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How to avoid PCB warping problem?

There are two main types of PCB warping problems: bow and twist.

What does bow mean?

Bow refers to the bending of the edge of the PCB board, forming an arc-shaped bulge or depression. Bowing usually occurs on two opposite edges of the PCB, and the entire PCB board presents a bow-like shape, so it is called bowing. Bowing may be unilateral or bilateral.

Measurement method: Place the circuit board flat on the marble with the four corners touching the ground, and then measure the height of the arch in the middle.

Calculation method: Bow degree = arch height / PCB long side length * 100%.

What does twist mean?

Twist refers to the rotation of the PCB board in the diagonal direction, forming a convexity or concave on the diagonal. Twist usually occurs on the diagonal of the PCB, making the PCB present a twisted shape, so it is called Twist.

Measurement method: Put the three corners of the circuit board on the ground and measure the height of the raised corner from the ground.

Calculation method: Twist = Height of single corner lift / PCB diagonal length * 100%.

What are the reasons for PCB board warping?

01
Stress exceeds the material tolerance
Warping occurs when the stress applied to the PCB board is greater than the material can withstand. This stress is usually caused by uneven stress on the board, such as uneven soldering temperature or uneven chemical influence.

02
Thermal stress
During the manufacturing process, if the PCB board is subjected to uneven thermal stress, such as an overheated reflow process, it will cause the PCB board to warp. When the PCB board is overheated and reaches the upper temperature limit of the material's Tg, the material will soften and cause permanent deformation.

03
Chemical stress
Chemical stress refers to the stress caused by the PCB material being attacked by chemicals. For example, factors such as chemical solvents, humidity, and climate change may have an impact on the PCB material, which in turn may cause the board to warp.

04
Improper manufacturing process
Improper PCB design and manufacturing process may also cause PCB board warping. For example, uneven copper foil surface area distribution and overheating during the manufacturing process may cause PCB board warping.

How to reduce PCB warping during the design stage?

01
Choose the right sheet material
Choose a sheet material with a higher Tg (glass transition temperature). This type of sheet material has better dimensional stability at high temperatures and can resist deformation caused by thermal stress.

02
Increase the thickness of the circuit board
Properly increasing the thickness of the circuit board can improve its ability to resist bending and warping without affecting the overall performance of the product.

03
Optimize the circuit board design
In the circuit board design, minimize the number and overall size of the boards to reduce the sagging deformation caused by their own weight. At the same time, arrange the component positions reasonably to avoid placing heavy components on one side of the circuit board to reduce the warping problem.

04
Choose the right stacking structure
Reasonably select the stacking structure of the PCB and use a symmetrical lamination process to reduce uneven stress inside the board.

05
Add support structure
Add support structures to the design of the PCB board, such as adding connecting plates and support columns, to enhance the structural strength of the PCB board and reduce the possibility of warping.

How to prevent PCB warping during circuit board manufacturing?

01
Reduce the number of PCBs on the narrow side of the production panel
On the production panel, try to reduce the number of PCBs on the narrow side to reduce the problem of board warping in the reflow oven. The reflow oven uses a chain to drive the circuit board forward. The larger the size of the production board, the greater the possibility of concave deformation in the reflow oven. To reduce board warping, place the long side of the production panel on the edge of the reflow oven chain, which can reduce the impact of deformation on the narrow side of the PCB.

02
Use a fully automatic depaneling machine for PCB depaneling
If the V-CUT is manually split, the stress structure of the board may be destroyed, causing the circuit board to warp. The use of a fully automatic depaneling machine can avoid this problem.

The advantages of the fully automatic depaneling machine include:

No circuit board warping will be generated during the depaneling process.

It eliminates the impact of manual depaneling on component soldering, and manual depaneling may also cause the risk of PCBA scrapping.

The appearance of the circuit board after depaneling is smoother and free of burrs.

Because the connection with the edge guide is more stable than the half hole, the production panel is smoother during the assembly process.

03
Bake the circuit board before electronic assembly
The purpose of baking the circuit board before electronic assembly is to remove moisture from the circuit board. At the same time, the resin in the laminate is fully cured and the residual stress in the laminate is further eliminated, which helps prevent the circuit board from warping.

04
Control the temperature during the manufacturing process
Pay attention to controlling the temperature during the manufacturing process to avoid overheating the PCB board, which will cause the material to soften and permanently deform.