Métodos de Teste de PCB: Um Guia Abrangente para Testar e Caracterizar PCBs

In the design and manufacturing process of electronic equipment, PCB (Printed Circuit Board) warpage is a common but troublesome problem. PCB warping not only affects the performance and reliability of the device but can also cause connectivity issues and other serious consequences. Therefore, it is crucial to understand how to prevent and resolve PCB warping. This article will explore some best practices in design and manufacturing to help you effectively prevent PCB warpage problems from occurring.

What is PCB warpage?

PCB warpage refers to the bending or twisting of a circuit board (also known as bow or twist). It occurs when the originally flat board has its ends or middle slightly raised when placed on a flat surface.

In actual production, most PCBs exhibit some degree of bow or twist rather than being perfectly flat. Warpage is typically measured as a percentage using IPC-TM-650 Method 2.4.22, by calculating the maximum deviation relative to the board's diagonal length.

According to IPC standards, the permissible warpage for a PCB board with surface mount components is 0.75%, whereas for a PCB board without surface mount components, it is 1.5%. However, in order to meet the demands of high-precision and high-speed placement, some electronic assembly manufacturers have stricter requirements for warpage. Some requirements allow for 0.5% warpage, and a few even require 0.3%.

The Dangers of PCB Warping

It hinders the installation of SMT electronic components, leading to poor contact between the components (including integrated chips) and the solder joints of the PCB board.

It increases the difficulty of mounting electronic components and makes it difficult to cut off the pins.

During the wave soldering process, some pads may not effectively connect to the solder surface due to warping.

These problems not only affect production efficiency but may also lead to equipment performance degradation and reliability issues. Therefore, it is crucial to prevent and solve PCB warpage to ensure the stability and reliability of electronic devices.

Causes of PCB Warping

Deformation caused by the weight of the circuit board.

The weight of the circuit board causes it to dent and deform. In the reflow oven, the two sides of the board act as fulcrums to support the entire board. If there are heavy objects on the board or if the size is too large, it will cause the board to dent in the middle, leading to bending.

The Effect of V-Cut on Panel Deformation

The V-cut is a significant factor in compromising the structure of the board as it creates grooves in the original large piece of board, making deformation more likely at the V-cut.

Warpage caused by the PCB pressing process

Warpage caused by the PCB pressing (lamination) process mainly results from residual thermal stress, uneven pattern distribution across layers, excessive prepreg usage, increased overall thickness without proper balancing, and challenges in fully eliminating internal stresses during curing. JLCPCB recommends designing symmetric stackups (equal copper distribution and core/prepreg thickness on both sides of the board) to minimize these issues.

Improper Storage

Stacking boards or inadequate shelf support during storage can result in mechanical deformation, especially for thin boards.

The copper surface area is uneven

With more on one side and less on the other, resulting in uneven surface tension and excessive temperature causing the board to warp.

How to Prevent Circuit Board Warping

● Optimized design:

Ensure that the design complies with standard size and weight requirements and avoid excessively large or heavy designs.

Reasonably arrange the V-Cut design to reduce damage to the board structure. Per JLCPCB guidelines, maintain a minimum panel size of 70×70 mm, keep components and copper at least 1.0 mm away from V-scores, and ensure the V-cut angle is 25° with straight-line scoring only.

● Material Selection:

Use a high-quality PCB substrate, such as fiberglass-reinforced FR-4, to reduce the possibility of warping. JLCPCB offers standard FR-4 with board thicknesses from 0.2 mm to 3.2 mm; we recommend the common 1.6 mm thickness for optimal balance between mechanical rigidity and cost, as thicker boards provide greater resistance to bending and warping under thermal or mechanical stress.

● Proper storage:

During the storage of semi-finished products, it is important to avoid stacking and improper storage of panels, especially thin ones.

● Uniform distribution of copper foil:

Evenly distribute copper foil on both sides of the PCB (and across all layers) to ensure consistent heat conduction and material performance. Significant copper density imbalance between layers can cause uneven thermal expansion; JLCPCB advises balancing copper coverage (ideally within 30–70% per layer) to minimize warpage during lamination and reflow.

Solution to Circuit Board Soldering Warping

1. Control temperature: During the reflow soldering process, maintain uniform temperature across different areas of the board. This can be achieved by optimizing the reflow profile (e.g., balanced top/bottom convection) or adjusting zone settings. Ensure full solder wetting to prevent voids.

2. Optimize the structure: The structural design of the circuit board will also impact soldering warpage. Therefore, we can reduce the thermal disturbance during the welding process and minimize the warpage of the circuit board by modifying the circuit board structure and adjusting the position of the solder joints.

3. Reinforce the circuit board: To address soldered warps on circuit boards, reinforcement can be applied as a solution. For example, support can be reinforced in areas where the circuit board is severely warped, or solder joints can be added in the warped areas.

4. Replace the material: If none of the above measures are effective, consider replacing the material. Choosing materials with lower thermal expansion coefficients or implementing surface bonding technology can help minimize circuit board solder warpage.

FAQ

Q: What is the maximum allowable warpage according to IPC standards?

For SMT boards it is 0.75%; for non-SMT boards it is 1.5%. JLCPCB can achieve tighter tolerances (down to 0.5%) upon special request.

Q: Does using a thicker PCB help reduce warpage?

Yes. The standard 1.6 mm thickness offers the best rigidity; thicker boards (up to 3.2 mm) are significantly less prone to bending under thermal stress.

Q: How does V-Cut panelization affect warpage?

V-Cuts weaken the panel structure. Follow JLCPCB guidelines: minimum panel size 70×70 mm and keep 1.0 mm clearance from components and copper.

Q: What should I do if my assembled PCB still warps?

Re-check copper balance and symmetric stackup in the design. During reflow, use board fixtures or optimize the temperature profile for uniform heating.