SMT Magazine

SMT-Nov2017

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28 SMT Magazine • November 2017 ly, 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 function- al test or JTAG." Assembly Front HDI concerns are primarily a fabrica- tion concern. 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 lay- er—and that's mostly what the assembler will consider. Typically, what the assemblers would care about is the assembly profile. The assem- bler has three agents: the bare board, the com- ponents, and the bonding agent of solder. As- semblers want to ensure that land pattern pro- vides 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 res- idue, 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, the electrical integ- rity, and the manufacturability. SMT Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have reported significant advances in the thermo- electric performance of organic semiconductors based on carbon nanotube thin films that could be integrated into fabrics to convert waste heat into electricity or serve as a small power source. The research demonstrates significant potential for semiconducting single-walled carbon nano- tubes (SWCNTs) as the primary material for effi- cient thermoelectric generators. The discovery is outlined in the new Energy & En- vironmental Science paper, "Large n- and p-type thermoelectric power factors from doped semi- conducting single-walled carbon nanotube thin films". Advantages of this research include the promise of solution- processed semiconductors that are lightweight and flexible and inexpensive to manufacture. According to the researchers, the introduction of SWCNT into fabrics could serve an important function for "wearable" person- al electronics. By capturing body heat and converting it into electricity, the semiconductor c ould power portable electronics or sensors embed- ded in clothing. The paper was authored by Jeffrey Black- burn and Andrew Ferguson, both senior scien- tists in NREL's Chemical and Materials Science and Technology center. Other NREL authors are Bradley Ma- cLeod, Rachelle Ihly, Zbys- law Owczarczyk, and Kather- ine Hurst. The NREL authors also teamed with collaborators from the University of Denver and partners at International Thermodyne Inc. Research Yields Significant Thermoelectric Performance THREE PERSPECTIVES ON HDI DESIGN AND MANUFACTURING SUCCESS

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