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Knocking Down the Bone Pile: X-ray Inspection of Ball Grid Array Solder Joints

Ball grid array (BGA) devices are ubiquitous in electronic products primarily due to their advantages, including compact packaging, greater durability than leaded devices, and improved performance from shorter signal paths.

BGAs improve electrical performance by reducing inductance and capacitance between the internal silicon die and the PCB, resulting in superior signal integrity and faster operation speeds. BGAs also offer superior thermal performance by dissipating heat more effectively, reducing the risk of overheating.

BGAs provide high density interconnection because, unlike perimeter-only packages such as quad flat pack (QFP) devices, BGAs utilize the entire bottom surface of the device, allowing for a much higher number of I/O connections in a smaller footprint. However, since the solder interconnections of a BGA are hidden beneath the device, inspecting solder joints is challenging.

Solder Joint Inspection Methods
A major advantage of BGAs in the PCB assembly process is their ability to self-align during reflow soldering, as the surface tension of molten solder balls helps position the component on the PCB pads. Several methods can be used to inspect BGAs after solder reflow on a PCB, including visual, endoscope, and X-ray inspections.

• Visual inspection: Used with an optical microscope to view the outer rows of a BGA for wetting and ball-to-pad alignment
• Endoscope inspection: Used to view some inner rows of a BGA for wetting and alignment otherwise inaccessible by an optical microscope
• X-ray inspection: Used to inspect internal solder joints, detect voids within solder balls, and identify bridging between solder balls that cannot be seen visually
• Electrical testing: Functional testing to ensure BGA connectivity, but requires specialized equipment and skilled technicians, and is a time-consuming process

While optical microscopes and endoscopes for visual inspection are useful as an initial check of a BGA assembly, they are limited in the ability to detect other critical parameters, such as voiding and solder bridging.

Acceptance Criteria
The key acceptance criteria for BGA assembly per the IPC-A-610 and J-STD-001 standards include:

• Alignment: Solder balls are centered and show no offset of the ball-to-land centers or spacing violations
• Solder ball spacing: Solder balls do not violate the minimum electrical clearance
• Solder connections: BGA solder balls contact and wet to the land, forming a continuous elliptical round connection; no solder bridging, opens, missing balls, or unsoldered connections
• Solder ball uniformity: While not considered a defect, solder balls that are not uniform in size, shape, coloration, and color contrast are considered a process indicator for Class 2 and Class 3
• Solder voids: 30% or less of any ball in the X-ray image area
• Solder ball appearance: Solder balls should show evidence of proper reflow, wetting, and uniform collapse across the array
• Head-in-pillow: No head-in-pillow defects
• Foreign object debris (FOD): No signs of conductive or loose debris that violates clearance or can affect function under or around BGA

Popcorning

The term “popcorning” refers to a failure mode associated with moisture ingress into surface mount components such as BGAs. If moisture-sensitive devices, including BGAs, are not retained in a nitrogen-dry storage cabinet, they are susceptible to the popcorn phenomenon. Popcorning occurs when a small amount of moisture trapped within the polymer molding of a BGA is converted into a large volume of steam during the solder reflow process. This causes the BGA package to expand like a kernel of cooked popcorn, potentially leading to cracks and field failures.

Popcorning of BGAs can be indicated by bridging between adjacent solder balls beneath the device, since the bottom of a BGA is its thinnest point. This occurs when the package expands during solder reflow, causing it to “dish” downward; the underside of the package deforms and presses down on the solder balls. As the solder is liquid at the time, it allows solder from adjacent balls to coalesce, thereby forming a solder bridge underneath the BGA device.

The most typical cause of popcorning is the hygroscopic sensitivity of the molding compound used to protect the die. Device manufacturers have been aware of package moisture sensitivity levels (MSL) for many years and have procedures in place to treat components prior to their use in the reflow process.

With the move to lead-free solders, which have substantially higher reflow temperatures, BGA packages are more susceptible to popcorn. This is due to the peak temperature a BGA is exposed to, with lead-free alloys approximately 20°C higher than tin-lead solders. As a result, it is suggested that a typical MSL will increase between one and three levels for the same device when used in a lead-free process.

Common Causes for Rejection

Several anomalies can occur during the assembly of a PCB containing BGA devices. The most common are incomplete solder melting, excessive voids, misalignment, or missing solder balls.

• Partial reflow or cold solder: Incomplete melting of solder balls during the solder reflow process, resulting from inadequate time-temperature profile or insufficient dwell time
• Excessive voids: Large or clustered excessive voids within solder balls, greater than 30%. Alternatively, this voiding requirement can be as low as 10% for mission-critical applications such as aerospace, defense, and medical products, etc.
• Misaligned or shifted BGA: Device package should be located over its intended PCB land/pad with no gross offset that risks contact with the wrong land, insufficient connection, or reduced electrical clearance
• Missing or broken balls: Missing or damaged solder balls prior to placement

X-ray Inspection
Transmissive, real-time X-ray with a tilt function should be used to document several potential defects during BGA inspection. Solder bridges are the most common defect detected by this technique. Gross voids are visible at higher power and magnification levels.

Defects such as opens and voids can sometimes be difficult to discern even with high-quality X-ray equipment. Inspection technicians need adequate training to interpret X-ray images and identify all types of defects.

In addition, the concentricity, pitch, circularity, and solder ball diameter can also be measured as part of the BGA inspection process. With the proper equipment, which either tilts the sample or the detector, even head-in-pillow defects can be noted.

Endoscopic inspection is a valuable supplemental tool for BGA inspection because it allows inspectors to view beneath the BGA and assess visible solder joint surfaces that may not be fully characterized by X-ray alone. It can help determine whether reflow produced an acceptable joint condition or signs of defects by revealing surface characteristics such as texture, uniformity, smoothness, color, brightness, wetting, and possible micro-cracking. Endoscopes also provide useful documentation through captured images and video, supporting inspection findings, customer review, process improvement, and failure analysis.

Conclusion
The ability to rework and inspect hundreds of BGA types across a wide range of board designs, using both industry-standard and customer-specific acceptance criteria, is essential. A robust BGA inspection protocol should combine proven X-ray inspection expertise with the latest X-ray tools and techniques to reliably identify the full range of potential BGA defects.

It is also important to avoid excessive X-ray exposure during inspection, particularly when working with commercial off-the-shelf semiconductor devices, where radiation effects may be a concern. Methods for minimizing the impact of X-ray radiation will be discussed in a future column.

This column originally appeared in the July 2026 issue of SMT007 Magazine.