Inspection Methods for BGA Solder Ball Wettability and Soldering Defects | I am a Manufacturing Process Engineer (MPE) (2025)

This is a summary post aimed at providing some guidance to newcomers in the electronics assembly field on methods for analyzing the solderability of BGA (Ball Grid Array) solder balls. Workingbear decided to write this after seeing someone ask about this topic in an online forum. Some of the content might overlap with topics already covered in previous blog posts.

Inspecting the solderability of BGAs can be tricky because the solder joints are located underneath the component body, making it difficult to use conventional visual or optical inspection methods. As a result, special inspection techniques are required.

When inspecting BGA solder ball quality, if the issue involves solder bridging or short, it’s usually sufficient to use X-ray equipment to detect the problem. (Note: Short don’t always happen due to solder issues; they can also occur within the BGA chip itself or in the PCB’s internal layers.) However, if you’re looking for issues like non-wetting, cold solder joints, or cracks in the solder, the process becomes more complex, and X-ray might not be enough.

Currently, the industry commonly uses three main methods for inspecting BGA solderability:

1. X-Ray Inspection
2. Red Dye Penetration Test
3. Micro-sectioning (Cross-section Analysis)

Of these, only X-ray inspection is non-destructive. Therefore, it’s highly recommended to always start with X-ray inspection when analyzing BGA solderability. Even if the results are inconclusive, X-ray is still worth trying as it doesn’t damage the sample. Destructive methods like red dye testing and micro-sectioning should only be used when X-ray inspection fails to identify the problem.

It seems I forgot to mention another method—using optical equipment to inspect BGA solder balls, such as microscopes or fiber-optic cameras. However, as BGA pitch continues to shrink, this method becomes less effective. Fiber-optic cameras can usually only inspect the outer two rows of solder balls, leaving the rest inaccessible. Additionally, this method can only evaluate the solder on the PCB side, not on the component side. In some cases, mirrors can be used to reflect the image and inspect the solder joints on the BGA component side, but the increasing number and proximity of components surrounding the BGA make this even more challenging. Due to these limitations, we won’t go into detail about this method here.

Let’s Discuss the Three Methods for Inspecting BGA Solderability:

Inspecting BGA Solderability with X-Ray

Most standard X-ray machines can only provide 2D top-down images. However, 2D images often make it difficult to determine issues like non-wetting or solder cracks since the image only shows the overall shape of the solder ball. That said, 2D X-rays still have their advantages. The equipment is more affordable, and it works well for detecting shorts or voids in the solder balls. If the voids inside a solder ball are too large or excessive, it can lead to cracking issues.

Additionally, with a slight investment, the equipment can be upgraded to 2.5D functionality, allowing the product to be tilted at different angles for inspecting solder joints. This makes it easier to detect issues like Head-in-Pillow (HIP) or Head-on-Pillow (HoP) defects. For experienced users, it’s even possible to estimate soldering issues by comparing the outer diameters of BGA solder balls. If one solder ball is noticeably larger or smaller than its neighbors, it could indicate insufficient soldering. However, this is more of an experience-based judgment. For more details, refer to the article linked below.

Further Reading:

• Using 2D X-Ray to Diagnose BGA Solder Void Issues

• How to Use 2D X-Ray to Identify BGA Issues like Non-Wetting, Cold Solder, and HIP or NWO

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Recently, newer X-ray machines have been introduced in the industry, capable of generating 3D images similar to CT (Computed Tomography) scans used in hospitals. These 3D images provide a more detailed view to identify solder defects. However, these machines are expensive, making it unlikely for most assembly factories to purchase them. A more practical approach is to rent 3D CT X-ray equipment from external labs for initial inspections. If these machines can successfully detect BGA soldering defects, there’s no need to proceed with destructive testing methods.

Recommended Reading:
3D CT X-Ray Non-Destructive 3D Tomography Analysis for PCBA and BGA Failures

Red Dye Penetration Test

The Red Dye Penetration Test is a destructive inspection method typically used when non-destructive techniques fail to pinpoint the issue. Once this test is performed, the PCB and BGA component must be scrapped, and even the original evidence might be destroyed.

This method is particularly useful for examining the soldering condition of all the solder balls beneath the entire BGA. The principle involves applying a red dye solution to the underside of the BGA. The dye penetrates any small and micro cracks and remains there. After curing, the BGA is removed from the printed circuit board, and the distribution of red dye on the solder balls is examined. Both the PCB pads and the BGA’s solder balls are inspected for dye residue.

The results are typically recorded on a chart corresponding to the layout of the BGA solder balls, and the dye distribution for each ball is noted. For more detailed information, refer to the article below.

Further Reading: Red Dye Penetration Test to check for BGA solder joint cracking

Cross-Section Analysis of BGA Solderability

The cross-sectioning method is another destructive test, and it’s even more labor-intensive than the red dye test. It usually requires precise pre-analysis to narrow down the area of interest. Electrical testing is often used first to locate the suspect solder ball or specific location.

You can think of cross-sectioning as cutting into the problem area with a blade. The cut surface reveals the internal structure of the solder ball, as well as the PCB’s circuitry and pads. Sometimes, the issue might not be with the BGA soldering itself but with the PCB traces or nodes. Cross-sectioning allows for analyzing both.

However, cross-sectioning requires careful handling. If the cutting or grinding is too aggressive, it can destroy the connection structure between the BGA and the PCB. The process must be done slowly and meticulously to expose the desired cross-section. Once the section is prepared, it needs to be cleaned of dust and treated with a special solution to enhance visibility under a microscope.

Because of its time-consuming and skill-intensive nature, cross-sectioning is often outsourced to specialized labs where experienced technicians can perform the analysis.

Related Posts:

• How to Resolve HIP or HoP Soldering Issues in BGA Solder Balls
• Why BGA soldering ball always crack(1)? Stress > bonding-force
• When does SMT require Reflow Carriers and Full Process Carriers?
• How to Implement Paste-In-Hole (PIH) Process for Through-Hole Devices and Solve Solder Filled Percentage for PTH