Printed circuit board assemblies (PCBA) buckling is a common problem in electronics manufacturing that involves the deformation of the board during the reflow oven process. This phenomenon can affect both the functionality of the product and the quality of the solders, which can lead to failures during operation or a complete Not OK condition. To understand the causes behind this problem, it is necessary to analyze various factors related to the design, materials and assembly processes.

Factors contributing to PCBA buckling

Buckling usually occurs when the printed circuit board (PCB) experiences a deformation during the assembly process. The reason behind this failure mode is that the board is subjected to different temperatures or pressures instead of being uniform for various reasons, causing different cohesive forces on the board. This difference in forces, if high, causes a deformation. During cooling, this delta can also occur.

Since the PCB is multilayer, in addition to the bonding forces, the copper-resin interface can also cause problems with deltas or moisture accumulation, even only on the copper layer.

This deformation can be upward (when it curves towards the center) or downward, and it presents as a curvature of the board. Although the processes and materials used in PCB assembly remain constant, buckling can be caused by variations in any of the following factors:

1. Copper Imbalance: One of the most common factors causing PCBA buckling is an imbalance in the copper distribution on the top and bottom layers of the board. If the design does not distribute copper evenly, the thermal expansion during the soldering process will be different on each layer, which can result in warping of the board. Using a PCB supplier that does not properly control copper balance can exacerbate this problem (or uses different methods for your NPI PCB and your Production PCB).
2. Moisture Absorption: PCBs are hygroscopic materials, meaning they can absorb moisture from the environment. When these wet boards are subjected to high temperatures during the reflow process, they can expand unevenly, resulting in warping. This problem is particularly common when boards are not stored properly or when there are conditions that change humidity and temperature in the environment.
3. Changes in PCB Materials: Materials used in PCB manufacturing, such as resins and copper, can vary between batches or suppliers. If a supplier changes their materials without notification, the thermal behavior of the PCB could be altered, which can lead to buckling. Additionally, changes in the thickness of the copper or resin used can also affect the PCB's ability to withstand thermal stresses during the soldering process.
4. Reflow Oven Setup: The reflow oven is a crucial component for PCB assembly. If the oven does not maintain a uniform temperature or is not calibrated correctly, thermal differences can occur, causing the board to warp. Three-zone reflow ovens, although commonly used, have limited ability to control the thermal profile, especially when assembling large or complex components such as BGA (Ball Grid Array) packages with heat sinks.
5. PCB Design and Panelization: PCB design, including the choice of panelization (how the boards are cut), also influences its propensity to warp. If an improper panelization method is used or the board orientation during the reflow process is incorrect, thermal stresses may not be evenly distributed, which promotes warping. Larger PCBs must be properly oriented in the oven to prevent temperature differences between the edge and center of the board from causing warping. (We recommend you consider V-Scoring.)
6. Environmental ConditionsFactors such as factory humidity and temperature also influence buckling. Humidity in the environment can cause PCBs to absorb water, which increases the risk of warping during the reflow process. Additionally, seasonal conditions, such as the temperature change between winter and summer, can affect how boards respond to temperature during assembly.

Expert Comments on PCBA Buckling

Experts agree that a low-quality reflow oven or three-zone reflow oven should not be blamed alone. There are many other factors, such as the materials used and the process conditions, that can influence PCB warping. Here are some key recommendations:

• Verification of material specifications: It is essential to verify that the PCB specifications, such as the Tg (glass transition temperature) value, are suitable for the reflow process. If the Tg value is insufficient, the board may not be able to withstand the high temperatures of the process, increasing the risk of warping.
• Use of mechanical reinforcements:To prevent buckling, some companies rely on mechanical reinforcements in the PCB design, such as stabilizers or additional stiffeners that help maintain the shape of the board during assembly. These reinforcements must be part of the PCB design to be effective.
• Review of reflux profiles: It is important to confirm that the reflow profile used has not changed over time. The oven temperature and speed must be kept constant to ensure proper assembly. Experts recommend performing validation testing using thermocouples and test profiles to ensure that the process remains the same as when it was initially set up.
• PCB Material TestingPCB materials must undergo rigorous testing before production begins. This may involve checking the humidity of the boards or checking that the supplier has not changed materials without informing them. Additionally, the boards must be stored properly to prevent moisture absorption.

Conclusion

PCBA buckling is a complex problem involving a variety of factors, from design and materials to assembly process and environmental conditions. While reflow oven and thermal control are critical, it is also important to consider the impact of humidity, PCB design, and material changes. Addressing these factors systematically will help reduce the risk of buckling and improve the quality and reliability of electronic products.

For more information on preventing PCBA buckling, it is recommended to take a preventative approach that includes constant monitoring of processes and validation of the materials and equipment used.

If you want to learn more about PCBA buckling, here are some useful references:

1. Hi Electronic - A source that provides detailed information on common causes of PCBA failures, including buckling issues related to copper imbalance and humidity. You can find more on these topics here here.
2. Circuit Insight - You can check out the article on Board Warp and other articles of interest here.
3. Compufab Inc. - This company has been solving PCB buckling problems for over 40 years. On their website you will find mechanical solutions to prevent PCB warping. here.
4. MicroCare Corporation - They offer specialized products to clean reflow ovens and improve thermal performance, which can help prevent warping. More details on their website here.

These resources provide additional information and recommendations for troubleshooting PCBA buckling issues, helping to maintain the quality and reliability of your assemblies.