To the uninitiated, the type and number of available vision inspection systems may seem endless. Acoustic? Laser scanning? Optical? X-ray? Or perhaps an interferometer-based system or one employing a combination of technologies is the best choice.

And once you’ve made your technology choice, you’ll have to consider price, which is often not a small issue, since available systems can escalate into the mid six figures.

Obviously, the key to selection is your application and what you need (and expect) the system to be able to accomplish. The demands of production line inspection of PWBs for solder paste placement are substantially different from inspecting for solder bump placement on flip-chips.

In package inspection, higher density and lower cost demands are leading to a wide variety of new packages that generate new needs in real 3D inspection combined with 2D measurement, says Stephane Duranleau of Canada’s SolVision, maker of inspection systems that utilize fast moiré interferometry.

The future in package inspection, Duranleau predicts, is a high-speed, single vision station that performs high precision co-planarity, height and x-y positioning, warpage, peel off, glob top, chip out, voids and circuitry inspection.

Unfortunately when we’re talking inspection, we’re moving into the same gray area as test. That is, unlike a wire bonder, where you absolutely need to interconnect the die to the package leads, you don’t need to inspect the solder joints. (Who’s gonna know, anyway!)

Inspecting the solder joints on a board doesn’t add value per se, much as test doesn’t add value that you can see. However, a failed PWB can, at some point, speak volumes about your lack of interest in the inspection technologies!

We’re going to look mostly at machine vision systems that do a relatively dedicated job of inspection for one or several applications, not systems that are onboard other pieces of equipment—such as the inspection platform on a die bonder.

As Gary Wagner, president of Imaging Technology, points out, machine vision began as proprietary, embedded systems made essentially for capturing and processing images.

These early systems employed a unique operating system and programming language with processing performed by custom ASICs. "This highly customized approach made early machine vision solutions extremely expensive and difficult to use," Wagner observes.

One way to reduce costs is to adopt open standards. "In recent years, we have seen the evolution to Windows-based systems with standard interfaces, components and design rules."

The result of open computer architecture has been a trickle-down effect to most of the systems currently offered.

Automated vision inspection systems have moved into the production mainstream, says Richard Amtower, president and CEO of CR Technology. "Automated inspection has become a key step in the production process. It has transitioned from primarily offline to routine in-line use."

Amtower says that, "The yield management and process control value of automated inspection will increase as its use in multiple locations on the line—post-paste, post-placement and post-solder—grows." Users can expect, he adds, improved system performance and lowered capital cost.

With the increasing use of sophisticated vision systems across a broad spectrum of applications, there are new and as yet unresolved challenges facing equipment providers.

According to Tom Eskridge, vice president of advanced technology for Intelligent Reasoning Systems, Austin, Texas, the major challenge is how to handle the tradeoff between dectection accuracy and over-rejection of parts.

"With current generation detection processes, the false-call rate increases as the detection rate increases," Eskridge points out.

However, Eskridge adds, "new software technologies that include solder joint modeling and defect learning will decouple detection and false calls. Learning increases the detection while providing better discrimination—which, in turn, will decrease the number of false calls."

Bob Ries, marketing director at CyberOptics Corp., Minneapolis, contends that the major challenges in solder-joint inspection can be summarized in three words: cost, resolution and speed.

"A balance must be struck between resolution and throughput, as typically the higher the resolution, the slower the speed. This is compounded when the system must also perform three-dimensional inspection to detect the majority of possible defects," adds Ries. "One solution is to employ a 100% solder-paste inspection system," which costs far less than a joint failure detection system.

Mark Norris, CEO of ViTechnology, Haverhill, Mass., sees programming and debugging time as "the first issues being raised at this time. New technologies are now available which can replace gray scale correlation and pixel counting with synthetic models or vector images," Norris observes. Keep in mind, he suggests, "A vision inspection machine or AOI is nothing more than a data collection system. The way in which the data is used is the secret to the successful implementation of the process."

Model transportability is also an important challenge, as mechanical and optical differences from machine-to-machine may make it necessary to fine tune the program, and even the library for each individual system—dramatically increasing implementation time," Norris adds.

In addition, the continued shift to outsourcing assembly and test will continue to place pressure on systems makers for machines that offer easy changeover and are able to handle new and different parts types, according to Michael Gray, vice president of sales and marketing for RVSI.

These systems, typified by entries from Sonoscan (cover photo) and Sonix, have developed a small but loyal niche following in the package inspection field.

The use of what is often termed "scanning acoustic microscopy or acoustic micro-imaging" can reveal a relatively narrow, but ultimately useful range of package defects, such as popcorning and delamination.

To date, the use of sound has been mostly a laboratory tool, not a production tool. The use of ultrahigh frequency acoustic imaging (sonography, which has also been used on pregnant women for decades), also offers the key benefit to both unborn child and uninstalled IC of imaging non-destructively.

