-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Unlocking the Power of AOI
December 31, 1969 |Estimated reading time: 6 minutes
Things have changed in the AOI world since the boom years of 1999 to 2000. But if defect detection coverage, repeatability, false alarm rates and ease of use are still imperatives, what remains as key evaluation criteria?
By David Doyle
Today, inspection speed is a more relevant parameter at the high-volume facilities of the Far East. Serving them are a limited number of automated optical inspection (AOI) companies having the necessary core technology and infrastructure with sufficient critical mass to support a global industry in a declining market. Despite the continued resilience of AOI demand over that of other assembly-related capital equipment groups, including X-ray, some AOI vendors have responded to the industry changes with tailored solutions that provide the right performance/value combination for specific applications.
However, the biggest change in current commitments is that AOI investments are now more frequently based on a manufacturer's interest in his own efficiency, quality and cost-reduction rather than any existing OEM relationships. Maturing visions of AOI implementation allow for the next steps of evolution to occur in which the power and full potential of automated inspection can be released.
AOI as the Quality "Engine"
It often is asked, "Why AOI?" The answer is clear: The technology offers early detection of defect trends that permits process-control implementation and lower repair costs. In fact, here lies the key to how AOI has changed to meet the needs of the emerging assembly industry in 2003/2004, i.e., defect detection is not enough but that AOI must deliver defect prevention.
AOI is the first and most cost-effective part of a complementary test infrastructure. It enables redeployment of resources as component technology exceeds human inspection capabilities and provides a more cost-effective platform for defect repair. If a well-defined feedback loop exists for solder paste printing, component placement and the solder reflow processes, continuous improvement in first pass yield can be achieved.
Second-generation AOI vendors now provide solutions that incorporate defect detection and rework prevention tools. For example, as growing segments of activity such as automotive adopt AOI rapidly, it may be appropriate to describe the new AOI design elements in the following analogy:
"AOI equipment is merely the engine on which an aerodynamic framework is constructed." Its goal is to keep pace with ever-changing process variances and product design considerations that are confronted. The design elements of this framework are the data analysis tools that permit intelligent decision making on the performance and the operating conditions around it. The interface relates to the user the key parameters of interest without requiring an understanding of all the intricate details of the system's workings, i.e., a "black-box" approach.
Early Defect Detection
The equipment selection process requires a good understanding of the capabilities of the equipment under varying "operating conditions," i.e., a specific assembly process and/or product mix. Preparation and visibility of an existing defect spectrum help to focus on weaknesses with a view toward advancing the return on investment. Sudden or frequent changes must be handled easily or automatically.
By far, the most effective process control solution involves AOI at solder paste inspection and at solder reflow or wave. Paste print-related defects that can be detected early in the process are screened out of production while post-solder or post-wave inspection provides the most effective end-of-line coverage. (Some segments continue to require post-placement AOI but at a declining rate.)
Component size reductions, like that of 0201 parts on cellular products, mean that "touching up" the board before reflow is a precarious operation at best. RF shield requirements that prevented post-solder inspection are lessening. Also, the solder reflow process self-corrects the majority of placement offsets.
Figure 1. Second-generation AOI solutions focus on data analysis and process optimization using SPC, defect analysis and Cpk analysis tools. Preventing process excursions has become the primary objective.
Feeding information back to the process engineers using defect-analysis tools or real-time statistical process control (SPC) trend analysis helps to focus resources on the core issues, e.g., paste-volume control or pick-and-place capability (Figure 1). Minimizing the escape rate to other complementary test points — X-ray, In-circuit (ICT)/functional test, etc. — dramatically reduces repair costs and sets up a platform for decreased ICT fixture complexity. Without the evolution and implementation of second-generation AOI solutions, the goal to reduce ICT costs or to ensure coverage for limited-access boards cannot be achieved.
Integrated Second-generation Enhancements
Repair. A defect review or repair interface is now integrated with AOI. Most important is the operator's ability to identify the location of the defect and to verify the nature of the fault or repair actions. Laser pointer and video camera technology at the repair station (with graphical or photo representation of the board layout) help to direct the operator. Defect images with color views also help the operator verify what has been found.
Repair solutions should be flexible for operation in off- or in-line configurations with a good system for handling good and bad printed circuit boards (PCB). A dual repair path may be an advantage to permit good boards to continue downstream while work is carried out on the defectives. Often when the process has been "pipe cleaned" and process performance issues addressed, single repair workstations can deal with multiple AOI machines. For example, automation elements such as barcode reading improve the efficiency of data retrieval and the source of data can vary.
Figure 2. How yield improvement may be established over time using AOI and process advisor tools. (The black curve shows anticipated improvement if an integrated yield management strategy has not been implemented.) Time-to-market and profitability metrics benefit directly from proactive process improvement.
Finally, verification using corrected false-alarm information from the operator in a controlled way is important to permit self-learning software to optimize algorithm performance.
SPC comes in two forms: real-time SPC software monitors defect trends in the process or other parametric information; historical attribute data analysis provides root-cause analysis and clues to design for manufacturing improvements.
Solder paste inspection typically demands paste registration, 2-D coverage and 3-D height data analysis. Component placement control and improvement can be gained via X-Y offset and skew-measurement analysis either with production or sample data. Post-solder or wavesolder applications provide defect classification data that can be analyzed on a Pareto and trend-analysis basis.
Finally, automated alarming features and configurable expert messaging systems can help improve the early warning systems and point manufacturing personnel toward the source of process excursions. Time-based shift reports or production-based batch reports consolidate process and AOI performance information on a regular basis to production management.
Process Capability (Cpk). The industry is seeking to embrace the IPC-9850 standard that provides a well-defined procedure for pick-and-place machine certification and acceptance. This has been generated with the participation of the equipment vendors for the purpose of standardizing on an agreed machine-acceptance methodology. It may well be of considerable value but it is not the most efficient way to determine in-process placement equipment performance issues — the core issue for contract manufacturers today.
AOI can be harnessed to provide this capability by eliminating the sources of non-pick-and-place variances in the inspection process. Using a glass plate and ceramic parts with local fiducials is the key, i.e., imaging the necessary features to generate accurate measurement information is the role AOI plays.
Integrated Yield Management. It is an easy term to use but what does it really mean? Integrated yield management is a combination of tools seamlessly integrated with AOI equipment to provide rapid troubleshooting of the PCB processing line. It also is a process for utilization of the valuable data made available via AOI systems, a process that is defined by the customer in conjunction with the AOI consulting partner in order to tailor the solution toward their manufacturing environment. This may include interfacing with other test equipment, third-party CIM vendors or other factory control systems.
David Doyle may be contacted at Orbotech Inc., 44 Manning Rd., Billerica, MA 01821; (978) 901-5114; Fax: (978) 667-9969; E-mail: davidd@orbotech.com.