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.
Another critical factor to be considered for PCB inspection is the integration of image processing algorithms within the camera to increase system performance. Processing speed for inspection algorithms can be a bottleneck. New cameras, such as the ISG LightWiseTM, contain CPUs, FPGAs, and on-board memory to accelerate the algorithms (see Figure 3). In some cases, inspections can be done completely inside the camera, eliminating the need for a PC.
Keep in mind the lighting needs and how well image detail is captured at the current lighting level. It may be necessary to add illumination. If designed correctly, a light, or in some cases multiple lights, can insure that key areas of the board are more visible for image capture, while de-emphasizing areas not of inspection interest. Many situations will require lights that may need setup time to be directed to the right location. Multiple inspection stations may also be used to provide an inspection after key operational steps in the PCB manufacturing process.
Depending on the manufacturing environment, the durability and reliability of the cameras and support system may need to be considered. Extra efforts may be called for to protect the inspection system and to insure ongoing operation and less downtime. Some PCB manufacturing steps, for example, include washes and may require a hardened enclosure with forced air to keep the outer window clean and dust free.
A more sophisticated system may also require some advanced training for the manufacturing team and maintenance staff. As an option, some firms rely upon the imaging integrator or installation team to oversee this component of inspection systems as an additional service.
Installing a System
There are many variables and considerations that quickly make an installation very complex. Thus, the design and installation will require very deep experience with different options and systems; an imaging integrator or installation partner can play an important role in the installation process. Utilizing an imaging industry partner with design experience with systems used for PCB inspection can help get a project to completion, offering a range of options to meet PCB manufacturing needs.
A well-designed PCB inspection system is a requirement in the competitive market. An incorrectly designed system, even with minor inefficiencies, can cause a financial drain on board manufacturing and adversely affect customer relationships. The reward in working with an inspection system expert and choosing the proper equipment will lead to a mutually beneficial outcome for all concerned.
This article was written by Brad Gouldthorpe, President of expand Business Resources (Rochester, NY). For more information, Click Here .