PCB manufacturing is a very competitive market, and manufacturers must be able to confidently ship accurate printed circuit board (PCB) products. Delivery of incorrect boards may have a significant, negative impact on the company reputation, which can directly result in lost business. Inspection of the boards before shipment is required.
Many customers specify PCB inspection requirements so they can be assured of receiving a high-quality product. As PCBs become smaller, more complex, and more quickly manufactured, inspection has become more difficult to successfully accomplish. Magnifiers and microscopes have been used for inspection, but the methodology often cannot meet the speed and/or resolution requirements to accurately inspect high-volume PCBs. Suppliers, therefore, are moving quickly to a more sophisticated approach using cameras to automate the various inspections required.
A growing, alternate PCB assessment approach uses cameras for automatic optical inspection of the boards. Of particular interest is to find the faults in the manufacture of the boards by comparing each board to an ideal standard. It is critical to identify these errors early so the manufacturing can be modified before bad products are shipped. Cameras have been very successful in identifying short circuits, voids, misalignment, missing or incorrect components, and other defects, such as scratches and stains. Online inspections during soldering are also vital as issues with solder paste integrity and profiles affect successful outcomes (see Figure 1).
Why a Camera?
Microscope inspection worked for years when production speed for PCBs was not as demanding and when the PCBs were larger and less complex. As the trends toward faster, smaller, and more complex products took hold, the inspection step became too slow.
In addition, operator eye strain and fatigue distort the inspections. Cameras used for quality monitoring have made it easier for operators or inspectors to view product and for the inspection system to make comparisons to a standard.
Cameras offer the following advantages over microscope inspection:
- Autofocus capability
- Larger depth of field
- Improved resolution
- Better board access during re-soldering work and repair
Cameras have shown to be highly successful with bare board and solder paste inspection, as well as for evaluation of component placement prior to reflow, post-reflow component conditions, and open or short circuits (see Figure 2). Problems in the solder and assembly area of a PCB can be seen early in the production process.
There are a number of design factors one must consider for PCB inspection cameras, including the choice of color or monochrome, resolution level, frame speed, and line or entire frame capture method. These decisions, of course, will influence the speed of image capture and processing.
Historically, Charge Coupled Device (CCD) cameras were preferred to capture the detail needed for an accurate inspection at faster production speeds. CCD sensors are inherently global shutter by nature as every pixel is captured simultaneously. Because of their low noise floor relative to other types of sensors, the CCD cameras are quite desirable. A few limitations do exist, however, and need to be considered. CCD sensors are generally slower and more expensive than other types of sensors, such as CMOS image sensors.
Comparing cell phone CMOS image sensors to today’s industrial CMOS image sensors is not a fair comparison. While cell phone CMOS imagers offer acceptable images for consumers, they are not suitable for inspection purposes. CMOS imagers designed for the industrial market offer much larger pixels for improved sensitivity (better image quality) and very fast performance (high frame rates or line rates).
CMOS cameras are a valid alternative to CCD cameras that are capable of higher speeds at an overall lower cost. Many camera vendors offer both CCD and CMOS sensors. The options enable customers to choose the proper camera for the application and make the best trade-offs. Very large PCB boards can be inspected using high-resolution CCDs (~29Mp) in a single pass, while some inspection lines run very fast and examine boards at hundreds a second, which usually are accomplished with high-frame-rate CMOS imagers.