Ball Grid Array (BGA) is a widely adopted packaging technology in the modern electronics industry. By distributing pins in the form of a grid of spherical solder points beneath the package, BGA enhances both chip heat dissipation and the miniaturization of electronic devices. However, this technology presents two significant challenges: soldering quality detection and rework. Typically, X-ray inspection is used to identify BGA soldering defects. In this article, TechSparks will showcase potential BGA soldering defects under X-ray equipment, analyze their root causes, and share effective methods for avoiding these issues.
Why Use X-Ray for Detecting BGA Soldering Defects
In the preceding content, emphasis was placed on must use X-ray to detect BGA soldering defects. You might wonder: why not use AOI, a cheaper and more commonly used option?
Let TechSparks briefly explain this question.
Typically, assembly factories equip both AOI and X-ray to meet high-quality inspection requirements from customers. AOI is integrated into the production line and is used to inspect the quality of the assembled PCBA board after soldering. It can quickly and effectively identify component installation issues such as missing, polarity, and misalignment, as well as soldering quality issues like solder bridges, voids, and cold joints. In fact, these defects can be detected with careful observation using a magnifying glass, and the purpose of AOI is to expedite the inspection process and avoid human errors.
In contrast, X-ray is an offline detection method primarily used for inspecting specific features rather than the entire assembly. Leveraging the penetrating properties of X-rays, it can delve into the inspection of the solder ball connections beneath the devices, making it considered the most effective means of BGA soldering quality inspection to date.
Solder Ball Bridging
Solder ball bridging is a severe soldering defect that can lead to short circuits, significantly impacting the normal functionality of the circuit. In the above image, you can clearly see two or more BGA solder balls connected together, forming a bridging phenomenon.
Analysis of Causes:
- Excessive and uneven distribution of solder paste.
- Inappropriate soldering parameters, such as excessive temperature or prolonged soldering time, ultimately causing excessive flow of solder paste.
Preventive Measures:
- Prioritize the selection of solder paste with stable quality.
- Strictly control soldering parameters, consistently monitoring the soldering process.
- Implement cooling measures after the completion of each production batch.
Missing Solder Balls
This refers to the unintended absence or dislodgement of solder balls in the packaging, which directly results in a lack of electrical connection, leading to power failure issues. This is perhaps the most easily identifiable defect since BGA arrays are typically uniform, and missing portions are clearly visible under X-ray inspection.
Analysis of Causes:
- Thermal cycling leading to solder ball fatigue.
- Use of low-quality solder or materials.
- Omissions during the ball placement process.
- Solder balls flowing into vias on the printed circuit board.
Preventive Measures:
- Use reliable material suppliers.
- Optimize the design to ensure the size of PCB pads comply with standards.
- Employ a process with a moderate temperature gradient to avoid abrupt temperature changes.
Solder Ball Shift
This manifests as the overall distortion of BGA solder balls in a particular direction, eventually resulting in incomplete alignment between BGA solder balls and PCB pads. Typically, this type of relative displacement defect does not impact electrical performance but affects soldering reliability.
As shown in the diagram, where L is the distance from the solder ball center to the pad center and D is the pad diameter, the formula for calculating the offset of solder ball displacement is L/D. The judgment for the need for rework is often based on acceptance standards set by the project, with a common criterion being L/D < 25%.
Analysis of Causes:
- Misalignment during the placement process.
- Microscopic shifts caused by thermal expansion and contraction.
- Quality issues with the BGA component itself.
- Improper pad design or mismatched dimensions during the design phase.
Preventive Measures:
- Optimize soldering processes, such as using equipment with more temperature zones.
- Prototype manufacturing to validate the design’s feasibility.
- Source components from reliable suppliers.
- Implement auxiliary positioning techniques.
Solder Ball Voids
Voids refer to the presence of air pockets within BGA solder balls, and these bubbles can have a certain impact on both signal transmission and mechanical performance. For such BGA defects, X-ray imaging systems have integrated features in their software to calculate the area of solder ball voids, helping you determine whether rework is necessary. Typically, void areas less than or equal to 25% of the solder ball’s projected area are considered acceptable, while anything beyond 25% may require rework.
Analysis of Causes:
- Volatile components in the solder material decompose into gas during soldering and are not promptly removed.
- Unstable soldering temperatures or temperatures exceeding the recommended range for the solder material.
- Presence of dirt, oxidation, or inappropriate surface coatings on the pad surface.
Preventive Measures:
- Match solder material and processes, controlling soldering temperatures based on material characteristics.
- Provide good ventilation in the soldering area.
- Ensure proper surface treatment of pads, free from dirt, oxidation, and other issues.
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