BGA Voiding Cause and Solutions

BGA voiding is a serious problem that affects the structural integrity of its delicate solder joints. It also reduces their thermal and electrical conductivity, making them less efficient. Here, you will learn what causes the defect and how to prevent it.

Key Takeaways

• Root Cause: Voiding in BGA stems from trapped gases. It occurs during the soldering process.
• How It Affects BGA Joints: It weakens the joints and reduces their electrical and thermal conductivity capabilities.
• Poor Designs Can Cause Voids: Some voids result from poor PCB design features that prevent gases from escaping, such as open vias in pads.
• Process Control Matters: Proper control of the reflow process helps prevent voids.
• The Need for X-Rays: X-ray inspection is the most popular and effective inspection method for BGA voids.

What is Voiding in BGA Assembly?

BGA voiding is the formation of empty spaces in its solder balls. The voids are usually gas-filled. They compromise the electrical, thermal, and mechanical reliability of the soldered connections.

Voids primarily occur when trapped air or volatiles fail to escape during soldering. After cooling, the solder solidifies around the gaseous materials, trapping them inside the joint.

Excessive voiding can cause critical problems. Understanding its effects and the limits of its acceptability is, therefore, an essential part of any PCB project. Before that, let’s discuss its various causes.

BGA Voiding Cause Factors

Voiding in the solder balls of a ball grid array (BGA) can stem from a poor PCB design, wrong material choices, board contamination (including moisture), and assembly process errors. Here is how each factor leads to the fault.

Poor Board Design

Some board design features encourage voiding by promoting the trapping of gases during soldering. They include incorrect pad geometries, unfilled vias, and inadequate thermal management.

Moisture and Contamination

Moisture is another leading cause of voids. It expands during soldering, forming air-filled pockets in the joints. Various other factors, such as surface oxidation and excessive flux, can release volatile gases that remain trapped in the solder.

Solder Paste Factors

Inconsistent solder paste application or too much paste can also lead to the formation of voids. Sometimes, the paste formulation is inappropriate and more likely to outgas, releasing volatiles that get trapped in the solder.  

Reflow Profile Issues

Reflow process errors can cause BGA solder joints to develop voids by hampering the efficient escape of gases. They include short soak times or low soak temperatures, high ramp rates or peak temperatures, and rapid cooling.

What are the Dangers of BGA Voids?

Voids affect BGA connections in various ways, from causing structural weaknesses to reducing the thermal and electrical performance of the soldered joints. That, in turn, affects the overall PCB’s reliability.

Weak Connections

The empty spaces reduce the mechanical strength of the BGA joints. Such joints are more likely to crack in harsh conditions, such as excessive heat and vibrations, creating reliability risks.

Reduced Thermal Conductivity

Another major effect of voiding is reduced thermal performance. The spaces can act as thermal insulators, blocking the free flow of heat from the chip to the PCB and causing localized heating.

Board Reliability Risks

Voiding also affects the electrical conductivity of BGA joints. In sensitive, high-speed circuits, even slight disturbances caused by voiding can cause signal-integrity issues.

How is Voiding Detected in BGA Solder Joints?

BGA void detection is a complex process that requires internal imaging. That’s because the device forms hidden connections. Below is a look at the most common inspection methods and how they work.

X-Ray Imaging

X-ray imaging is a highly dependable inspection method. It works by scanning the package using high-energy electromagnetic waves. Dense areas (solder) absorb more of this energy, while empty parts absorb less.

The difference in absorption helps create an image that shows voids as lighter areas surrounded by darker regions. The above image shows how the images appear on a screen during inspection.

Other Methods

Other BGA joint inspection techniques include visual and optical methods, thermal imaging, and acoustic microscopy. In some cases, micro-sectioning is necessary to provide a more detailed analysis of the failure method.

Visual inspection uses magnification equipment to view the outer edges of the joints, while optical inspection uses imaging equipment to visualize the peripheral solder connections.

Thermal imaging works by scanning the package with infrared pulses and analyzing their absorption to create an image of the soldered connection. Acoustic imaging relies on high-frequency sound waves to visualize joints.

How to Prevent BGA Voiding

A BGA’s soldered connections are prone to voiding. Preventing the defect is, therefore, essential when working with the device. The following strategies can help minimize void formation during assembly.

Design Solutions: Use tented or plugged vias, especially for via-in-pad designs, to prevent air from reaching the molten solder and becoming trapped. 

Board and Package Storage: Store PCBs and BGAs in dry environments where they cannot absorb moisture from the atmosphere.

Moisture and Cleaning: Bake the PCB at up to 125°C to remove moisture. Ensure all soldering surfaces, including PCB pads, are free of contaminants before assembling the package.

Solder Paste Measures: Use high-quality, low-voiding formulations. Apply the paste in precise, optimal amounts and ensure an even distribution.

Solder Type: Lead-free solder has a high surface tension that can trap gases and prevent them from escaping. Ensure correct temperatures that encourage the escape of air and volatiles.

Stencil Design: The stencil for applying solder paste to BGA assemblies should be thin enough and have appropriate apertures.

Reflow Profiling Control: Extend soak times and use appropriate temperature ramp-ups. The peak temperatures should also be high enough for gases to escape, and the cooling time should be optimal.

Vacuum or Nitrogen Assembly: Vacuum-based SMT assembly reduces bubbles, while a nitrogen environment reduces oxidation to minimize voiding.

BGA Void Acceptable Levels and Criteria

A certain degree of BGA voiding is usually acceptable. These are usually negligible voids that do not affect the package’s functionality and performance. It also depends on other factors, such as the location and type of the void.

For instance, voids that form between the solder balls and PCB pads are more dangerous than internal types. A collection of tiny voids is also more critical than a single but comparatively larger void.

The IPC-7095 standard outlines the acceptance criteria for voids.  The standard specifies that voids that are less than 25% of the joint’s projected area are generally acceptable, depending on the PCB class.

– Class 1 and 2 PCBs: Generally acceptable if the total void area is less than 30% of the solder ball’s X-ray image area.

– Class 3 PCBs (High Reliability): Stricter limits apply, typically requiring the total void area to be less than 25% (and sometimes <20% for specialized aerospace/medical applications).

Conclusion

BGA voiding affects the component’s connections in many ways, from making them weak to reducing their thermal and electrical efficiency. While you cannot completely prevent the defect, assemblers must take measures to minimize its occurrence.

At Jamindo, we understand the need for reliable solder joints. We assemble and inspect your BGA components using advanced surface mount technologies and years of experience. Partner with us for all your PCB manufacturing and assembly needs.