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Search :      Thursday, August 28, 2008

HDI: Our Newest "Old" Technology
By Sandra A. Swanson
Bannockburn, IL — Miniaturization, increased reliability, better connectivity — those are just a few advantages of high density interconnect (HDI) with microvias, when compared with plated-through hole technology. The downside: for many OEMs, it's too costly to include in systems. "And that's a pretty big downside," says Martin Rausch, general manager of systems manufacturing technology development at Intel Corp.

How big? "In general terms, today HDI costs more than 2 times more than equivalent plated-through hole designs," says Rausch.

HDI 23 Years Old
In decades past, large OEMs produced their own printed circuit boards, such as Hewlett-Packard, which first developed HDI boards in 1985. Today, OEMs that want to incorporate HDI must look primarily to Asian fabricators. Approximately 80 percent of the HDI market is handled by suppliers in Japan, Taiwan, Korea and mainland China, estimates Happy Holden, who worked on HP's PC boards for more than 30 years and is now a senior PC board technologist at Mentor Graphics Corp.

North American fabricators haven't invested much in HDI — in part because OEMs in the region have focused more on medical, military and aerospace electronics. Unlike consumer products, those markets don't have a strong demand for miniaturization, says Holden. "We kind of lost sight of (HDI's) benefits because we weren't using it," he says. "Now, it's become imperative in military and aerospace. This is an important technology, and we're way behind."

Making It Work
Holden maintains that with proper design, OEMs can make HDI cost-effective. He addresses four primary design principles to break through the HDI price barrier:

  • To make the price go down, the microvia must replace the through-hole. That's something Holden learned through previous mistakes. "At HP, we discovered that we were not designing HDI boards in an optimal fashion. We were using a through-hole board with a bunch of microvias on them, and that would make a board always cost more."
  • Rethink the stack up — the assignment of electrical layers. "New innovative ways of ordering your layers have to be envisioned, and by doing this, you eliminate the need for vias of any kind," says Holden. "Especially the larger through-holes."
  • Use the microvia within the BGA footprint to form channels on the inner layer of the PC board. This allows fewer layers to break out the ball grid arrays (BGAs).
  • Careful placement of microvias in the channel creates even larger channels, or boulevards. These boulevards allow for significant reduction in layers of a PC board, says Holden. "And as the layers go down and the size goes down, so does the cost."
    Better Performance
    HDI can deliver higher performance and lower cost than a conventional board, says Holden. But the path to those benefits is no cakewalk.

    "With HDI, the number of variables in how you design a PC board goes up terrifically, and designers don't know how to juggle all of these things," he says.

    To make HDI a viable, cost-effective option, designers need predictive and planning software. "The good news is, if you don't use the right HDI variables, the board will probably still work. The bad news is that it will probably cost more," Holden says. Holden also notes that IPC provides standards that cover performance, materials and design specifications for HDI. "That takes away a lot of the burden of having to guess at it," he says. IPC is now addressing the instruction and training issues, Holden says.

    Design changes alone, just like process optimizations, won't be enough to prompt HDI proliferation in the mainstream personal-computer market, says Rausch. "OEMs aren't including it in their specs for mainstream PCs because the process costs more and fabricators aren't tuning processes because design changes haven't closed the economic gap. For proliferation to happen, both parties will need to collaborate to deliver sufficient volumes to justify tuning processes and designs in order to overcome the economic gap," he says.

    Learn more about microvia technology at IPC Printed Circuits Expo/APEX and the Designers Summit courses and conference sessions. Check the Show directory or visit

    http://www.GoIPCShows.org for details.  

    (Reprinted courtesy IPC®)

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