- Email: [email protected]
Computer-aided visual inspection for integrated quality control
ELSEVIER
Computers in Industry 30 (1996) 185-192
Computer-aided visual inspection for integrated quality control Jing Bing Zhang
*
Gintic Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Auenue, Singapore 2263, Singapore
Abstract This paper first briefly introduces the fundamentals of automated inspection techniques, followed by a categorization of typical applications. AI. its focus the paper elaborates on issues relating to the need for, the approach to and the benefits of integrating automatic inspection systems within the context of computer integrated PCB manufachuing. It argues that automatic inspection systems should assume the role of not only quality inspection systems but also important quality information generators on the shop floor. In particular, integrated quality control will be realized through an integration of inspection systems with process/quality control systems such as statistical process control @PC). Keywords: Automatic inspection; Integrated quality control; System integration
1. Introduction The automation of manual visual inspection processes has become an imperative step toward advanced quality contrIal in printed circuit board (PCB) manufacture. There are several factors that fuel the demands for such automation. Firstly, with rapid technological advances in design, fabrication, assembly and packaging, F’CB products arc being produced with increasingly more complex functionality and higher component mounting density. At the same time, the dimensional measurement of both the boards and the components used have been reduced considerably. These have posed great challenges and difficulties to manual inspection: the features are simply beyond the capability of human visual inspection. Secondly, as more and more advanced, high-speed PCB manufacturing and assembly machines are installed to increase production capacity and productivity, manual inspection is found increasingly unable
= Corresponding
author. Email: [email protected]
0166-3615/96/$15.00 Copyright PII SO166-3615(96)00012-7
to meet the requirements of high speed production. Thirdly, the increased costs of labors (which accumulate as the human operators are being employed) become another important driving force for automating the labor-intensive manual inspection operations by exploiting automatic inspection technologies and systems. As such, computer-aided automatic inspection techniques have been developed and now evolved into an enabling and indispensable means of quality control for contemporary PCB manufacture.
2. Fundamentals
of automated inspection
Automated inspection begins with image acquisition which is now commonly done with solid state CCD (charge coupled device) cameras and associated circuitry. Specially designed illumination equipment is usually needed to improve the quality of the acquired image of the inspected objects so as to facilitate a consistent inspection result. The acquired image is then digitized and quantized into a digital
0 1996 Elsevier Science B.V. All rights reserved.
186
J.B. Zhang/Computers
form and stored in frame buffers of the computerized inspection system. Prior to more comprehensive and complex image processing and analysis, some forms of relatively simple yet important preprocessing (e.g., filtering) are often performed on the digitized image. The purposes of such preprocessing are manifold: to reduce noise level, to enhance contrast, and to highlight certain features (e.g., edges). Statistical computation may also be applied, for example, to perform histogram analysis and/or equalization, or to calculate the minimum, maximum and/or mean gray values, etc. Further processing follows to perform edge detection, thresholding, contour tracking, region formation, feature extraction, object recognition, and so on. Information regarding these features, edges, contours, regions, or individual objects is analyzed in detail by inspection algorithms, and decisions can then be made as to whether the PCBs inspected are up to the given specifications.
in Industry
30 (19961 185-192
3. Exploiting automated inspection cycle of PCBA manufacturing
Proper applications of automatic inspection systems can result in increased product inspection throughput, improved inspection reliability and more consistent inspection results. More importantly the earlier in the manufacturing cycle an automatic inspection system is used, the better the PCB quality will be and the less the product scraps will result. As the structure of PCB product is becoming increasingly complicated, a single missed defect in any of the inner layers of a board would cost the entire board to be scraped. The philosophy is therefore to detect defects as early as possible so as to avoid adding additional values to defective boards or layers. As illustrated by Fig. 1, numerous opportunities exist for applications of automatic inspection systems in various stages of PCB manufacturing (from bare board fabrication to PCB assembly). A detailed
nt
1 Solderino
and Cleaninn
23 Testing
Fig.
in the full
1.Applications of automatic inspection.
1
J.B. Zhang/Computers
review of such applications is beyond the scope of this paper. However, the author believes that these seemingly diverse applications can be classified into essentially four major categories, these being panel solder paste inspection, component inspection, placement inspection, and solder joint inspection. A classification of these various applications will help to gain an insight of the automatic inspection technologies and their applications in electronics industry.
