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November 2017 • The PCB Design Magazine 27 THREE PERSPECTIVES ON HDI DESIGN AND MANUFACTURING SUCCESS if you are using the small-pin package, you don't have room for that. I design boards that have thousands of nets that would need to come out to the outer surface and often there is no room. HDI does not lend itself well to that. With these high-density challenges test- ing companies tend to go more into functional test or JTAG." Assembly Front HDI concerns lie primarily with designers and fabricators. However, one of the biggest is- sues when it comes to assembly is component placement feasibility, according to Creeden. "A via-in-pad, even if it's a through-hole via, is a form of HDI. Therefore, it's high-density from a placement perspective because now I can fit all the parts on the board with good DFA con- siderations. Because I can put a via in its pad, I can route that board. Assemblers typically don't know what it takes to route a board. They don't know what's internal. They see the outer layer—and that's mostly what the assembler will consider. Typically, what the assemblers would care about is the assembly profile. The assembler has three agents: the bare board, the components, and the bonding agent of solder. Assemblers want to ensure that land pattern provides a robust solder connection to match the component, especially getting a good solder connection underneath any BGA," he explains. "That's critical to them. One of the concerns they are seeing now is package types known as landless grid arrays (LGA), which are like BGA but without elevated solder balls on each contact pin. I see this utilized with a lot of power sup- ply devices. The problem, because of its coplanar mating, they cannot disperse the solder flux resi- due, causing it to form a barrier on the edge of the device. It's not a normal perception of an HDI issue, but when you're talking about 100 amps coming from a power-supply device, that's a high-density power issue. People have this way of thinking that HDI means only microvias; now I put to you that HDI can be something with larger features such as a landless low-profile part, which may become an HDI assembly concern. So, what a good designer might do is to add vent holes in between the pins of the landless grid arrays so they can outgas the flux residue." HDI is here to stay, and there are three key perspectives of concern: layout solvability for the geometry and density, electrical integrity, and manufacturability. PCBDESIGN Researchers from DTU, Aarhus University, IBM, and Brookhaven have for the first time made self- assembly work in the thinnest films that exist: two- dimensional materials. The most well-known one of these is graphene that consists exclusively of carbon atoms and has outstanding electrical prop- erties. This creates new hope for super-storage media in extremely small scale. At DTU, Professor Peter Bøggild is thrilled and believes that the new discovery has a huge potential. "We know that pattern- ing of thin films is one of the keys to produce new proper- ties, and—in my opinion— this is a breakthrough. We already know how we can stack materials one layer at a time, and now it appears that we can also pattern them in something approaching atomic resolu- tion. It will be exciting to see how far we can get with this strategy." Bøggild believes, however, that it is still too ear- ly to say anything about the applications. "We've basically discovered a new way of con- trolling nanomaterials at atomic scale. Graphene islands behave like small artificial atoms and can be used for many different applications within the energy sector, biosen- sors, and optoelectronics. But where it gets really excit- ing is if we can learn to do the same with some of the hundreds of other atom-thin films that we know today." says Bøggild. Carbon Atoms Assemble Themselves on Command