Yes, the word groovy may bring a smile of recognition to those of us old enough to remember this once over-used word, a substitute for okay or cool. So groovy just had to pop into mind when I noted with great interest a paper on grooved circuits authored by three companies and given by Amkor’s Ron Huemoeller at the annual Pan Pacific conference in Hawaii in late January. The title was “Unveiling the Next Generation in Substrate Technology.”

Grooved circuits, or recessed traces, where the board’s traces in cross section are U-shaped, are a proposed new geometry at the heart of a major advancement in PWBs, at least for HDI. The concept is to add metallization after grooves or channels are formed in the dielectric versus having traces only on top of the dielectric surface. The subsequently applied metal either coats the sidewalls and bottom of each groove or plates them solid with copper. Provisions for subsequent drilled through holes simply use a recessed pad.

As Karl Dietz of DuPont has pointed out in several articles, this recessed geometry allows for superior processing because the surface is more distinguishable from the dug-in metal traces. Trace metal is protected on three sides and therefore has better total adhesion (e.g., more abuse resistant) as opposed to only one adhesive surface with traditional traces. This makes it easier to etch away unwanted metal (all flat surface metal is removed) while leaving the trace of recessed metal. Incidentally, semiconductor interconnections have used this U-shaped grooved circuit geometry for many years.

Also noteworthy is that the exact same process used to form the grooves (Amkor’s supplier used lasers, other methods can be employed as well) can concurrently make microvias. This means the currently decoupled processing (whereby trace making is separate from hole making) can more efficiently streamline, using one process step to achieve multiple purposes. This clearly saves processing steps. It also means, due to the accuracy of the processes employed, padless or near-padless microvias are viable and can be simultaneously formed during trace (groove) formation. This is almost the Holy Grail of HDI.

So why did Amkor select a new way, after many years of research? It started with frustration, the paper citing, “The electronic packaging industry has been crippled by the incremental technology advancement produced by the substrate manufacturers over the past decade.” And more to the point, in their industry, “... the IC industry is in dire need of a significant change at the substrate level to remove the innovative barrier that exists today and allow chip designers to continue their efforts in reducing size and cost, while increasing the functionality.” Of course, Amkor’s interest is at our industry’s leading edge, high-density substrates for IC packaging.

Who else has made positive, published comments about the viability of recessed traces for our industry? A growing, impressive list: besides Amkor, it includes Samsung E-M, Intel, and AT&S. The group has made glowing comments about the prospects, touting the samples made, processing/cost advantages, and design attributes. So these could be the channel drivers, large companies whose backing of an innovation can be extremely instrumental to drive it forward. Because “anything is possible if you don’t have to do it yourself,” it’s also good to see some PWB fabricators embracing the concept.

Now we need to see who goes into production, and when. The invention-to-production time lag is far, far longer than we would like to think it is, the timing delays being understood by what academics call diffusion of innovations (515,000 Google hits). So let’s hope they all started many years ago because, since 5-10-15+ years from concept to market penetration is not uncommon. In this respect, the semiconductor industry proposes a new method to continue to be groovy, by nanoimprinting (over 40,000 Google hits on this word), which the industry has worked on since the mid-1990s. But nanoimprinting is now on the industry roadmap (ITRS).

Can our industry use this concept more broadly? Yes and no. Not necessarily by Amkor’s methods, because its work is directed at the far leading edge, traces well below 25 µm, with special, high aspect ratio plating and expensive laser machines.

So it’s aimed at a single-digit percent of the market, for now. And also because Amkor has apparently patented parts of the process to control who gets to buy these HDI substrates.

But yes, “going groovy” is a better idea. After years of groovy work, I’m very biased to think so, especially if other ways to make the recessed features are also considered, such as by hot-embossing. (Disclosure: My own company, Dimensional Imprint Technology, has issued patents in this method.) Whether making traces having widths of 10 or 100 µms, recessed traces dramatically cut the number of process steps—read cost reduction—and allow improved electricals, as well as higher density. A study we commissioned shows that depending upon the application, 30 to almost 50 percent of the process steps can be eliminated by en masse imprinting of the critical features. Yet embossing is performed in an ordinary laminating press. We hope to announce more about this viable, drop-in process (no new capital equipment needed) later this year.

What should we say about all this? No question, this topic is groovy!