classes of applications and associated technologies 4. Four
4.1. Panel inspection The term panel 21sused in this paper refers to a representation of a PCB artwork in the form of artwork films, production master films, individual layers (e.g., signal layers, the ground layer, etc.), or bare boards. Panel inspection thus refers to the inspection of these various artwork films, master films, inner layers, bare boards, and so on. Basically panel inspection is concerned with the inspection of features that are essentially of a two-dimensional nature. The requirements are to verify track width, via hole size and shape, soldering pad shape and size. In addition, it is also required to verify the spacing clearances between track and pad, pad and pad, or track and track. In fact this class of automatic inspection represents the most common applications of what is known as automatic optical inspection (A011 systems during printed circuit board fabrication. Conventionally used techniques for panel inspection can be classified as falling into one of the following categories, namely, (i) image comparison based techniques, (ii> rule checking based techniques, (iii) combined approaches. 4.2. Solder paste inspection Entering the cycle of PCB assembly, the first operation (for SMT) is screen printing of solder paste onto the bare board. Solder paste application has a direct and often decisive effect on the final quality of the solder *joints (and thus the functionality
in Industry 30 (1996) 185-192
187
and reliability of the PCBs). Thus the inspection of solder paste application is a must for PCBs involving SMT. The requirements are to check, prior to component placement and soldering, the volume and the three-dimensional distribution of the solder paste applied by the screen printing process [l]. This is in effect to make sure that the right amount of solder paste has been supplied at the right place and in good alignment with soldering pads. Depending on the requirements of inspection, two basic classes of inspection techniques can be differentiated, namely, two-dimensional (2D) based and three-dimensional (3D) based methods. If only alignment and area need to be verified, then 2D techniques will be enough. However, if volume and/or spatial distribution of the solder paste need to be checked, then some 3D inspection techniques will be required. In addition, special illumination and filtering techniques may also be required to facilitate the acquisition of an image with a high contrast between the metal solder paste and any other background materials (e.g., the substrate). 4.3. Component placement
inspection
Where there is less confidence in the placement equipment or where complex components are used, it is often necessary to perform component placement inspection both pre-soldering and post-soldering [2]. Typically, this class of application is concerned with (i> checking the presence/absence of components, (ii) verifying that the right component (i.e., its type and value) is placed on the right place and with the right orientation. Pre-soldering inspection helps isolate misplaced components before they are physically fixed on the board (i.e., by solder joints) and thus helps minimize post-soldering rework/repair. On the other hand, post-soldering placement inspection performs a final inspection, before in-circuit test and functional test, to verify the presence of certain components as required. This has an additional advantage of limiting the range of possible variables affecting the electrical functionality and thus assists test equipment in making decisions regarding the board tested. As far as inspection techniques are concerned, optical character recognition (OCR) methods may be needed to recognize and establish the identity of
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components, 3D analysis may be needed in order to verify the spatial orientation of a component, whereas some 2D techniques may suffice if only the presence/absence of a component must be checked. 4.4. Solder joint inspection Automatic inspection of solder joint is probably the most difficult and demanding, yet important inspection task to tackle. The objectives are to isolate soldering defects such as insufficient/excess solder, poor wetting, missing lead, bridging/opens, solder balls, and so on. Due to the shape and the high reflective nature of the metal joints inspected, proprietary built illumination systems are almost always a prerequisite to obtaining any useable images of solder joints. For instance the use of structured light [3], tiered illumination [4], fluorescent-diffused illumination [5] were reported. The introduction and increasing application of surface mount technology contributed further to the difficulties of solder joint inspection. Not only the number of solder joints to be inspected has increased dramatically, but also image acquisition of solder joints becomes more difficult. In some cases the solder joints may not even be visible at all as a result of using some special surface mount packages such as plastic leadless chip carriers (PLCCs) and leadless ceramic chip carriers (LCCCs). In these cases, a special X-ray lighting and X-ray sensitive image acquisition subsystem will be required to obtain images of solder joints that are underneath components
in Industry
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185-192
filling the emerging and broadening requirements of modern computer integrated PCB manufacturing. More often than not, inspection systems are now required not only to detect and locate defects, but also to classify the defects and render probable causes of the defects. This information can be of significant value to manufacturing process diagnosis, control and optimization. To support more advanced, and intelligent, applications of automated inspection, it is necessary to achieve a level of integration between inspection systems and other manufacturing facilities and process control functions, such as design, fabrication, assembly, quality control, production management, planning and control. Ultimate objectives of systems integration are to achieve a higher level of information sharing and supporting amongst those systems. In this sense, inspection systems should therefore be treated as an integral element of this broader integrated manufacturing environment so as to assume a pro-active role of important on-line generators of product quality information. Two basic issues must be addressed in order to achieve an integration of computer-automated inspection system within the PCB manufacturing environment, and to gain the benefits of such integration. These two issues reflect the need for two-way traffic of information flow to and from the inspection system. 5.1. Information systems
support
to automated
inspection
M. 5. Towards integrated applications of automatic inspection systems Traditionally the life cycle of automatic inspection tends to end with the detection of defects, and the rejection (via some other mechanism) of the faulty boards. The role of automatic inspection is mainly considered for and limited to the reactive tasks of inspection. The system is used either to automatically give inspection results or to provide information to assist an operator to make decisions about the results. This type of standalone application scenarios are found increasingly inadequate for ful-
