the flattened PCB board can be taken out.
After the arch mold is flattened, the PCB board has low warpage; Even after wave soldering, it can basically maintain a flat state; The impact on the appearance color of the PCB board is also minimal.
PCB board warping is a headache for PCB manufacturers. It not only reduces production, but also affects delivery time. If a curved mold is used for thermal leveling and the leveling process is reasonable and appropriate, the warped PCB board can be leveled to solve the delivery time problem.
The above is a brief introduction to PCB warping, the causes of PCB warping, and solutions. We hope it can be useful to everyone and welcome your guidance.