Figure 2. Intelligent Reasoning Systems' IOI-6000 optical-based PWB inspection system can be either in-line or stand-alone.	Figure 3. CyberOptics' SE 300 is a high-speed, high-resolution 3D solder paste inspection system.

To date, the most extensive use of AMI has been in failure analysis laboratories. Both Sonoscan and Sonix perform an extensive amount of failure analysis for customers. Both companies also contend that developments in AMI automation now make the technology capable of handling several thousand packages/hour[1]. "It is important, even for those familiar with the (AMI) technology, to give it another look," says John Goings of Sonix [2].

Acoustic microscopes employ ultrasound at 15-260 MHz to for an internal view of package integrity. Since frequencies in this range don’t travel through air, water has been used in the past as the coupling medium [3]. However, the recent development of Sonoscan’s "waterfall" transducer makes immersion of the sample unnecessary.

A tiny module rides on the Sonoscan transducer, delivering a micro-stream of water only to the area where acoustic coupoing is needed, exposing the sample to fluid for only a very short time. Because of the fluid’s small volume, other liquids (including alcohol), can also be employed.

Although optical inspection systems have achieved a high level of maturity in the market, the demands of the semiconductor industry are keeping suppliers constantly on their toes, especially for disclosing defects in solder ball placement of PWBs.

"The biggest challenge we face, as makers of AOI systems, is creating an optical BGA inspection system able to inspect the center interior of solder balls, under the lowest chips—and on the heaviest of populated boards--with the clarity and resolution possible on external rows of balls," according to Graham Ross, sales manager of O.C. White Co., Three Rivers, Mass.

Ross contends this technology is necessary due to the "never-ending demand for smaller and smaller boards—generated primarily by the telecommunications industry."

Figure 4. GSI Lumonics' Model 8200 3-D inspection system employs scanning laser technology.	Figure 5. RVSI's Model LS-7700 is a laser-based package inspection system for a variety of applications, including checking lead integrity.

Inspection equipment that employs X-ray technology appears to be one of the fastest growing segments of the market. X-ray inspection equipment spans the market from large, manual systems under $50,000 to high-end ($350,000+) automated units.

Currently, there are no less than 10 companies vying for a chunk of this market, including such heavyweights as Agilent Technologies (formerly an H-P division headquartered in Loveland, Colo.) and Nicolet Imaging Systems (now part of Genrad).

Several companies offer equipment that combines visible light with micro-focus X-ray inspection. These are admirably suited to advanced package designs and circuit assembly configurations that have concealed areas, connections and components, which are becoming more commonplace," says Paul J. Handler, director of North American sales for Viscom USA.

"These hidden connections often cannot be reliably inspected with traditional opitical inspection equipment alone. However, by combining visible and X-ray inspection, typical faults in surface mount, BGA and plated through-hole components can be detected in one operation, according to Handler

Figure 6. Glenbrook Technologies' Jewel Box 90-C is an X-ray-based, stand-alone system for inspecting advanced IC packages.	Figure 7. The X-Tek VTX system employs a high energy X-ray source of 160 kV@20 watts, making it suitable for applications that require the highest magnification with fine resolution.

Among X-ray’s key applications in IC packaging, is the technology’s ability to reveal solder-joint defects on PC board assemblies. X-ray can detect such anomalies as excess or insufficient solder, briding, no reflow, voids, missing balls and skewed placement of solder balls. [4]

Three-dimensional imaging and hidden joint inspection are driving the X-ray inspection industry, according to Christine Oliver, marketing manager for FeinFocus USA. Focal spots and defect detectabililty are rapidly moving towards sub-fractional micron requirements, she adds.

While only manual systems currently take advantage of high-resolution microfocus X-ray, users can look for fully automated inspection systems that have overcome speed, cost and inspection quality issues, Olive observes.

X-ray inspection systems are now being incorporated into manufacturing systems, according to Joseph E. Pascente, president of Lixi Inc. Pascente also notes that X-ray inspection incurs many of the same inspection requirements as optical systems, since the output of the X-ray sensor is either analog or digital.

Unfortunately, there is no single, ideal technology, no one-size-fits-all system, even when you limit your inspection task to a single, dedicated application, such as CSP inspection. Like most things in life—and in the industry—it’s a judicious tradeoff of cost versus capabilities.

1. T. Adams, "Acoustic Microimaging of Components in Strips," HDI, May 2000, p. 42.
2. J. Goings, "Acoustic Microscopes Provide a Unique CSP View," Chip Scale Review, September-October 2000, p. 65.
3. Goings, ibid.
4. R. Iscoff, "Putting X-Rays to the Test: Inspecting for Solder-Joint Failures," Chip Scale Review, May-June 1999, p. 26.

Write the editor at chipscale@cs.com.