The first issue is concerned with providing information to the inspection system so as to support inspection operations. Such information may be derived, for example, from CAD, CAM or any other relevant manufacturing systems/processes. In essence, “inspection” is the process of comparing detected features with expected features of a PCB product. Detected features are features extracted from a PCB under inspection (i.e., through image acquisition, processing and feature extraction), whereas expected features are product specifications with tolerance. The ultimate utilization of the supporting information is to establish a local model of the PCBs to be inspected. This model will contain specifications of all the expected features (e.g., in
J.B. Zhang/Computers
the form of inspection specifications or generic rules) to which an acceptable product should confirm. However, it needs not be a complete representation of the product, but rather contains only specifications used for the purpose of inspection. In other words, it is a local view of a complete PCB model, being taken from the perspective of inspection. Among the potential information sources, CAD is one of the most important for such purposes. The CAD data generated during design can be considered as representing the original product specifications, and are thus defect-free in nature. Use of such data to establish inspection criteria will be much “safer” and more reliable than the conventional “known good board” approach. Where CAD data are not available for such purpose, CAM data (often Gerber files for NC Drills or placement machines) can be an important alternative from which necessary information can be extracted to establish inspection criteria, or used for other reference purposes (e.g., nominal locations of components, etc.). With the availability of this off-line generated information the time re-
in Industry 30 (1996) 185-192
189
quired to set up the inspection system will be reduced dramatically. Requirement for manual on-line teaching processes (which are often time-consuming and error-prone) will be minimal, if not totally eliminated. For situations where a broad-ranging system integration is not in place, the author proposed that an ad hoc approach be adopted to establish the information link and to provide CAD/CAM based information support to inspection systems. This approach would entail the following logical steps, namely: Analyze the target inspection application and identify information requirements of the inspection system, Based on the information requirements identified, determine (a) the content of the information to be extracted from CAD/CAM database, and (b) an information representation format suitable for the inspection system, and Design and code the necessary software program to fulfil the requirements. Implementations of the above software program
Information Requirement (Application-dependent)
Fig. 2. Key issues in providing
CAD/CAM
information
to support automatic
inspection.
190
J.B. Zhang / Computers in Industry 30 (1996) 18% 192
may differ from each other, depending on the endsystems involved, hardware platform and operating system, and programming language used. However, the author believes that a generic functional model can be defined as guideline for developing such software information links. As illustrated in Fig. 2, the model consists of three functional layers, viz.: 1. Parsing and interpreting the incoming data from CAD/CAM database, 2. Extracting the necessary information items, and 3. Representing the extracted information in a format deemed suitable for the inspection system in question. 5.2. Information feedback from inspection systems The second issue of integration is concerned with communicating inspection information to various upand/or downstream manufacturing stream systems/processes, so that the inspection information could be utilized more intelligently. Since the inspection information is generated and stored in an electronic form, it can be easily accessed, transferred and further processed for other applications. This can be considered as an added advantage of automatic inspection over human inspection where it can be
Level_4:
Level3:
extremely difficult (if not impossible) to systematically accumulate comprehensive real-time information, let alone to further process such information for advanced uses. The availability and intelligent utilization of the inspection information can open additional opportunities for process improvement and better quality control. For instance, information generated during bare board inspection can be utilized as feed-forward information to provide calibration support for subsequent placement process. More importantly, this information can be utilized as feedback information to support the diagnosis of process malfunctioning in etching, drilling, or plating. This makes it possible to fine-tune and to better control corresponding manufacturing processes. However, it is important to point out that such application potentials (or benefits) of automatic inspection may not come as granted until a supporting integration scheme is realized and additional features are built into the automatic inspection systems to facilitate wider-scope information sharing and utilization. On the one hand, the integration scheme is to provide the infrastructural mechanisms for the controlled access of data of a common interest, and for the seamless flow of meaningful information
“I Application Oriented Algorithms (AOAs)
Global Board Analysis
(CBA) Algorithms
Level_2:
Level-1 :
Level-O:
Fig. 3. A reference architecture
for automatic
inspection systems [8].
J.B. Zhang/Computers
among a variety of heterogeneous manufacturing subsystems in the whole PCB manufacturing environment [7]. This provides the enabling capability for interconnecting existing “island of automation” and building information links among them. On the other hand, the additional features expected of an automatic inspection system are concerned with facilitatmg (i) the effective utilization of supporting information in the inspection processes and (ii) the generation of inspection information that is readily and easily usable for purposes beyond for “ inspection’ ’ . These are obvious requirements the inspection systems as benefits of integration and information support cannot be realized unless the provided information is utilized to the maximum advantage of the inspection system. In literature [8], Zhang and Weston proposed a reference architecture and some guidelines for building open automatic inspection systems. In addition to systemizing the reorganization of inspection system software, the reference architecture specifically emphasized the need to build open automatic inspection systems that have a built-in enabling layer of Application-Oriented Algorithms to provide inspection information support to more advanced applications.
