What Is Visual Inspection in PCB and PCBA?
PCB Visual Inspection is to inspect whether the printed circuit board, or its specimen/coupon, and PCB assembly (PCBA) meets customer’s requirements and related quality acceptance standards either with naked eyes or through optical equipment. The visual inspection goes throughout the design, manufacturing and assembly processes
Nowadays, PCBs are complex. While this complexity makes room for lots of new capabilities, it also creates more chances for failures. As PCBs have gotten smaller, denser and more advanced, so have the inspection techniques and technologies used to ensure their quality.
Manual Visual Inspection
The simplest form of PCB inspection is manual visual inspection. The technicians will compare the board to the Gerber files to ensure that all specifications were met. They’ll also look for common defaults. The type of defects they look for vary depending on the kind of board they’re checking and the components on it.
It can be useful to perform a manual visual inspection after nearly every step in the PCB fabrication process, including assembly. MADPCB conducts an manual visual inspection under magnification of every board we produce. You may also wish to follow manual visual inspection with a more advanced, comprehensive examination.
Inspection staff members inspect nearly every aspect of a board and look for various common defects on each of them. A typical visual PCB inspection checklist might include the following:
- Ensure that the board’s thickness is correct, and check for surface roughness and warpage.
- Check the dimensions of the assembly match the specification, and pay particular attention to dimensions related to electric connectors.
- Inspect the integrity and clearness of conductive patterns, and check for solder bridging, open circuits, burrs and voids.
- Check the surface quality, then look for the existence of pits, dents, scratches, pinholes and other defects on printing traces and pads.
- Verify that all vias are in the correct positions. Make sure they aren’t missed or inaccurately punched, that their diameter matches design specifications and that there are no voids or nodules.
- Inspect the firmness, roughness and brightness of pad plating, and check for any bulge defects.
- Assess coating quality. Check the color of plating flux and whether it is even, firm and in the correct position.
Automated Optical Inspection
You can also use a optical inspection machine to conduct a visual inspection. This method is called automated optical inspection (AOI).
An AOI system conducts an inspection using several light sources and one or more still or video cameras. The light sources light up the board from various angles. The cameras then take still images or video of the board, compiling them to create a complete picture of the unit. The system then compares the images it captures with information about what the board should look like taken from design specifications or an approved completed unit.
An AOI can find many of the same defects that a manual visual inspection can, including nodules, scratches, open circuits, solder thinning, missing components and more. At MADPCB, we sometimes use AOI, especially for more complex boards and higher-volume production runs. Our AOI system can reveal lead-level defects for components as small as 16-mil pitch. It can accurately detect bridging, insufficient solder, reversed polarity and damaged parts.
AOI is a well-established, accurate technology and can detect many well-known faults in PCBs and PCB assemblies. It’s useful at many stages of the PCB fabrication process. It’s also faster than manual visual inspection and eliminates the possibility of human error. Like manual visual inspection, it can’t be used to inspect components that are out of the line of sight, such as connections that are hidden underneath ball grid arrays (BGAs) and other types of packaging. It might also not be effective for PCBs that have a high concentration of components because some of them may be hidden or shadowed.
X-Ray Inspection
With the rise of surface mount technology, PCBs have become more complex. Boards can now be more densely populated, feature smaller components and include chip packages such as BGAs and chip scale packages (CSPs) through which you cannot see hidden solder connections. These features present challenges for visual inspections such as manual visual inspection and AOI.
To overcome these challenges, you can use x-ray inspection equipment. Materials absorb x-rays according to their atomic weight. Heavier elements absorb more, while lighter ones absorb less, enabling you to distinguish between materials. Solder is made of heavy elements such as tin, silver and lead, while most other components on a PCB are made from lighter elements such as aluminum, copper, carbon and silicon. As a result, solder is easy to see during x-ray inspection, while most other components, including the substrate, leads and silicon integrated circuits, are hardly visible.
Rather than being reflected like light, x-rays travel through an object, forming an image of it. This process allows you to see through chip packages and other components to inspect the solder connections beneath them. X-ray inspections will also let you see inside solder joints to reveal bubbles that you couldn’t see using AOI.
Besides, it’s an important approach to use X-ray measurement device to measure the copper plating and copper reduction thicknesses in the Via-in-pad process, especially for HDI board and rigid-flex board production.
X-Ray systems can also see the heels of solder joints, which are masked by leads during AOI. Also, with x-ray inspection, there are no shadows to get in the way. For this reason, x-ray inspection works well for densely populated boards. You can conduct manual x-ray inspection using x-ray inspection equipment, or you can conduct automated x-ray inspections (AXI) using an automated x-ray system.
X-ray inspection is ideal for more complex boards and offers some capabilities that other inspection methods do not, such as the ability to see through chip packages. It also works well for inspecting densely packed PCBs and allows for a more detailed inspection of solder joints. The technology is a bit newer and more complex, and it can be more expensive. It might only make sense to invest in x-ray inspection equipment if you have a high volume of densely populated boards with BGAs, CSPs and other such packages.
If the design of your PCB requires a unique form of inspection, MADPCB can offer specialized inspection services to meet your needs. Our in-house team of experts knows the importance of quality control and we will work with you to make sure your PCB assembly meets all expectations of performance. We would love to work with you to determine if our PCB X-Ray inspection service is the right process for your product.