Why PCB Warpage Happens During Reflow and How to Reduce It

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Manufacturing question:

“We have boards warping during reflow. Can you help us understand what’s causing it?”

A representative question from the production floor

A printed circuit board (PCB) can bend, twist, or bow as it heats and cools during reflow soldering. This is called PCB warpage. It can interfere with solder paste printing, component placement, conveyor support, and solder-joint formation. Warpage usually reflects several influences together: board construction, moisture and stored stress, the heating and cooling profile, and how the board is supported. The most reliable way to reduce it is to review design, fabrication, storage, and assembly together rather than changing the reflow oven alone.

What causes PCB warpage during reflow?

Different materials expand at different rates

A PCB is a composite structure made from resin, glass reinforcement, copper, and other materials. These materials expand and contract differently as temperature changes. Their properties also vary by direction; laminate expansion through the thickness differs from expansion along the board. During reflow, these differences can create internal stress and cause a board to bow or twist.

Copper distribution is uneven

Copper carries heat and adds stiffness and mass. When copper coverage differs substantially from layer to layer or from one side or region of a board to another, the board may heat and expand unevenly. Large copper areas, sparse regions, and abrupt changes in copper density can contribute. Review copper balance across the whole design; the goal is not necessarily to make every layer have the same copper percentage.

The layer stack-up is asymmetric

A multilayer board with an unbalanced arrangement of copper and dielectric layers can respond unevenly to heat. A stack-up that is reasonably symmetric around the board’s center plane generally reduces bending forces. Electrical, impedance, and manufacturing requirements still govern the final stack-up, so review symmetry with the PCB fabricator rather than treating it as a rule that overrides the design.

Moisture is present in the laminate

PCB materials can absorb moisture during storage and handling. At soldering temperatures, moisture can contribute to swelling and, in serious cases, blistering or delamination. Moisture-related damage is not the same as ordinary board bow, but moisture conditioning can affect dimensional stability and should be considered when investigating a board that changes shape during heating.

Manufacturing stress is released during heating

Lamination, pressing, drilling, routing, and other fabrication steps can leave residual stress in a board. Heating during reflow may allow some of that stress to relax, revealing bow or twist that was less visible at room temperature.

The reflow profile or support conditions are unsuitable

A rapid heating rate, uneven temperatures across the assembly, an unsuitable peak temperature, or inconsistent cooling can increase thermal gradients and stress. Thin, large, or heavily populated boards may also sag if the conveyor or process does not support them adequately. The profile must suit the solder paste, components, board construction, and equipment.

How to reduce PCB warpage

• Review the stack-up for symmetry around the center plane, including copper layers and dielectric thicknesses.
• Balance copper distribution across layers and major board regions; discuss substantial imbalances with the fabricator.
• Select laminate materials for the application and assembly temperature. Consider Tg and other thermal properties together; a higher Tg alone does not guarantee low warpage.
• Store and handle bare boards according to material and supplier requirements. Control humidity and use a bake procedure only when applicable guidance calls for it.
• Measure and tune the reflow profile using the actual assembly. Avoid unnecessary thermal gradients and verify heating and cooling behavior at representative board locations.
• Use appropriate panel design, conveyor support, or a support fixture for thin, large, or otherwise distortion-prone boards when the process requires it.
• Measure boards before and after assembly. Separating incoming board condition from changes after reflow helps identify whether the main contributor is fabrication, storage, design, or assembly.

A practical way to troubleshoot warpage

First confirm when the deformation appears. Check representative bare boards before assembly, then measure the same design after reflow. Record board location, orientation, support method, and the measured profile. Compare results across panels and lots, and review the stack-up, copper distribution, material data, storage history, and oven profile with the PCB fabricator and assembly team. Changing several process settings at once makes it harder to identify the real cause.

Frequently asked questions about PCB warpage

Need help investigating a manufacturing problem?

PCB warpage prevention starts during design and fabrication, continues through storage and handling, and is confirmed during assembly. J&K Custom Electronics provides engineering and manufacturing support for electronics production challenges. Share the board details, when the warpage appears, and any measurements or process data so we can discuss practical next steps.

Email: kevin@jk-customelectronics.com
Phone: (260) 224-3267
Based in Roanoke, Indiana | High-Performance EaaS Support Nationwide.