6. Inspection
for integrated
quality control
Quality control applications such as SPC have already been widely used by many PCB manufacturing companies as a means of process and quality control. Although most of the SPC systems can have a network interface to link them to relevant factory offices, they are seldom linked to inspection systems which are important quality data generators on the shop floor. This situation can be partially attributed to the fact that traditionally automated inspection systems have been designed to operate standalone. In other words, they have not been designed with an open architecture to share the inspection information with other systems. As illustrated in Fig. 3, the Application-Oriented Algorithms (AOAS) proposed in [8] are necessary software features of open inspection systems. These algorithms, though not directly involved in or contributing to making inspection decisions, are important information intsegration enablers which facilitate
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the re-processing of the inspection results and make them easily available to applications with a need. (See literature [8] for a detailed functional description of each layer of the reference architecture.) There are several ways in which data generated by an inspection system can be made available to and utilized by quality control software packages. For example, with an electronic data transfer mechanism in place the inspection results can be communicated to the quality control software either real-time or in a batch sampling mode. The data can then be utilized to alarm, Pareto analysis to identify correlation between defects and process parameters, and so on.
7. Conclusions Automatic inspection has now become an important enabling technology as well as an indispensable quality control means for modem PCB manufacturing industries. Their application has already proven to be beneficial to PCB manufacture. A conclusion that can be drawn is that such inspection systems should be applied as early in the manufacturing stages as possible. This approach will largely prevent the addition of value to any defective PCBs that ultimately will be scraped, and hence help reduce production costs. This has become increasingly important with the adoption of advanced PCB manufacturing technologies and philosophies such as multilayer technology, fine and ultra-fine pitch technology, and surface mount technology. Another conclusion is that with the emerging requirements of computer integrated manufacture, automatic inspection systems are now required to assume a more pro-active role as valuable inspection information generators to help achieve intelligent process and quality control, in addition to fulfilling their traditional role as “passive” quality control means. To make this possible, inspection systems are required to be integrated and to co-operate with many other heterogeneous subsystems in the host manufacturing environment. The implication is that a new generation of automatic inspection systems should be designed by taking into account the following considerations, namely:
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. They should be easily integrated with existing environment to facilitate information sharing and reutilization, and * They should be built with the capability of generating more comprehensive inspection information that can be easily shared with and utilized by other systems, for example, to achieve improved process and quality control.
References it1 R.P. Prasad, Surface Mount Technology: Principles and Practice, Von Nostrand Reinhold,
1989. Surface Mount Technology, IFS publications and Springer-Verlag, 1987. visual inspection of solder joint on t31Nakagawa, “Automatic printed circuit boards”, SPIE, Vol. 336, 1982, pp. 121-127. [41D.W. Capson and SK. Eng, “A tiered-color illumination approach for machine inspection of solder joints”, IEEE Trans. PAM, Vol. PAMI-10, No. 3, 1988, pp. 387-393. [51Y. Hara et al., “A system for PCB automated inspection using fluorescent light”, IEEE Trans. PAMI, Vol. PAMI-10, No. 1, 1988, pp. 69-78. Elec[61R. Nameth, “X-ray inspection aids process control”, tronic Production, July 1989, pp. 33-37.
Dl C.H. Mangin and S. McClelland,
[7] J.B. Zhang, “On flexibly integrating machine vision inspection systems in PCB manufacture”, Ph.D. Thesis, Department of Manufacturing Engineering, Loughborough University of Technology, U.K., March 1992. [8] J.B. Zhang and R.H. Weston, “Reference architecture for open and integrated automatic optical inspection systems”, International Journal of Production Research, Vol. 32, No. 7, 1994, pp. 1521-1543. ,/li;:,, Jing Bing Zhang received his Bachelor of Engineering degree in Industrial Automation from the Tsinghua University, P.R. China in 1985, and his Ph.D. degree from Loughborough University of Technology, U.K. in 1992. Jing Bing . Zhang is presently a Research Fellow with the Shop Floor Integration (SFI) Group of Gintic Institute of Manufacturing Technology, Singapore. He has been involved in several industrial projects involving the design and implementation of a flexible assembly line for electronic products; enterprise-wide CIM feasibility study and architectural design; design and implementation of shop floor SPC systems, shop floor data collections and information systems. His main R&D interest covers shop floor control and factory information systems, SPC/SQC, shop floor networking and communication, as well as client/server computing for computer integrated manufacturing.
