Ultra-high-density interconnects are more smoke than fire right now, but they won’t be that way for long. Driven by high-density BGAs and RF products, UHDI is finding its way into the mainstream.

Given the number of conferences, webinars and the like, readers would be forgiven if they thought UHDI was already standard, however.

First, of course, means agreeing on what, exactly, UHDI is. The working definition of UHDI is product with line widths and spaces of fewer than 50 microns, dielectric thickness of less than 50 microns, and a microvia diameter of less than 75 microns. That’s not a standard definition – yet – and the lower lever parameters have yet to be defined. At some point, there stands to be overlap with semiconductor technology. Stay tuned as the definition evolves.

I am reminded – to a degree – of the chaos surrounding UHDI’s (slightly) larger cousin, high-density interconnects, which hit widespread production in the late 1990s (although the original concept dates much earlier). Then, the issues could be boiled down to two:

• Which process to use: etch or drill (and if drill, mechanical or laser)?
• The demand signal.

As to the former, some of the processes of the day included photovia, plasma etching and the eventual winner, laser. As to the latter, time will tell as to the ultimate market size, but orders are already coming.

It’s mostly lost to history now, but not every company banked on laser at the start. Motorola, for instance, produced millions of boards using a sequential build HDI process that mated photoimageable dielectrics and semiadditive copper metallization on a PWB substrate.

And even fewer companies – or regions, really – saw the need. North America and Europe were all about big boards in those days. Japan controlled IC substrate production. Taiwan and especially China were still relatively marginal players. Still, the signs were ominous. Key industry observers were already piping up that investment in microvia formation, specifically in laser drills, was needed.

The West wasn’t listening. Indeed, upon seeing the buoyant forecasts for the PCB industry in North America for the early 2000s, I recall Jack Fisher, then the CTO of the Interconnection Technology Research Institute, remarking that it would be years before Americans started pursuing HDI. How right he turned out to be. And when the major telecommunications and IT companies started canceling orders en masse for their so-called “surfboards,” the Western PCB shops weren’t left with much to fall back on.

Asia, on the other hand, led by Taiwan and Japan, invested heavily on leading-edge drilling capacity. Today, we all know the Pacific Rim dominates high-end PCB fabrication.

Could the script be flipped? Unlike with HDI, additive fabrication processes look like they will have a significant role in UHDI. It’s unlikely a single solution will dominate.

We have a long way to go to mainstream this technology. IPC task groups are working on various specifications for fabrication and qualification, among others. Designers need to understand component choices, signal integrity implications, impedance matching, and thermal management. Assemblers will have to master the use of smaller powder sizes, different stencils, and so forth.

According to John Johnson at American Standard Circuits, one of the handful of PCB fabricators in the US capable of UHDI production, when lines and spaces reach less than 25 microns, UHDI is needed. To be sure, that’s really small. But while conventional technology will continue to have a long runway, those who master UHDI will be better-positioned to take advantage of the smaller, thinner, lighter products consumers of all stripes demand.

A special free panel on UHDI at PCB West this year will outline the basic various ways UHDI processing takes place, and to help designers and design engineers understand how their decisions might affect yield. Among the panelists are Michael Gleason of GreenSource Fabrication; Meredith Labeau of Calumet Electronics, Anaya Vardya of American Standard Circuits/Sunstone, and Jay Vyas of SigmaTron International.

The panel takes place at the Santa Clara Convention Center on Oct. 9 from 9-10 a.m. We invite all our readers to join us for this and several other free sessions that day, and to take in the more than 100 leading electronics design, fabrication and assembly companies on hand during the one-day exhibition.

See you at the show!

P.S. The video from our forum on UHDI & Substrates: From Design to Package will be available at PCEA.net later this month. Also, a series of videos on printed circuit board design will be available on our YouTube channel.

Mike Buetow is president of PCEA (pcea.net); mike@pcea.net.

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