Machine Vision in the Electronics and PCB Inspection Industry

Introduction

In this short report, we will review the current exploitation of machine vision in the electronics industry. We will do this both from the perspective of existing and emerging markets and from a scientific and technological perspective. Throughout, we will concentrate on one of the largest sectors in the industry: printed circuit board inspection.

The Electronics and Printed Circuit Board Industry

The electronics and printed circuit board industry comprise three broad classes of company. The original equipment manufacturers (OEM) such as Siemens, Intel, Motorola, Nokia, and Ericsson; the large PCB manufacturers who specialise in large- scale production of electronic components, and the local PCB assembly subcontractors. In terms of volume, these three classes form a pyramid with the OEMs at the top and the local subcontracts with their high total production volume at the bottom. As PCB manufactures continues to adopt increasingly higher levels of integration and achieving higher and higher levels of component density, tolerances on assembly become tighter and tighter. This has caused an increased need for visual inspection, especially in companies at the top and middle of the pyramid.

There are three main functions of machine vision in printed circuit board assembly:

Solder paste inspection

Component placement

Post-reflow inspection

Solder paste inspection is an in-line inspection process and, hence, systems must be capable of achieving production rates in real-time, delivering area, height, and volume of solder paste deposits following screening and prior to placement of components. Philips (NL), SVS (USA), and CyberOptics (USA) are the three market leaders in this sector. Both the Philips Triscan and the SVS 8100 systems used scanning laser-based triangulation to yield the required measurements and, common to all triangulation techniques, occlusions have to be catered for by exploiting more than one light source. Deposits of solder paste are visually-complex objects comprising collections of specular microscopic balls of solder. The resultant reflective surface causes significant difficulties for vision systems and make it difficult to achieve the high tolerances PCB Assembly on accuracy, repeatability, and reproducability required by the electronics industry. In addition, the need to deploy active illumination techniques places a severe load on the mechanical transport systems, a load which not all vendors are capable of bearing. The CyberOptics CyberSentry system, while slower than the other two systems, uses structured light to compute the local geometry and achieves acceptable repeatability and reproducability (R&R) measures.

MV Technology Ltd. (IRL) is the world's largest supplier of component placement systems, shipping between 45 and 60 units annually. This system owes its success to its ability to achieve the strict industry accuracy and R&R requirements through the use of very high quality high-speed mechanical sub-systems and high-speed imaging. The vision technology is based on grey-scale feature-based template matching using, naturally enough, proprietary algorithms to achieve the necessary robustness. The U.S.-based Theta Group also have a presence in this market.

In the area of post-reflow measurement, where the assembled PCB is inspected after the components are electrically and mechanically bonded to the PCB following a heat- induced phase transitions of the solder from solid to liquid and back to solid, four classes of systems can be distinguished. These are

Detection of the presence or absence of components (this is effectively a subset of component placement task)

Inspection of the solder joint characteristics using image processing based in reflected light

Inspection of solder joint characteristics using 2-D x-ray imaging 4. Inspection of solder joint characteristics using 3-D x-ray laminography.

In the case of inspection of solder fillets using reflected light, systems tend to offer only binary go/no-go functionality, mainly because of the complexity of the object being inspected, the difficulty in characterising good fillets, and the need to use complex variable-illumination inspection heads. Elimination of false reject errors pose some of the main difficulties. Market leaders in this sector include Control Automation (US); Shue (D), and Grundig (D). On the other hand, Nicolet (USA) and IRT(USA) lead the field in 2-D x-ray systems. In the case of 3-D x-ray laminography, the clear market leader is the US company Four Pi (which is owned by Hewlett- Packard).

Before completing this very short sample of PCB inspection systems, it is worth mentioning a number of other applications of machine PCB Assemblyvision in this general area. These include the use of vision systems on the component placement machines themselves. Typical functions in this area include component recognition, localization (position and orientation, typically in 2-D), and PCB fiducial mark (i.e. registration mark) identification and location.