Computers in Industry 30 (1996) 185-192
Computer-aided visual inspection for integrated quality control Jing Bing Zhang
*
Gintic Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Auenue, Singapore 2263, Singapore
Abstract This paper first briefly introduces the fundamentals of automated inspection techniques, followed by a categorization of typical applications. AI. its focus the paper elaborates on issues relating to the need for, the approach to and the benefits of integrating automatic inspection systems within the context of computer integrated PCB manufachuing. It argues that automatic inspection systems should assume the role of not only quality inspection systems but also important quality information generators on the shop floor. In particular, integrated quality control will be realized through an integration of inspection systems with process/quality control systems such as statistical process control @PC). Keywords: Automatic inspection; Integrated quality control; System integration
1. Introduction The automation of manual visual inspection processes has become an imperative step toward advanced quality contrIal in printed circuit board (PCB) manufacture. There are several factors that fuel the demands for such automation. Firstly, with rapid technological advances in design, fabrication, assembly and packaging, F’CB products arc being produced with increasingly more complex functionality and higher component mounting density. At the same time, the dimensional measurement of both the boards and the components used have been reduced considerably. These have posed great challenges and difficulties to manual inspection: the features are simply beyond the capability of human visual inspection. Secondly, as more and more advanced, high-speed PCB manufacturing and assembly machines are installed to increase production capacity and productivity, manual inspection is found increasingly unable
= Corresponding
author. Email: [email protected]
0166-3615/96/$15.00 Copyright PII SO166-3615(96)00012-7
to meet the requirements of high speed production. Thirdly, the increased costs of labors (which accumulate as the human operators are being employed) become another important driving force for automating the labor-intensive manual inspection operations by exploiting automatic inspection technologies and systems. As such, computer-aided automatic inspection techniques have been developed and now evolved into an enabling and indispensable means of quality control for contemporary PCB manufacture.
2. Fundamentals
of automated inspection
Automated inspection begins with image acquisition which is now commonly done with solid state CCD (charge coupled device) cameras and associated circuitry. Specially designed illumination equipment is usually needed to improve the quality of the acquired image of the inspected objects so as to facilitate a consistent inspection result. The acquired image is then digitized and quantized into a digital
0 1996 Elsevier Science B.V. All rights reserved.
186
J.B. Zhang/Computers
form and stored in frame buffers of the computerized inspection system. Prior to more comprehensive and complex image processing and analysis, some forms of relatively simple yet important preprocessing (e.g., filtering) are often performed on the digitized image. The purposes of such preprocessing are manifold: to reduce noise level, to enhance contrast, and to highlight certain features (e.g., edges). Statistical computation may also be applied, for example, to perform histogram analysis and/or equalization, or to calculate the minimum, maximum and/or mean gray values, etc. Further processing follows to perform edge detection, thresholding, contour tracking, region formation, feature extraction, object recognition, and so on. Information regarding these features, edges, contours, regions, or individual objects is analyzed in detail by inspection algorithms, and decisions can then be made as to whether the PCBs inspected are up to the given specifications.
in Industry
30 (19961 185-192
3. Exploiting automated inspection cycle of PCBA manufacturing
Proper applications of automatic inspection systems can result in increased product inspection throughput, improved inspection reliability and more consistent inspection results. More importantly the earlier in the manufacturing cycle an automatic inspection system is used, the better the PCB quality will be and the less the product scraps will result. As the structure of PCB product is becoming increasingly complicated, a single missed defect in any of the inner layers of a board would cost the entire board to be scraped. The philosophy is therefore to detect defects as early as possible so as to avoid adding additional values to defective boards or layers. As illustrated by Fig. 1, numerous opportunities exist for applications of automatic inspection systems in various stages of PCB manufacturing (from bare board fabrication to PCB assembly). A detailed
nt
1 Solderino
and Cleaninn
23 Testing
Fig.
in the full
1.Applications of automatic inspection.
1
J.B. Zhang/Computers
review of such applications is beyond the scope of this paper. However, the author believes that these seemingly diverse applications can be classified into essentially four major categories, these being panel solder paste inspection, component inspection, placement inspection, and solder joint inspection. A classification of these various applications will help to gain an insight of the automatic inspection technologies and their applications in electronics industry.
