Problem Overview

During the reflow soldering process of BGA devices, the PCB board is prone to deformation（warpage） due to high-temperature thermal stress. This deformation can lead to poor contact between the BGA solder balls and pads, causing false soldering, short circuits, or fracture of the solder joints due to stress concentration in the later stage of use. The following table summarizes the core solution approaches:

��️ How to prevent PCB board deformation(bending and Warpage)

Prevention is key and can be approached from three aspects: materials, circuit board design, and process.

1. Material selection and PCB design

Select high Tg CCL: Tg (glass transition temperature) is the critical point at which a material changes from the glassy state to the rubber state. The higher the Tg value, the stronger the board's ability to maintain rigidity at high temperatures during reflow soldering, and the stronger its resistance to deformation. However, the cost of high Tg sheets is usually also higher.

· Appropriately increase the board thickness: Many thin and light electronic products use a board thickness of 1.0mm or even 0.6mm, which is prone to deformation during reflow soldering. If there is no strict thickness limit, it is recommended to increase the plate thickness to 1.6mm, which can significantly enhance rigidity and reduce the risk of deformation.

· Optimize the circuit board layout and panel design：

o Reducing the size of the plates and the number of assembled units in a array: The larger the panel size, the more severe the depressions and deformations caused by the self-weight on the reflux oven chain. Reducing the size of the panel of the board and the number of spliced units can help alleviate this problem.

o Pass through the oven with the long side perpendicular to the oven direction： Using the long side of the PCB as the edge to contact the reflow oven chain can reduce deformation caused by weight.

o Avoid V-Cut shallow grooves：V-Cut can significantly damage the structural strength between PCB units. Its use should be avoided as much as possible or its depth should be reduced. It is a better choice to switch to a Router for board depanelizing.

2.  Process optimization and auxiliary tools

· Adjust the reflow soldering temperature curve： Temperature is the main cause of stress. Deformation caused by thermal stress can be reduced by appropriately lowering the peak temperature of reflow soldering or slowing down the heating and cooling speed. However, it should be noted that an overly slow cooling speed may cause problems such as solder short circuits.

· Using fixture：This is a very effective method for dealing with bending and warpage or high-precision PCB.

o Single-layer tray：It can provide support for the PCB and reduce softening deformation above the Tg temperature point.

o Cover tray：If the single-layer tray is not effective, the upper and lower trays can be used to clamp the PCB. This method can greatly reduce deformation when passing through the reflow oven, but the pallets themselves are expensive and require manual placement and recycling.

�� Standards and detection methods for PCB warping 

1. Warpage standard

According to the IPC standard, there is a clear acceptable range for the warpage of PCBS:

· For boards with surface mount devices (SMT) : The maximum allowable warpage is 0.75%.

· Other boards (such as pure plug-in components) : The maximum allowable warpage is 1.5%

It is worth noting that for some more demanding applications, such as BGA packaging, customers may request that the standard be raised to 0.5% or even 0.3%.

2.Measurement and calculation of warpage

The measurement methods of industry standard is follows

Place the PCB to be tested freely and flat on a standard calibration platform such as marble or glass

1. Measure the vertical distance between the highest point of the PCB warpage and the platform, that is, the arch height

2. Calculate by the formula: Warpage = (arch height/PCB long side length) × 100%

�� PCB Warpage and stress testing equipment

To accurately measure the deformation and stress of a PCB, professional equipment is required.

· Bending and warping inspection machine：It is suitable for rapid and efficient full inspection of finished products leaving the factory to ensure that the delivered PCBS meet the warpage standards.

· Thermal warpage measuring instrument：It is highly suitable for the R&D and process optimization stages, used to evaluate the anti-deformation ability of specific PCB or designs at simulated reflow soldering temperatures.

· Stress-strain tester：This type of equipment can precisely measure the microscopic strain (με) that the circuit board actually bears under various processing and testing conditions such as panel separation, insertion, screwing, and drop tests by attaching strain gauges to the PCB. This helps to identify the specific processes that cause excessive stress and make improvements.

��  Remedial measures after the PCB has deformed

If the PCB already has slight deformation and cannot be changed, the following measures can be taken to ensure the reliability of the BGA solder joints:

· Underfill：

o After the BGA soldering is completed, a special epoxy resin adhesive is penetrated the space between the bottom of the BGA chip and the solder balls through capillary action.

o After the glue cures, it can firmly bind the BGA chip, solder balls and PCB together to form an integrated structure.

o This can significantly enhance the mechanical strength of the solder joints, resist the stress caused by the subsequent deformation of the PCB, and effectively prevent the fatigue fracture of the solder joints. This is a key technology commonly used in high-reliability products.

· Optimize the steel mesh design to adjust the amount of solder paste： 

o For the known PCB deformation areas, the amount of solder paste printed in the recessed areas can be increased by locally adjusting the opening size and shape of the steel mesh.

o The additional solder can provide a larger solder joint volume during reflow, compensating for the poor coplanarity caused by board warping and ensuring that all solder balls can form reliable connections.

�� Summary and Suggestions

If deformation issues caused by BGA reflow soldering, it is recommended that you adopt a systematic approach:

1. Prioritize prevention: During the product design stage, give priority to choosing high Tg base material and plates with a thickness of 1.6mm, and avoid designs that are prone to deformation in terms of layout.

2. Process Optimization: Carefully adjust the reflow soldering temperature curve. For complex PCB boards, investing in the use of oven trays is a highly cost-effective solution.

3. Strict monitoring: With the aid of professional testing equipment and in accordance with IPC standards, the warpage of incoming materials and finished boards is monitored, and stress tests and control are conducted on key manufacturing processes.

4. Post-reinforcement：For BGA that has been deformed or has high reliability requirements, bottom filling is an effective means to enhance long-term reliability.