How IBM sees Automated Optical Inspection
“Inspect/repair” strategy for advanced MLC substrates cuts costs, eliminates waste, adds profits -
Ben Arnold, HTCC/LTCC Market Manager,
MIDAS Vision Systems
IBM Corporation has produced MLC (Multi-Layer Ceramic) substrates at its East Fishkill, NY, manufacturing headquarters for over 25 years. This site is one of the largest MLC facilities in the United States and produces a variety of high-end products for a worldwide base of internal and external customers.
Over the years, IBM has maintained a consistent focus on building leading edge, highly complex circuits. Cost effective production of these circuits requires advanced manufacturing strategies throughout the process line. One critical component of the process at IBM, East Fishkill is the use of sophisticated automated optical inspection (AOI) equipment to monitor and improve process yields. This article will explore how AOI equipment is used by IBM, and the benefits that make it so important to the overall process.
20 years of AOI experience
IBM has employed AOI technology at its Fishkill facility since production began over 20 years ago, and actually helped develop many of its capabilities (both on their own and in cooperation with AOI system manufacturers). IBM engineers determined that AOI technology, coupled with an equipment set to facilitate verification and repair of defects, could increase production yields substantially. Locating and repairing circuit faults at the individual layer level prevented defective circuits from being laminated into a multi-layer package that would then fail electrical test.
Large scale implementation of this inspect/repair equipment set was (and still is) a significant production advantage for IBM. The traditional approach used today by most circuit producers is to rely solely on process design and “ad-hoc” repairs to control production yields. John Garant, a senior engineer at the Fishkill facility states, “Print defects are a fact of life in a high volume production plant; there will always be some level of defects present. If these defects are not repaired, high yield production is simply not possible. The only way to effectively find these defects for repair is to use AOI.”
Third generation AOI systems
Today IBM is producing complex, high density MLC substrates with 100 or more lamination layers, thanks in large part to its third generation AOI systems. The systems are used for 100% automated inspection of MLC substrates, and form an integral part of the company’s innovative “inspect and repair” strategy. The AOI systems provide the data to not only repair a defective part before it continues onto the next stage of the processing cycle, but also to catalog all defects and help determine root process problems. This benefits IBM since, by determining root process problems, the company’s engineers and technicians can implement corrective actions to remove them at their source.
AOI is now used in two stages of the MLC print process at IBM. First, it is used as an on-line process control - sampling the output of screen printers while individual layers are being printed. This ensures that any major repetitive process defects (i.e., bad screening mask, poor paste quality, etc.) are detected and repaired immediately. By closely monitoring their process, IBM prevents as many print defects as possible.
The second AOI step occurs after the printing run is complete. This is a 100% inspection of each sheet to detect “random” defects within the lot. During this step, each layer is serialized for repair. By individually coding each sheet, IBM is able to maintain a link between the sheet and the defect locations noted during AOI. The material handling during this step is highly automated to reduce production costs. See figure 1.
While IBM has employed 100% AOI in the repair application for many years, its use as a process control tool is relatively recent. Historically IBM operators had used microscopes on sample sheets to identify process flaws. This left a lot to be desired with regard to finding all defects (such as broken lines or voids) since it requires human intervention with highly specialized experience and judgment abilities. The consistency and accuracy gained through AOI implementation had an immediate impact - most notably on the amount of downstream repair work that was required for each lot processed.
Another major advantage of AOI based process control is the ability to detect several types of “near fatal” defects. These defects may not cause an immediate circuit failure, but could fail later when the circuit is in service. The “worst case” scenario of a field circuit failure can often be avoided through the use of AOI tools.
tracking circuit defects for repair
After the 100% AOI step defective layers are sent to a separate workstation where a technician verifies each defect site, makes any required repairs, or removes the sheet from the process flow (if the defect cannot be repaired). Because the “defect database” generated by the AOI system accompanies the defective part, the repair and/or decision making process is both simple and efficient. The repair workstation steps through all of the defect locations identified by the AOI tool.
AOI/repair drives competitiveness
Historically, MLC substrates manufactured by IBM have been used to support its internal line of products including mainframe computers, peripheral devices, and PCs. In recent years, they have also been sold separately to a worldwide OEM market. IBM credits use of its AOI/repair equipment set as a key factor in the company’s competitiveness in this market. The ability to maintain high manufacturing yields is driven by ensuring the lowest possible number of defects in every package (see sidebar illustration - Considering AOI - illustrating a basic example of how package yields can suffer, even when an individual layer is 95% defect free).
Other applications for AOI
While IBM’s Fishkill facility employs AOI technology for the “inspect/repair” loop described above, it also benefits from this technology in other ways. In fact, manufacturing of many of the sophisticated MLC circuits at IBM is possible only because of AOI technology in many phases of production. From the earliest process steps of creating pattern masters, through the entire process of green sheet blanking and punching, AOI technology ensures a high yield work-flow.
Even when IBM was producing less complex circuits than today, AOI techniques clearly demonstrated their practi- cality and usefulness. For example, using human inspection techniques a very large “open” was virtually impossible to detect; even under a microscope it may look like two different lines instead of a line with a break in the middle. Other circuit characteristics such as design revision levels are also not recognized by human inspection. By implementing AOI, IBM uses the actual design data for each layer as the inspection reference. In this way any variance from the accepted design will be flagged.
Space requirements should be considered
In the most recent “process control” implementation for AOI, IBM was very concerned with the footprint of the equipment it would use since its manufacturing lines were already in place. It was critical to place these tools as close to the printer as possible to get feedback quickly. The equipment selected for this application (Midas Vision Systems, Foxboro, MA) was compact enough to fit into existing space on the process lines. The tools also had the capacity to support multiple process lines in sampling mode further reducing floor space requirements.
Since its early exposure to AOI systems, IBM has added many new applications for this technology that have benefited the company in a number of ways. Garant said that, in general, it takes about 6-9 months to develop and implement new applications for AOI. This is mainly based on the complexity of the circuits the company is producing. “Of course,” he added, “we get help from the AOI equipment manufacturer on an as-needed basis.” Systems used today at IBM generate significant return on investment (ROI), based on the savings associated with added yields and lower scrap material costs.
There are now over 50 individual AOI systems at IBM’s East Fishkill MLC facility, used in the process control and final inspection phases of circuit production. An equal number of verification and repair stations are also in service to keep production flowing. Using the AOI/repair strategy has enabled IBM to profitably produce large quantities of extremely high density MLC substrates with fine lines and complex patterns demanded by today’s advanced systems and equipment. Close cooperation with leading AOI manufacturers will be required to continue to meet IBM’s requirements for ever-more-complex circuit design and production capabilities.
Conclusion
After years of experience, IBM can make a strong case that the investment in an equipment set to support an “AOI/repair” strategy is both practical and profitable for the MLC manufacturing process. While full-scale implementation is not yet a common practice in the industry, the trend is growing - especially as circuit designs become more complex. Most leading manufacturers have recognized the need for better management of production yields, and are exploring the role that AOI can play. These forward thinking companies will lead the way in realizing the reduced manufacturing costs necessary to support wide acceptance of MLC based commercial applications.
It is natural in closing to draw parallels to the printed circuit board industry. In this industry the AOI/repair manufacturing strategy has been used as a standard production step for over a decade (see sidebar 2). PCB manufacturers, like IBM, who had the vision to adopt this technology early have remained competitive over the years. A similar adoption cycle is expected in the Multi-Layer Ceramic world over the coming years.