classes of applications and associated technologies 4. Four
4.1. Panel inspection The term panel 21sused in this paper refers to a representation of a PCB artwork in the form of artwork films, production master films, individual layers (e.g., signal layers, the ground layer, etc.), or bare boards. Panel inspection thus refers to the inspection of these various artwork films, master films, inner layers, bare boards, and so on. Basically panel inspection is concerned with the inspection of features that are essentially of a two-dimensional nature. The requirements are to verify track width, via hole size and shape, soldering pad shape and size. In addition, it is also required to verify the spacing clearances between track and pad, pad and pad, or track and track. In fact this class of automatic inspection represents the most common applications of what is known as automatic optical inspection (A011 systems during printed circuit board fabrication. Conventionally used techniques for panel inspection can be classified as falling into one of the following categories, namely, (i) image comparison based techniques, (ii> rule checking based techniques, (iii) combined approaches. 4.2. Solder paste inspection Entering the cycle of PCB assembly, the first operation (for SMT) is screen printing of solder paste onto the bare board. Solder paste application has a direct and often decisive effect on the final quality of the solder *joints (and thus the functionality
in Industry 30 (1996) 185-192
187
and reliability of the PCBs). Thus the inspection of solder paste application is a must for PCBs involving SMT. The requirements are to check, prior to component placement and soldering, the volume and the three-dimensional distribution of the solder paste applied by the screen printing process [l]. This is in effect to make sure that the right amount of solder paste has been supplied at the right place and in good alignment with soldering pads. Depending on the requirements of inspection, two basic classes of inspection techniques can be differentiated, namely, two-dimensional (2D) based and three-dimensional (3D) based methods. If only alignment and area need to be verified, then 2D techniques will be enough. However, if volume and/or spatial distribution of the solder paste need to be checked, then some 3D inspection techniques will be required. In addition, special illumination and filtering techniques may also be required to facilitate the acquisition of an image with a high contrast between the metal solder paste and any other background materials (e.g., the substrate). 4.3. Component placement
inspection
Where there is less confidence in the placement equipment or where complex components are used, it is often necessary to perform component placement inspection both pre-soldering and post-soldering [2]. Typically, this class of application is concerned with (i> checking the presence/absence of components, (ii) verifying that the right component (i.e., its type and value) is placed on the right place and with the right orientation. Pre-soldering inspection helps isolate misplaced components before they are physically fixed on the board (i.e., by solder joints) and thus helps minimize post-soldering rework/repair. On the other hand, post-soldering placement inspection performs a final inspection, before in-circuit test and functional test, to verify the presence of certain components as required. This has an additional advantage of limiting the range of possible variables affecting the electrical functionality and thus assists test equipment in making decisions regarding the board tested. As far as inspection techniques are concerned, optical character recognition (OCR) methods may be needed to recognize and establish the identity of
188
J.B. Zhang/Computers
components, 3D analysis may be needed in order to verify the spatial orientation of a component, whereas some 2D techniques may suffice if only the presence/absence of a component must be checked. 4.4. Solder joint inspection Automatic inspection of solder joint is probably the most difficult and demanding, yet important inspection task to tackle. The objectives are to isolate soldering defects such as insufficient/excess solder, poor wetting, missing lead, bridging/opens, solder balls, and so on. Due to the shape and the high reflective nature of the metal joints inspected, proprietary built illumination systems are almost always a prerequisite to obtaining any useable images of solder joints. For instance the use of structured light [3], tiered illumination [4], fluorescent-diffused illumination [5] were reported. The introduction and increasing application of surface mount technology contributed further to the difficulties of solder joint inspection. Not only the number of solder joints to be inspected has increased dramatically, but also image acquisition of solder joints becomes more difficult. In some cases the solder joints may not even be visible at all as a result of using some special surface mount packages such as plastic leadless chip carriers (PLCCs) and leadless ceramic chip carriers (LCCCs). In these cases, a special X-ray lighting and X-ray sensitive image acquisition subsystem will be required to obtain images of solder joints that are underneath components
in Industry
30 (IS’%/
185-192
filling the emerging and broadening requirements of modern computer integrated PCB manufacturing. More often than not, inspection systems are now required not only to detect and locate defects, but also to classify the defects and render probable causes of the defects. This information can be of significant value to manufacturing process diagnosis, control and optimization. To support more advanced, and intelligent, applications of automated inspection, it is necessary to achieve a level of integration between inspection systems and other manufacturing facilities and process control functions, such as design, fabrication, assembly, quality control, production management, planning and control. Ultimate objectives of systems integration are to achieve a higher level of information sharing and supporting amongst those systems. In this sense, inspection systems should therefore be treated as an integral element of this broader integrated manufacturing environment so as to assume a pro-active role of important on-line generators of product quality information. Two basic issues must be addressed in order to achieve an integration of computer-automated inspection system within the PCB manufacturing environment, and to gain the benefits of such integration. These two issues reflect the need for two-way traffic of information flow to and from the inspection system. 5.1. Information systems
support
to automated
inspection
M. 5. Towards integrated applications of automatic inspection systems Traditionally the life cycle of automatic inspection tends to end with the detection of defects, and the rejection (via some other mechanism) of the faulty boards. The role of automatic inspection is mainly considered for and limited to the reactive tasks of inspection. The system is used either to automatically give inspection results or to provide information to assist an operator to make decisions about the results. This type of standalone application scenarios are found increasingly inadequate for ful-
