How to Repair PCB Warpage?

What Defines PCB Warpage?

According to the IPC standard "Qualification and Performance Specification for Rigid Printed Boards", the maximum allowable warpage and twist for PCBs is between 0.75% and 1.5%.

Based on IPC standards, PCBs requiring surface-mount components must have a warpage degree of ≤0.75% to qualify as acceptable. For PCBs that only include through-hole components (no surface-mount devices), the flatness requirements are less stringent, allowing a warpage degree of ≤1.5%.

Calculation formula for PCB warpage:
Warpage (%) = (Warp Height / PCB Diagonal Length) × 100%

PCB Warpage Does Not Only Occur During Manufacturing？

Most people habitually assume that warpage is caused by manufacturing errors, but this is not entirely true.

1. Material-related issues: Variations in the physical and chemical properties of materials can lead to thermal and mechanical stress during the PCB fabrication process, causing warpage. Examples include:

1. Warpage in the copper-clad laminate material itself.

2. Warpage caused during PCB pressing.

3. Baking during solder mask and silkscreen printing.

4. Rapid heating and cooling in the production process.

5. Excessively deep V-cuts.




2. Storage issues: Improper storage can exacerbate warpage, including:

1. Moisture absorption of copper-clad laminates during storage.

2. Improper storage conditions such as excessive weight or overly tight bundling.




3. Engineering design issues: Inefficient design can lead to warpage:

1. Uneven copper distribution causing imbalanced stress between the PCB layers.

2. Asymmetric stack-up structure leading to localized deformation.

3. Structural and layout design causing warpage due to weight distribution.

4. Excessive panelization resulting in sagging during reflow soldering.

5. Lack of consideration for mechanical stress during circuit board design.

How to Prevent PCB Warpage

1. Engineering design improvements:

1. Align prepreg arrangements between layers.

2. Use the same supplier for all core and prepreg materials.

3. Ensure outer layer copper coverage is balanced, potentially using a grid pattern.

2. Reduce PCB size and panelization quantity: Smaller PCBs and fewer panels reduce warpage caused by weight during high-temperature processes.

3. Use high-Tg materials: High-Tg materials enhance resistance to stress deformation.

4. Use thicker PCBs: Thicker boards reduce the risk of bending and warping.

5. Bake the board before cutting: Typically at 150°C for 3 hours to remove moisture and internal stress, and to further cure the resin.

6. Stress relief after lamination: Allow natural cooling after lamination to release stress.

7. Avoid mechanical brushing for thin boards: Use chemical cleaning instead.

8. Natural cooling after solder leveling: Allow boards to cool naturally on flat steel plates before cleaning to reduce thermal stress.

9. Use a router instead of V-Cut for depaneling: V-Cuts weaken structural strength; if used, the depth should be minimized.

10. Use reflow trays or fixtures: Trays can stabilize PCBs during reflow.

11. Lower reflow oven temperature: Reducing oven temperature minimizes stress on the PCB.

PCB Warpage Repair

1. Roller straightening machine: For significantly warped boards, a roller straightening machine can flatten the PCB before further processing.

2. Heat press leveling: Use a hot press to flatten the PCB under controlled pressure and temperature.

3. High-temperature flattening for small quantities: Place the PCB in an oven, heat to the material’s glass transition temperature, and then clamp it in a flat mold until cooled.