Automated Optical Inspection (AOI)

Automated Optical Inspection (AOI) was a step implemented several decades ago to improve the PCB product yields. It was originally started to inspect PCB layers.  The layers could be scrapped at an early stage reducing cost of manufacturing. The defective layers could also be reworked to an acceptable level.  

AOI, automated optical inspection is an automated vision inspection of PCB during the manufacturing process. It is used to scan the inner layers and outer layers of a PCB after the processes of etching and stripping. After scanning by an AOI machine, the defects which do not meet the manufacturer’s requirement will be identified by the machine. In this way, AOI can detect problems early in the production process, so faults would not be passed to next production process and production cost could be saved. Therefore, AOI can increase the quality of PCBs and reduce the production cost.

About the AOI Machines

The basic principles of the AOI machine are to have a motion system in combination with a camera and specialized software. The market has older generation machines utilizing monochrome cameras that can detect transition from substrate to copper surface finish, this allows for the image to be compared to the CAD data or the learned image.

The Camera

There are colour and monochrome camera types.  For AOI machines, they only need to classify copper and substrate, so a monochrome type of camera is enough for them.  Modern AOI cameras are digital using the full colour spectrum. This allows for finer and finer lines/traces to be inspected.  In digital imaging, a pixel (or picture element) is a single point in a raster image. The pixel is the smallest addressable screen element; it is the smallest unit of a picture that can be controlled. Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a two-dimensional grid, and are often represented using dots or squares. Each pixel is a sample of an original image. More samples typically provide more accurate representations of the original.

The intensity of each pixel is variable. In colour image systems, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.

The number of pixels is also an important parameter of cameras. Larger number of pixels does not mean it can detect smaller defects. To detect smaller defects, smaller pixel size is required.  Pixel size is calculated by scan width / number of pixels.

For example, the scan width is 2 inches and number of pixels of camera is 2000. Pixel size = 2i inches/2000 = 1mil.

Some machines use cameras with fewer pixels.  But they can use more cameras to compensate.  Normally the exposure time for more pixels CCD is longer.  So, the scanning time of higher pixel CCDs is longer too.  Therefore, some machines use multiple CCDs with less pixels rather than using one camera with more pixels.

monochrome and color difference

The camera will only work with suitable light to enhance the surface finish of the inne rlayer or outer layer.  Older systems will use halogen lamps, infrared lighting or LED’s for modern machines.  Each light has a distinct advantage depending on the type of product you are trying to inspect. 

The light of AOI machines consists of specular and diffused light.

Specular light and diffuse light on flat surface

For specular light, light ray incident on the surface in a single angle and reflected into a single direction by the surface like mirror-reflection.   For diffuse light, the light incident on the surface is from many directions and reflected in many directions.

Consider a dent on a line, the surface of the dent is not flat, most of the specular light will not be reflected to the camera.  But diffuse light will be less affected by the surface and there will be some part of diffuse light being reflected to the camera. 

Figure 25 – Specular light and diffuse light on a dent

If the diffused light intensity of the dent is high enough, the brightness of the dent will be similar as the brightness of flat surface.  This small brightness difference may cause the machine miss detecting the dent.

On the other hand, if the intensity of diffused light is reduced, then it may cause some false catching, such as oxidation, dust, oil stain, etc.

The Software

Some machines use a basic pixel by pixel comparison.  Although effective, it can increase processing time.

Detect Defects by pixel by pixel

Some machines use logical methods to detect the defects.  They use different methods such as complex contour comparison and design base inspection to detect the defects on different features.  So, the process time would be longer.  To maintain short process time, the machine needs faster computers for comparison using pixel by pixel method. 

Detecting Defects

For AOI, the main two factors that determine whether the defects can be identified are the size and brightness/color of defects.  The pixel size of the AOI machines determine the minimum size of defects it can detect.  The pixel size can be seen as the basic unit of the scanned image.  Therefore, the minimum defect size detectable must be larger or equal to pixel size.  But even if the defect size is the same as the pixel size, the defect might be very difficult to detect.  For example, in the following case, defect size is same as pixel size. 

For case A, it is the best situation where the defect is inside the pixel, so the defect is easily detectable.  For case B, it is the worst situation as each of the four pixels can only see the 25% of the defect, so it may not detect the defect.  Therefore, if the defect size is very small and is close to the pixel size, then it may not be detected by the machine.

Case for defect size equal to pixel size

Case for defect size equal to pixel size

Another factor which makes the defect difficult to identify is brightness/color of defect.  If the brightness/color of the defects is close to normal area, then it will be difficult for the machine to detect the defects.  The common missing defects on AOI and AVI are scratches and dents. 

A verification station is used offline to the AOI to verify or repair defects.

Why Gardien’s Solution?

Your Gardien Local Service Centre may offer this service.  The AOI tooling and production services are controlled by Gardien’s proprietary job flow system called Ontrack (link to Ontrack).   This creates a seamless and error free flow of information from the customer supplied data to the Service Floors at Gardien.

Gardien’s team is trained and qualified on all internal process as documented Quality Management System.  The internal process has specific inputs for incoming, certified, and not good boards as well as descriptive educational programs on various board types, and surface finishes. 