The first issue is concerned with providing information to the inspection system so as to support inspection operations. Such information may be derived, for example, from CAD, CAM or any other relevant manufacturing systems/processes. In essence, “inspection” is the process of comparing detected features with expected features of a PCB product. Detected features are features extracted from a PCB under inspection (i.e., through image acquisition, processing and feature extraction), whereas expected features are product specifications with tolerance. The ultimate utilization of the supporting information is to establish a local model of the PCBs to be inspected. This model will contain specifications of all the expected features (e.g., in
J.B. Zhang/Computers
the form of inspection specifications or generic rules) to which an acceptable product should confirm. However, it needs not be a complete representation of the product, but rather contains only specifications used for the purpose of inspection. In other words, it is a local view of a complete PCB model, being taken from the perspective of inspection. Among the potential information sources, CAD is one of the most important for such purposes. The CAD data generated during design can be considered as representing the original product specifications, and are thus defect-free in nature. Use of such data to establish inspection criteria will be much “safer” and more reliable than the conventional “known good board” approach. Where CAD data are not available for such purpose, CAM data (often Gerber files for NC Drills or placement machines) can be an important alternative from which necessary information can be extracted to establish inspection criteria, or used for other reference purposes (e.g., nominal locations of components, etc.). With the availability of this off-line generated information the time re-
in Industry 30 (1996) 185-192
189
quired to set up the inspection system will be reduced dramatically. Requirement for manual on-line teaching processes (which are often time-consuming and error-prone) will be minimal, if not totally eliminated. For situations where a broad-ranging system integration is not in place, the author proposed that an ad hoc approach be adopted to establish the information link and to provide CAD/CAM based information support to inspection systems. This approach would entail the following logical steps, namely: Analyze the target inspection application and identify information requirements of the inspection system, Based on the information requirements identified, determine (a) the content of the information to be extracted from CAD/CAM database, and (b) an information representation format suitable for the inspection system, and Design and code the necessary software program to fulfil the requirements. Implementations of the above software program
Information Requirement (Application-dependent)
Fig. 2. Key issues in providing
CAD/CAM
information
to support automatic
inspection.
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may differ from each other, depending on the endsystems involved, hardware platform and operating system, and programming language used. However, the author believes that a generic functional model can be defined as guideline for developing such software information links. As illustrated in Fig. 2, the model consists of three functional layers, viz.: 1. Parsing and interpreting the incoming data from CAD/CAM database, 2. Extracting the necessary information items, and 3. Representing the extracted information in a format deemed suitable for the inspection system in question. 5.2. Information feedback from inspection systems The second issue of integration is concerned with communicating inspection information to various upand/or downstream manufacturing stream systems/processes, so that the inspection information could be utilized more intelligently. Since the inspection information is generated and stored in an electronic form, it can be easily accessed, transferred and further processed for other applications. This can be considered as an added advantage of automatic inspection over human inspection where it can be
Level_4:
Level3:
extremely difficult (if not impossible) to systematically accumulate comprehensive real-time information, let alone to further process such information for advanced uses. The availability and intelligent utilization of the inspection information can open additional opportunities for process improvement and better quality control. For instance, information generated during bare board inspection can be utilized as feed-forward information to provide calibration support for subsequent placement process. More importantly, this information can be utilized as feedback information to support the diagnosis of process malfunctioning in etching, drilling, or plating. This makes it possible to fine-tune and to better control corresponding manufacturing processes. However, it is important to point out that such application potentials (or benefits) of automatic inspection may not come as granted until a supporting integration scheme is realized and additional features are built into the automatic inspection systems to facilitate wider-scope information sharing and utilization. On the one hand, the integration scheme is to provide the infrastructural mechanisms for the controlled access of data of a common interest, and for the seamless flow of meaningful information
“I Application Oriented Algorithms (AOAs)
Global Board Analysis
(CBA) Algorithms
Level_2:
Level-1 :
Level-O:
Fig. 3. A reference architecture
for automatic
inspection systems [8].
J.B. Zhang/Computers
among a variety of heterogeneous manufacturing subsystems in the whole PCB manufacturing environment [7]. This provides the enabling capability for interconnecting existing “island of automation” and building information links among them. On the other hand, the additional features expected of an automatic inspection system are concerned with facilitatmg (i) the effective utilization of supporting information in the inspection processes and (ii) the generation of inspection information that is readily and easily usable for purposes beyond for “ inspection’ ’ . These are obvious requirements the inspection systems as benefits of integration and information support cannot be realized unless the provided information is utilized to the maximum advantage of the inspection system. In literature [8], Zhang and Weston proposed a reference architecture and some guidelines for building open automatic inspection systems. In addition to systemizing the reorganization of inspection system software, the reference architecture specifically emphasized the need to build open automatic inspection systems that have a built-in enabling layer of Application-Oriented Algorithms to provide inspection information support to more advanced applications.
