BGA Soldering: Techniques, X-ray Inspection, and Rework

The constant progress of technology has moved electronics toward lightweight, small products. To keep up with these customer needs, SMT was introduced. Furthermore, the increasing demand for these products requires the development of high-density technologies that could be quickly assembled. This push led to the development of ball grid array technology.

However, at first sight, BGA soldering may seem difficult because the solder balls are sandwiched between the circuit board and the BGA body itself. But, PCB assembly using BGAs has proved to be successful. The benefits of using BGAs are quite significant in terms of performance and reliability.

This article covers key BGA soldering techniques, equipment, processes, and best practices for assembling, inspecting, and reworking PCBs using BGA components.

What is BGA Soldering?

BGA is a very different package to those using pins, like a quad flat pack. The pins of the BGA packages are arranged in a grid pattern, giving rise to the name. Adding more pads with balls of solder is used instead of having the more traditional wire pins for the connections. On the PCB, onto which the BGA components are to be fitted, a matching set of copper pads offers the required connectivity.

Advantage of BGA

· BGA packages offer various advantages over their quad flat pack rivals. As a result, they're being used increasingly for manufacturing electronic circuits. A few of these advantages are:

· The BGA soldering is robust: Other packages have very fine pins, and these are easily damaged even with the most careful handling. And it's almost impossible to repair them once the pins are bent. However, BGAs don't suffer from this as the connections are provided by pads with the BGA solder balls on them, which are very difficult to damage.

· High-speed performance: In BGA packaging, the conductors are on the underside of the chip carrier. This means that the leads within the chip are shorter. So, unwanted lead inductance levels are low. In this way, BGA devices offer a higher level of performance than their rivals.

· Improved PCB design: Track densities around many packages become very high due to the very close proximity of the pins. A BGA spreads the contacts over the full package area, potentially reducing the problem.

· Low thermal resistance: BGAs offer a lower thermal resistance between the silicon chip. This lets heat generated by the integrated circuit inside the package be conducted out of the device onto the PCB faster and more effectively.

Applications of BGA

Ball Grid Array soldering is used in many industries, including aerospace, computer repair, and electronics manufacturing. It's also used to improve the performance of high-speed circuits. It's some other uses are:

· Electronic device repair: It's used to repair devices such as laptops, smartphones, gaming consoles, and tablets.

· Thermal management: BGA soldering can improve thermal management in electronic devices.

· Reduced electromagnetic interference: It can reduce electromagnetic interference in devices that operate at higher frequencies.

BGA Packages

BGA packages are a type of SMT assembly that is used for integrated circuits. They offer multiple benefits over traditional through-hole components, like reliability and the ability to track density and heat dissipation.

BGA Package Types

How is the BGA soldered to a PCB - BGA Soldering Process?

The BGA soldering process is very important in modern electronics manufacturing because it ensures reliable connections between the BGA package and the PCB. However, if you need proper execution, then you should adhere to best practices to achieve high-quality results. Below, we explore the essential steps and techniques involved in this process.

BGA Soldering Techniques

BGA soldering primarily employs two techniques: reflow soldering and manual soldering. The first one involves using a reflow oven to heat the solder paste, which will cause it to melt and form robust electrical and mechanical connections. The latter is used for repairs and prototypes and requires specialized tools like hot air stations and skilled operators to achieve the best possible results.

As I've mentioned earlier, selecting a BGA soldering technique depends on the application, production scale, and complexity of the PCB design.

BGA PCB Land Patterns

Proper land pattern design on the PCB is critical for effective BGA soldering. Land patterns should match the BGA ball pitch and diameter. These patterns feature non-solder mask-defined (NSMD) pads for better solder joint reliability. Furthermore, these patterns also include via-in-pad designs for high-density boards. At last, if you adhere to IPC standards, it'll help you ensure consistent results and reduce soldering defects.

BGA Solder Paste Printing

The most critical step is to apply solder paste accurately to the PCB. Some of the considerations include:

· Stencil Design: You should use a stencil with appropriate aperture sizes that match the land pattern.

· Paste Consistency: You should also ensure the solder paste has uniform viscosity to prevent voids and gaps.

· Alignment: You should be careful to achieve precise alignment between the stencil and PCB to avoid misprints.

Component Placement

Proper component placement of the BGA package on the PCB requires precision to align the solder balls with the corresponding pads. Fortunately, automated pick-and-place machines are typically used for this step. These machines will ensure accurate positioning, gentle handling to prevent damage to the BGA package, and verification using optical inspection systems to confirm alignment.