Gardien certifies each order processed with a Certificate of Compliance with details about how the order was processed, what specifications were used to certify the product, equipment used with calibration expiry date, team member who processed the order, quantity, and failure analysis.

Gardien strongly recommends that the test sequence and parameters are clearly stated on the manufacturing drawing or at a minimum agreed upon during the quoting or contract realization phase.    In addition, any PCB sent to us for processing should have a unique identifier on each peace for electronic traceability.

KEVIN SYVRET

FINANCE DIRECTOR, NORTH AMERICA

Mr. Syvret joined Gardien Services Canada as Financial Controller in 2001 and assumed the role of Finance Director North America in 2007. Over the past 20 years, Mr. Syvret has held senior finance and accounting roles, including with Canadian Pacific Ltd and Sprint Inc.

NIRAJ PATEL

VICE PRESIDENT, GARDIEN NORTH AMERICA

Mr. Niraj Patel joined the company on a part time basis in 1993 while attending college full time. He worked in various positions until he graduated from the Centennial College Computer Systems Technology program, whereupon he was hired full time. He progressed through several testing and CAM department positions and was appointed North American CAM Manager in 1998. After 18 years with the company, Mr. Patel was promoted to Senior Vice President of Operations, Gardien Services Canada Inc. in 2011. In January, 2015 Mr. Patel became Vice President NA of the newly combined organizational unit North America which includes the Canada and USA.

TODD KOLMODIN

VICE PRESIDENT QUALITY, NORTH AMERICA

Mr. Todd Kolmodin, a native to the Pacific Northwest was born in Seattle Washington. At a young age he moved to the Portland Oregon area. In March of 1986 he graduated from ITT Technical Institute with a degree in Electronics Engineering Technology. Todd began his tenure in the Electrical Test field that same year.  Todd spent eight years working in an independent testing facility in Wilsonville Oregon where he built skills in all aspects of Electrical Test and Quality Assurance. 

Todd then spent a short term with Yamamoto Corporation on their Engineering staff covering PCB drill, finishing and Electrical Test before joining Probe Test Fixtures in 1995. His charge was CAM and Fixuring Manager.  Todd joined the Gardien Group in September 1998 continuing the same roll and taking on the responsibilities of Quality Manager for Oregon operations. In 2008 Mr. Kolmodin was promoted to Vice President Quality USA. In this position Mr. Kolmodin oversees and maintains the Gardien ITAR registration, ISO Multi-Site registration and multiple site Lab Suitability with the Defense Logistics Agency Land and Maritime. Now also maintaining the Gardien QMS system Todd now serves as Vice President Quality North America.

Mr. Kolmodin is also a published columnist and Technical Presenter with multiple appearances at the IPC Apex Expo Conferences in both San Diego and Las Vegas. 

ROLAND VALENTINI

CHIEF OPERATING OFFICER

Mr. Valentini was appointed COO in July 2010 after successfully serving as head of North American operations since 2005.

Roland has over ten years’ experience in the PCB industry, including sales, service, and project management at Atotech GmbH, Germany mainly relating to PCB plating equipment.
He has significant sales experience from his tenure at Mack Rides GmbH and headed up worldwide installations for Lurgi Bischoff GmbH.

Mr. Valentini received a Master’s Degree in Mechanical Engineering from the University of Mannheim, Germany.

RICHARD HORSMAN

EXECUTIVE CHAIRMAN

Prior to Richard joining Gardien in September 2014, he was Chairman/CEO of tool chain vendor Atego Group Systems Limited which was sold to PTC Inc.

Prior to joining Atego, Richard held a number of Executive and Non-Executive roles including Senior Independent Non-Executive Director of Plethora Solutions, an AIM listed Pharmaceutical Company and CEO of AIM listed Cybit Holdings plc. where he grew the company from inception to revenues of over 25 million and took the company through multiple acquisitions before managing a sale of the business to Francisco Partners.

Richard previously held a number of senior roles in the Software industry.

RICK MERAW

GROUP VICE PRESIDENT QUALITY

Mr. Meraw was appointed Group Vice President of Quality in Jan 2015 after successfully  serving as Senior Vice President of USA operations since 2011.  Prior to being responsible for USA operations Mr Meraw was responsible for Canadian operations since 2001.

Rick has over twenty years’ experience with Gardien Group and has various roles including, service technician, customer liaison, Quality manager, technical support, project management, and software design/ implementation.

In June 1992 he graduated with an Electronic Engineering Technologist degree, with honors from a local community college.

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ROLAND VALENTINI

CHIEF OPERATING OFFICER

Mr. Valentini was appointed COO in July 2010 after successfully serving as head of North American operations since 2005.

Roland has over ten years’ experience in the PCB industry, including sales, service, and project management at Atotech GmbH, Germany mainly relating to PCB plating equipment.
He has significant sales experience from his tenure at Mack Rides GmbH and headed up worldwide installations for Lurgi Bischoff GmbH.

Mr. Valentini received a Master’s Degree in Mechanical Engineering from the University of Mannheim, Germany.