6. Inspection
for integrated
quality control
Quality control applications such as SPC have already been widely used by many PCB manufacturing companies as a means of process and quality control. Although most of the SPC systems can have a network interface to link them to relevant factory offices, they are seldom linked to inspection systems which are important quality data generators on the shop floor. This situation can be partially attributed to the fact that traditionally automated inspection systems have been designed to operate standalone. In other words, they have not been designed with an open architecture to share the inspection information with other systems. As illustrated in Fig. 3, the Application-Oriented Algorithms (AOAS) proposed in [8] are necessary software features of open inspection systems. These algorithms, though not directly involved in or contributing to making inspection decisions, are important information intsegration enablers which facilitate
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the re-processing of the inspection results and make them easily available to applications with a need. (See literature [8] for a detailed functional description of each layer of the reference architecture.) There are several ways in which data generated by an inspection system can be made available to and utilized by quality control software packages. For example, with an electronic data transfer mechanism in place the inspection results can be communicated to the quality control software either real-time or in a batch sampling mode. The data can then be utilized to alarm, Pareto analysis to identify correlation between defects and process parameters, and so on.
7. Conclusions Automatic inspection has now become an important enabling technology as well as an indispensable quality control means for modem PCB manufacturing industries. Their application has already proven to be beneficial to PCB manufacture. A conclusion that can be drawn is that such inspection systems should be applied as early in the manufacturing stages as possible. This approach will largely prevent the addition of value to any defective PCBs that ultimately will be scraped, and hence help reduce production costs. This has become increasingly important with the adoption of advanced PCB manufacturing technologies and philosophies such as multilayer technology, fine and ultra-fine pitch technology, and surface mount technology. Another conclusion is that with the emerging requirements of computer integrated manufacture, automatic inspection systems are now required to assume a more pro-active role as valuable inspection information generators to help achieve intelligent process and quality control, in addition to fulfilling their traditional role as “passive” quality control means. To make this possible, inspection systems are required to be integrated and to co-operate with many other heterogeneous subsystems in the host manufacturing environment. The implication is that a new generation of automatic inspection systems should be designed by taking into account the following considerations, namely:
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. They should be easily integrated with existing environment to facilitate information sharing and reutilization, and * They should be built with the capability of generating more comprehensive inspection information that can be easily shared with and utilized by other systems, for example, to achieve improved process and quality control.
References it1 R.P. Prasad, Surface Mount Technology: Principles and Practice, Von Nostrand Reinhold,
1989. Surface Mount Technology, IFS publications and Springer-Verlag, 1987. visual inspection of solder joint on t31Nakagawa, “Automatic printed circuit boards”, SPIE, Vol. 336, 1982, pp. 121-127. [41D.W. Capson and SK. Eng, “A tiered-color illumination approach for machine inspection of solder joints”, IEEE Trans. PAM, Vol. PAMI-10, No. 3, 1988, pp. 387-393. [51Y. Hara et al., “A system for PCB automated inspection using fluorescent light”, IEEE Trans. PAMI, Vol. PAMI-10, No. 1, 1988, pp. 69-78. Elec[61R. Nameth, “X-ray inspection aids process control”, tronic Production, July 1989, pp. 33-37.
Dl C.H. Mangin and S. McClelland,
[7] J.B. Zhang, “On flexibly integrating machine vision inspection systems in PCB manufacture”, Ph.D. Thesis, Department of Manufacturing Engineering, Loughborough University of Technology, U.K., March 1992. [8] J.B. Zhang and R.H. Weston, “Reference architecture for open and integrated automatic optical inspection systems”, International Journal of Production Research, Vol. 32, No. 7, 1994, pp. 1521-1543. ,/li;:,, Jing Bing Zhang received his Bachelor of Engineering degree in Industrial Automation from the Tsinghua University, P.R. China in 1985, and his Ph.D. degree from Loughborough University of Technology, U.K. in 1992. Jing Bing . Zhang is presently a Research Fellow with the Shop Floor Integration (SFI) Group of Gintic Institute of Manufacturing Technology, Singapore. He has been involved in several industrial projects involving the design and implementation of a flexible assembly line for electronic products; enterprise-wide CIM feasibility study and architectural design; design and implementation of shop floor SPC systems, shop floor data collections and information systems. His main R&D interest covers shop floor control and factory information systems, SPC/SQC, shop floor networking and communication, as well as client/server computing for computer integrated manufacturing.