Reflow Soldering Process

The reflow soldering process is usually used to solidify the solder joints, and it also establishes electrical connectivity. It involves:

1. Preheating: This step gradually raises the temperature, and as a result, it minimizes thermal shock.

2. Soaking: It stabilizes the PCB temperature and activates flux to clean the surfaces.

3. Reflow Zone: This step is designed to heat the solder paste above its melting point. As a result, it’ll allow it to flow and form joints.

4. Cooling: As it’s evident from this step’s name, it solidifies the solder and will prevent thermal stress on the assembly.

Hence, accurate soldering temperature profiling is important to avoid defects and mistakes such as solder bridging, tombstoning, or voids. By following the steps mentioned above, manufacturers can ensure robust connections and high-performance assemblies, and that's for applications that require ball grid array components.

BGA Solder Joint Inspection

BGA inspection is one area of the PCB assembly process that raised a considerable amount of interest when BGAs were first introduced.

Inspection Techniques

BGA inspection can’t be achieved in the normal way using simple optical techniques. Because, quite obviously, the solder joints are underneath the BGA components, and they're not visible. It created a considerable degree of unease about the technology when it was first introduced. Many manufacturers undertook test to ensure that they were able to solder the BGA components satisfactorily.

Furthermore, BGA solder joint quality requires specialized inspection techniques like visual inspection, X-ray inspection, cross-sectioning, and acoustic microscopy. These inspection techniques proactively identify any latent BGA soldering issues before products leave manufacturing.

X-ray Inspection for BGA

The solder joints can't be fully tested by checking the electrical performance. Though this form of the test of the BGA solder process will reveal conductivity at that time, it does not give a detailed picture of how the BGA solder process has succeeded. For this, the only test is a form of BGA inspection using X-rays.

X-ray inspection is able to look through the device at the soldered joint beneath. As a result, Automated X-ray Inspection became a main technology for checking PCBs that include BGAs. Fortunately, it’s noted that once the heat profile for the BGA soldering machine is set correctly, the BGA components solder very well, and few problems are encountered with the BGA solder process.

Troubleshooting Poor BGA Joints

Finding and addressing the root cause is key before attempting BGA rework. Potential root causes of bad BGA solder joints include:

· Thermal stress cracks

· Solder ball defects or damage

· Misalignment between balls and lands

· Moisture absorption under package

· Contamination preventing wetting

· Insufficient solder paste height or volume

BGA Rework and Repair

As anticipated, it isn't easy to rework BGA assembles unless the correct equipment is available.

Overview of BGA Rework

The steps in a typical BGA component rework process are:

1) Preparation: First of all, review the original assembly process for potential factors. Also, ensure that replacement tools and components are ready.

2) Removal: In this step, clean the pads thoroughly, leaving no residues. Then, reapply flux to prepare for new balls. Now, redress pads and land PCB land pattern.

3) Reballing: First, use a stencil to apply new solder balls to the BGA package and then reflow balls to attach to package terminals.

4) Replacement: Now, must use adhesive to secure component temporarily, carefully realign new BGA on site, and reflow to form connections.

5) Inspection: At the end, alignment and ball connections must be verified, and any collateral damage to pads or board must be assessed.

Rework Equipment

Typical BGA rework equipment includes:

· The preheater gradually heats the board to avoid thermal shock.

· The hot air nozzle directs the heated air stream for localized heating.

· The microscope provides high magnification to inspect alignment and joints.

· PCB support fixture secure board under the component to prevent heating.

· Closed loop temperature control of nozzles is also one of the reworked equipment.

· Convection rework oven is for small boards requiring a full oven thermal profile.

· The BGA toolkit provides alignment guides, fluxes, balls, stencils, and adhesives.

These specialty rework tools help to remove and replace BGAs properly with minimal collateral damage.

Rework Process

If a BGA component is suspected of being faulty, then it's possible to remove the device. This is achieved by locally heating the BGA component to melt the solder underneath it. In the BGA rework process, the heating is often accomplished in a specialized rework station. This comprises a jig fitted with an infrared heater, a vacuum device for lifting the package, and a thermocouple to monitor the temperature.

Great care is needed to ensure that only the BGA is heated and removed. Other devices nearby need to be affected as little as possible; otherwise, they may be damaged.

Conclusion

To sum up, BGA technology in general and the BGA soldering process in particular have proved themselves to be very successful since they were first introduced. They are now an important part of the PCB assembly process used in most companies for prototype PCB assembly and for mass production.

PCBasic offers BGA rework stations to help you set up your in-house end-to-end BGA rework process. Let us help you avoid common BGA rework mistakes that are familiar to those who have worked on these unique components.

Alternatively, you can contact us today to discuss our service offerings, including all aspects of BGA reworking.