Why does the PCB warp and how to prevent it?
In the PCB manufacturing process, PCB warping is a problem that manufacturers often encounter. When the printed circuit board warps, its performance and reliability will be affected. Warping can cause many problems, such as misalignment of components, poor welding quality, and even equipment failure. Therefore, for PCB manufacturers and PCB designers, it is essential to fully understand this issue. In this blog, we will explain the causes of PCB warping and more importantly how to prevent such problems. Let's continue reading.
What is PCB warping?
PCB warping is a defect, that is, the PCB board is deformed or bent, deviating from its original plane shape. This may happen on a bare PCB or on a finished PCBA. But in fact, every PCB will have a slight warping, which is inevitable in the lamination process of PCB manufacturing. For this issue, there are IPC standards (including IPC-6011 and IPC-6012) that specify an acceptable level of PCB warping. They also set up different categories with different maximum warping tolerances：
How to measure PCB warping?
There are 5 common methods for measuring PCB warping：
1. Measuring tape method
Use a measuring tape to measure the warping degree between the high and low points of the warped PCB. This is usually used for larger warping values.
2. Height ruler
In this method, we need to use a mechanical or digital altimeter to measure the height difference between the high point and the low point, which allows us to obtain accurate results of warping in inches or millimeters.
3. Profiler
The profiler is used to track the shape of the warped PCB. By placing the measuring instrument on the surface, it will be adjusted to match the contour of the circuit board, so that warping can be visually measured or measured. This method is particularly useful for warping irregular shapes.
4. Optical profiler
This method provides high-precision measurement, using an optical profiler to scan the curved PCB, and then create a 3D contour of the PCB surface. Therefore, we can identify small deformations of the circuit board.
5. Finite element method (FEM)
The finite element method is a calculation method that can simulate and analyze the warping of PCB under various stress conditions. By entering material properties and environmental factors, FEM can predict potential warping and help manufacturers design PCBs with the least risk of deformation.
The main cause of PCB warping
Asymmetric arrangement of PCB board layers can cause PCB warping. In this case, the thermal stress will not be evenly distributed on the circuit board during the heating process, causing the PCB board to warp or bend after cooling.
If the copper distribution of each layer is uneven, it will cause the PCB to warp. In a PCB, significantly more or less copper on one side than on the other will cause uneven expansion, causing warping.
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The coefficient of thermal expansion (CTE) and Tg of different materials are different, and the rates of expansion and contraction during heating and cooling are also different. This uneven stress can cause the circuit board to warp, especially during temperature changes in processes such as reflow soldering.
If the circuit board is clamped too tightly during wave soldering or reflow soldering, it may cause the circuit board to warp. When the clamped circuit board is heated, it needs to expand naturally. If the circuit board is clamped to prevent expansion, the circuit board may be permanently deformed, especially when there are heavy-duty components that need to be welded on the circuit board.
In the PCB manufacturing process, the temperature cycle of each step will cause the circuit board material to expand and shrink repeatedly. Excessive temperature cycling or improper process control can lead to serious warping problems.
How to prevent PCB warping?
We can reduce the possibility of PCB warping by implementing effective thermal management techniques. For example, optimize the reflow soldering curve to minimize the thermal gradient and ensure uniform heating and cooling. In addition, a heat dissipation function is added to the PCB design to dissipate heat more effectively.
In the PCB design process, it is essential to ensure the balanced placement of components and optimize the wiring. In addition, a symmetrical inter-layer prepreg and equal copper area on both sides of the board should be used to maintain a uniform heat distribution and thus prevent PCB warping.
When choosing materials, materials with compatible thermal and mechanical properties must be selected, which can minimize the risk of PCB warping. For substrate materials, please consider using FR4 because it has good stability. In addition, thinner substrates are better because they are less likely to deform.
Ensure that the copper foil is evenly distributed on both sides of the PCB, which helps maintain consistent heat conduction and prevents PCB warping. The complementary layers should have similar copper coverage, which may involve the use of copper filling to fill the unused area on one side. When doing this, it is very important to define appropriate gaps in the design rules, especially if the filling layer needs to support controlled impedance.
Before assembling components, baking the PCB board is a very critical step. On the one hand, this process can remove moisture from the circuit board, and on the other hand, it helps to fully cure the resin on the laminate and reduce the residual stress. , Thereby reducing the risk of circuit board warping.
Avoiding stacking and proper storage of panels can also prevent PCB warping. The storage environment should be dry and the temperature can be controlled. The panel should be placed flat and have sufficient support to prevent bending or deformation during storage.
Can PCB warping be repaired?
Once PCB warping occurs, it is usually difficult to completely “fix” the warping, especially when the problem occurs on the finished bare board or PCBA. Although the internal stress can be reduced by reflow or baking the circuit board under controlled conditions, thereby correcting some minor warping, severe warping is usually beyond repair and needs to be replaced. Therefore, instead of spending energy trying to repair a batch of warped PCBs, it is better to investigate the root cause and prevent this from happening again.
Final thoughts
PCB warping is a common challenge in the electronic manufacturing process. Once it occurs, it is difficult to repair, but it can be prevented by identifying the root cause and implementing key measures, as described above.
Technical Manager at Crosspoint Technologies
3 个月Love this, whatever is shared with the customer available in document form...thank you...
Quality Control & Electronics | Precision & Performance
3 个月Great insights into the causes and prevention methods for PCB warping! It's clear that proper design, material selection, and thermal management are crucial in minimizing this issue. I especially appreciate the emphasis on preventive measures like uniform copper distribution and baking the PCB before assembly. This knowledge will definitely help in improving the reliability and performance of PCBs in manufacturing.