As electronic components continue to evolve toward miniaturization, higher efficiency, and higher frequencies, traditional packaging technologies are gradually revealing some limitations. To meet the growing demand, BGA packaging technology was developed and quickly became a significant innovation in the electronics manufacturing industry.
However, the solder joints of BGA packages are located at the bottom of the component, making it difficult to directly observe the soldering details. This can lead to soldering defects, which severely affect product stability and performance. More critically, these defects may result in the need for rework. During the BGA rework process, the component often needs to be removed and subjected to multiple thermal cycles, which not only increases costs but may also cause the solder balls at the bottom of the package to detach or degrade. Therefore, a reballing procedure must be performed before re-soldering. To help you better understand this process, this article will introduce 4 common BGA reballing methods and their specific steps. Let’s get started!
What is BGA Reballing and its Benefits
Before delving into the BGA reballing process, it is essential to first understand the basic concept of BGA. BGA stands for Ball Grid Array, a common IC packaging method in modern electronic products, particularly suitable for high-performance and high-density circuit designs. The distinctive feature of BGA packaging is that its solder balls are located on the bottom of the chip and arranged in a grid pattern. These solder balls connect to pads on the PCB through soldering, ensuring stable electrical connections.
The core of the ball grid array reballing is to replace these solder balls in order to restore the precise electrical connection between the PCB and the chip. This process typically involves removing the aged, detached, or damaged solder balls and replacing them with new ones to restore their original functionality.
In the BGA assembly process, soldering defects are difficult to avoid completely. Manufacturers can only continuously optimize the process to improve the success rate and minimize the occurrence of issues. Since BGA components are relatively expensive, when problems arise, the reballing process allows for the reuse of BGA components after desoldering, avoiding the high costs of purchasing new components. Additionally, the reballing process can extend the lifespan of the original BGA components, helping companies save costs and improve production efficiency.
Desoldering Process
The process begins with removing the BGA component from the PCB. For this step, a dedicated BGA rework station is always the best choice, as it can improve work efficiency and significantly increase the success rate of desoldering. If such professional equipment is unavailable, a hot air gun can also be used for desoldering. However, when using a hot air gun, extra caution should be taken regarding the heating position, temperature, and duration. A slight misstep can lead to subtle damage to the component or PCB, even if there are no visible signs.
After desoldering, the pads need to be cleaned to remove residual solder, which could affect the quality of the subsequent soldering process. A common method is to place desoldering braid on the pad surface and gently press the braid with a soldering iron tip. The solder will be absorbed through the braid. However, special care should be taken not to overheat the same area for too long to avoid damaging the pads or PCB.
Material Selection
The choice of materials directly impacts the quality of reballing, especially the selection of flux and solder balls.
For flux, it is generally recommended to use a high-viscosity flux to ensure good performance in pre-tinning and soldering. Alternatively, you can use solder paste as a substitute, but ensure that the composition of the solder paste matches that of the solder balls.
When choosing solder balls, the ideal scenario is to select solder balls that match the composition of the original solder balls. Regarding solder ball size, if flux is used, choose solder balls that are the same diameter as the original solder balls on the BGA component. If solder paste is used, choose solder balls that are slightly smaller in diameter than the original solder balls, as the solder paste will cause a slight increase in solder ball diameter during the subsequent heating process.
4 Methods of BGA Reballing
Method 1: Using a Dedicated Reballing Tool
If you have a dedicated BGA reballing tool, the entire process becomes relatively simple. You only need to select a steel stencil that matches the BGA pad, with a hole size about 0.05 mm larger than the solder ball diameter, and follow these steps:
- Evenly distribute the solder balls onto the steel stencil, and gently shake the reballing tool to allow excess solder balls to fall into the collection tray.
- Ensure that each aperture only has one solder ball and place the reballing tool on the workbench.
- Use a vacuum nozzle to pick up the BGA component’s back (the side without the pads), which has been coated with flux or solder paste.
- Place the BGA component face-down, accurately aligning it with the solder balls on the steel stencil. Once in contact, the solder balls will attach to the BGA pads.
- Remove the reballing tool and inspect the BGA component for any missing or misplaced solder balls. Any defects can be corrected using tweezers.
Method 2: Using a Steel Stencil
This process follows the same principle as the dedicated reballing tool method and also requires a steel stencil that matches the BGA pads, with a hole size about 0.05 mm larger than the solder ball diameter. Follow these steps:
- Place the BGA component, which has been coated with flux or solder paste, face-up on the workbench.
- Prepare four spacer blocks and place them around the component to elevate the steel stencil.
- Align the steel stencil with the BGA pads and gently place it above the component, ensuring that the distance between the stencil and the component is slightly smaller than the solder ball diameter.
- Evenly distribute solder balls onto the steel stencil, and use tweezers to remove excess solder balls, ensuring that each aperture contains only one solder ball.
- Remove the steel stencil and inspect the BGA component for any missing or misaligned solder balls. Any defects can be corrected using tweezers.
Method 3: Manual Placement
As the name suggests, manual placement involves using tweezers or a vacuum pen to pick up each solder ball and place it onto the BGA pad. This method is suitable for small-scale operations and is relatively simple, requiring no extensive explanation. However, it is important to avoid applying too much force when using tweezers, as this could deform the solder balls.
Method 4: Solder Paste Ball Formation
This method involves directly printing solder paste onto the BGA pads. The solder paste will form solder balls due to surface tension after reflow.
However, this method is not recommended because controlling the amount of solder paste during printing is challenging, leading to inconsistent solder ball sizes. Additionally, the irregular shape of the formed solder balls can cause issues such as cold solder joints when later soldering to the PCB.
Generally, the aperture of the steel stencil should be slightly larger than the diameter of the pads. If the solder balls are perfectly round, the solder balls formed by this method will have a diameter roughly half the thickness of the steel stencil.
Reflow Soldering
At this point, the solder balls are installed onto the BGA pads, but further processing is required via reflow soldering. During the reflow process, the solder balls will securely attach to the BGA component. Even if the solder balls were slightly misaligned before, they will automatically adjust to the correct position due to the cohesive force of the solder.
Some workers may place both the steel stencil and the component into the reflow oven until the soldering is complete, and then remove the stencil. The benefit of this method is that it avoids the issue of solder ball misalignment when removing the stencil before the solder balls have fully solidified. However, the downside is that the steel stencil may get stuck in the solder balls after reflow, making it difficult to remove or potentially damaging the solder balls during removal.
Cleaning Process
The final step in BGA reballing is cleaning, as some flux residues may remain on the component regardless of how the process is performed. While these residues may not be noticeable in the short term, in the long run, they could cause short circuits and affect the reliability of the circuit. Therefore, thorough cleaning is required.
Additionally, the reballing process must be followed by re-soldering the BGA component within 24 hours, as prolonged exposure to air can lead to oxidation of the solder balls or component failure due to moisture absorption.
Ending
BGA reballing is a vital process that enables the repair and rework of BGA components, extending their lifespan and improving production efficiency. Whether you’re using a dedicated reballing tool, a steel stencil, manual placement, or solder paste, understanding the unique advantages and challenges of each method is essential to achieving high-quality results.
Through continuous learning and refinement of skills, engineers and technicians can consistently deliver precise and reliable BGA rework, contributing to the overall success of electronic assembly and repair.
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