While inspection techniques have advanced to a point where they can catch many defects before they proceed through the manufacturing process, it is always ideal to avoid irregularities. This requires careful planning from the design team. To maximize the chances of achieving successfully soldered joints on a BGA, design teams should consider the following in the PCB layout:
- BGA footprints: PCB designers need to make sure they have the correct footprint pattern for the BGA device they wish to use. Ensure the footprint is correct and take care to design the PCB footprint pad precisely to avoid shorts and solder issues during assembly.
- Thermal management: The PCB layout must consider thermal mass. Grouping BGAs closely together can cause a thermal imbalance between highly populated and less populated parts of the board. For example, applying enough heat to properly solder closely grouped BGAs may result in burns in less populated areas. Conversely, applying sufficient heat to avoid burning less populated areas may result in voids in the BGAs.
- Via placement: Vias or through-holes are common features of PCBs, but the placement of vias can affect the behavior of solder in the BGA assembly process. Vias adjacent to a BGA pad can result in excess solder running from the pad to the via, causing a short. To avoid this, tent any vias adjacent to a BGA pad with solder mask.
On top of these design considerations, you should also take care to develop and verify a proper reflow profile before assembly. This involves using a profiler to measure and log the temperature range at the BGA ball/pad junction when the board passes through the reflow oven. This can be done with scrap bare PCBs and BGA parts. This data can be used to ensure the reflow profile for the paste and the thermal profile of the PCB is correct, plus identifies any design issues before assembly.
What are the BGA inspection techniques?
Avoiding defects is the goal, but BGA inspection is always necessary to analyze the connections between a BGA and PCB. Because it is almost impossible to see the solder joints through direct visual observation, this analysis is accomplished using various inspection techniques and tools. These tools and techniques have evolved over the years to be dependable and user-friendly. While these tools cannot be used individually to cover all aspects of inspection, they can provide a comprehensive view of the BGA when combined.
Inspection tools and techniques are often organized into the following categories:
1. Electrical Testing
Electrical testing of BGAs is another inspection process that tests the board’s electrical properties. This is different from mechanical testing, which is a destructive process that subjects the BGA to shock and shear tests to evaluate the quality of solder joints under stress.
Electrical testing runs a current through a BGA to analyze the electrical stability of the board. The test determines whether the current is on or off after the BGA component is connected. This type of test is useful for identifying if there are any electrical problems such as shorts or opens. However, as previously discussed, electrical tests can only identify the presence of these defects — they cannot be used to locate where a defect is on a BGA.
While electrical testing is an important step in the inspection process, it is difficult to pick up test points under BGA components. It is also difficult to meet assessment requirements with electrical tests alone. Instead, electrical tests are often used in combination with other test techniques to identify issues in a ball grid array inspection.
2. Optical or Visual Inspection
Optical inspection, also referred to as visual inspection, is the use of optics technology to view a BGA and its connections. This technique historically used the naked eye or microscopes, which delivered limited results. However, introducing the endoscope has proven particularly valuable for this inspection technique.
The endoscope was originally designed for medical applications, but the technology’s ability to visually inspect tiny objects in confined areas makes it ideal for BGAs. The technology involves using a camera with a high magnification lens and a connected monitor to gain close-up images of an inspection area. This technology is often paired with visualization software for advanced image capture and measurement.
Visual inspection with an endoscope allows technicians to look at the outer row of connections between a BGA and PCB — with good lighting, technicians can also view some interior rows. This optical technique helps identify and assess the overall quality of a solder joint, including the joint’s shape, surface texture and appearance, which can all indicate different defects.
Optical inspection can also identify shorts, opens, debris and cold solder and is essential for assessing rework, as it can identify proper reflow coverage. However, the visual capability of this inspection technique is limited and does not allow for detailed analysis of internal connections.
One final note about endoscopes is that they are a highly versatile piece of inspection equipment. Endoscopes can be used to inspect BGA solder joints as well as various surface-mount and through-hole mount assemblies that can be difficult to analyze through normal methods.
3. X-Ray Inspection
The most advanced inspection method for BGAs is X-ray inspection. X-rays work by emitting X-ray energy from an X-ray tube onto the board. X-rays pass through the board at differing amounts, with more X-rays trapped in dense areas such as solder joints. The X-rays are then collected at a detector, which translates them into visible light and generates an image.
In this image, solder joints are darker than their surroundings, allowing the viewer to see solder patterns. The resulting image will show a uniform grid of identical dark circles in an ideal board, the circles being the solder joints.
The X-ray test results in an image where the ball grid and solder patterns are easily seen and inspected from a top-down angle. More advanced X-ray inspection technology features a tilt function, which can analyze the shape of solder joints from different angles. These images are particularly useful for seeing variations in solder patterns and shapes, making this a good technology for identifying solder bridges, popcorning, excess solder and similar defects. This technology, however, is not good at detecting opens.
Like endoscopes, X-ray inspection technology can be used in various applications outside of BGA inspection. Some examples include via and through-hole inspection, surface-mount defect analysis and unleaded solder inspection.