Limited access testing: the never-ending quest to keep up with testing challenges.

In my dealings with customers and coworkers in recent weeks, the latest buzzwords used repeatedly in association with test challenges experienced in the industry include "limited access." Why this surge in excitement? And why now? Access problems are not new. Limited access from a testing perspective, electrical and visual, has been an issue for years. Test providers have been focusing on limited access testing solutions and providing consult to that end. Design for manufacturing, design for test and establishing coverage maps for test plan optimization based on access have been underway for years as well. Why is limited access and limited access testing coming up so frequently now?

I suspect that if this question were posed to a panel, each individual would have his own answer. In my case, I believe it's because the market and industry developments are on an evolutionary trajectory, and issues like limited access are cyclical. In other words, we solve the issues of today and they carry forward for a time, but as the technology evolves, new solutions need to evolve with them.

Therefore, as components keep shrinking, board real estate becomes more coveted and cost pressures remain, unique solutions to limited test access need to arise. Let's look at four main areas impacted by limited access: solder paste inspection, AOI, automated x-ray inspection (AXI) and ICT.

As a 3-D solder paste inspection advocate, it is not surprising that I would suggest 3-D SPI is a critical tactic in the battle against limited access. In fact, I wonder if limited access is one of the primary drivers behind the rise in 3-D SPI solutions. The number of solutions has more than doubled over the past two years. Inspecting at paste deposition requires visual access, which, one could argue, is possible because components are not yet loaded. But as pads get smaller, this gets more challenging. Since volumetric measurement is known to be a good predictor of long-term joint quality, 100% 3-D SPI seems like a logical process step as access becomes more challenging. Companies providing 3-D SPI solutions are continuing to improve the inspection speed, capabilities, resolution and accuracy, which better positions 3-D SPI in the battle of access.

On the AOI front, there is a similar collection of competitive technologies for users to wade through as they decide on a strategy. Overall, AOI products are showing improved throughput, capability and resolution--all of which will improve the potential to use optical inspection as access becomes limited in traditional test. However, I believe it is the different AOI offerings' capabilities that will differentiate themselves when it comes to tackling limited access issues. The visual access limitations associated with a press-fit connector, for example, make it inherently difficult to inspect, and not all AOI systems can inspect for such components. Some AOI solutions include using angled cameras and multiple cameras to provide specialty measurements. In one case, an angled camera is used to measure pin height of PTH devices. Creating a solution to a point problem is the kind of activity evident in the AOI world today. Shrinking test access drives these activities.

Hidden joints, which by their nature have limited access, are the main driver for using automated x-ray inspection. Therefore, in the battle against access, AXI is a logical weapon. Another access trend that is slowly solidifying the focus on 3-D AXI is the steady increase in double-sided boards with growing overlap. I have recently witnessed overlap in boards under test ranging from approximately 10% up to 40% overlap! The need for 3-D AXI to access and evaluate those boards is solidified, as is the need for 100% 3-D AXI. The providers answer the calls differently. Some are adding AOI and AXI into one system shell promoting AXI sampling. Others are focusing on the growing need for 100% 3-D AXI and are focused on throughput and resolution.

Finally, we explore ICT. Driven by the ever-pressing need for more performance per square inch, ICT innovations have led the way in forging ahead in the access battleground. Bead probe is a new way of providing test access for an assembly using ICT. The bead probes are made of solder and sit on the signal trace, for which test access is needed. They can also be placed on a copper plane. They provide a flexible (i.e., layout independent) means to access the board. Many ICT test OEMs offer vectorless test and boundary scan solutions to attack this limited access issue.

While limited access will always be a challenge for testing, at each of the test insertion points, it seems there is no shortage of novel approaches to facilitating access, and those solutions are evolving at a rapid rate to keep up with the growing need and limiting factors.

Stacy Kalisz Johnson is product marketing engineer at Agilent (agilent.com); [e-mail].

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