Automated Optical Inspection has high return on investment.

It is vitally important for manufacturers to be able to confidently ship accurate printed circuit board (PCB) products.  PCB manufacturing is a very competitive market. Delivery of incorrect boards can have a significant negative affect on the company reputation, which can directly result in lost business. It is well known that in competitive markets, any business lost is difficult to gain back.


 

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Therefore, inspection of the boards before shipment to customers is required. Many customers specify PCB inspection requirements so they can be assured of receiving quality, working product. However, as PCBs become smaller, more complex, and manufactured faster, inspection has become more difficult to successfully accomplish. Magnifiers & Microscopes have been used for inspection but this methodology often cannot meet the speed and/or resolution requirements to accurately inspect high volume PCBs. Therefore, suppliers are moving quickly to a more sophisticated approach using camera to automate the various inspections required.

An Alternative Approach

A growing alternate approach is to use cameras for optical inspection of the boards. Of particular interest with inspection 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 so that bad products are not shipped. Cameras have been very successful 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.

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 affects 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.

It is generally accepted that cameras offer the following advantages over microscope inspection:

  • Auto focus capability
  • larger depth of field
  • improved resolution
  • For repairs, cameras allow better board access and watching during re-soldering work

inspecting-circuit-boardsCameras have shown to be highly successful with bare board and solder paste inspection, as well as for evaluation of component placement prior to reflow, for post-reflow component conditions, and to check for open or short circuits. Problems in the solder and assembly area of a PCB can be seen early in the production process.

Camera Selection Considerations for Inspection Applications

There are a number of design factors one must consider for PCB inspection cameras. Some of the more significant options are about whether the need for color or monochrome is required, the resolution level, frame speed, and line or entire frame capture method. Of course, these decisions will affect 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. CCD sensors characteristically have a low noise floor relative to other types of sensors. These make CCD cameras quite desirable. A few limitations do exist however, and these 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 decent 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 generally lower cost. Many camera vendors offer both CCD & CMOS sensors. This enables customers to choose the proper camera for the application and make the best trade-offs.

Pictured is the Imaging Solutions Group (ISG) LW-AL-CMV4000-USB3-C camera. It has 4Mp (2048 x 2048) and can operate up to 170 frames per second. The ISG LightWise™ Allegro USB3 family supports the CMOSIS 2, 4 & 12Mp sensors and also the OnSemi Truesense Imaging CCD family. This offers customers a wide variety of sensors in an inexpensive intelligent programmable camera platform.

Other Considerations

There are many other factors to be considered for PCB inspection. Only some of the more significant ones will be mentioned here.

One critical area where the Imaging Solutions Group (ISG) can be a valuable partner is with the integration of image processing algorithms inside the camera to increase system performance. Processing speed for these inspection algorithms can be a bottleneck. The ISG LightWise™ cameras contain CPU’s, FPGA’s and plenty of on-board memory to accelerate these algorithms. In some cases, inspections can be totally completed 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 set-up time to be directed to the right location. Another design factor is the use of multiple inspection stations during board manufacturing. This provides 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 required to protect the inspection system and to insure ongoing operation and less downtime.

A more sophisticated system may 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

As can be discerned from the prior text, there are many variables and considerations that can quickly make an installation very complex. Thus, the design and installation will require very deep experience with different options and systems. This is where an imaging integrator or installation partner can play a role. Utilizing an imaging industry partner with design experience with systems used for PCB inspection can help get a project to completion. They can provide a range of options that can likely meet just about any PCB manufacturing need.

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. The return on investment for automated optical inspections makes end users of products happy customers.

Author:
Brad Gouldthorpe
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