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May 2014 • The PCB Design Magazine 49 ensure uniform contact bonds for such parts. Immersion silver or immersion gold finishes are better alternatives if devices with tight contact pitches are involved. The immersion gold fin- ish is called ENIG (electroless nickel, immersion gold). Immersion silver does not require a layer of nickel underneath as does the immersion gold finish. The nickel is used as a barrier layer to prevent copper migration into the gold over time, which can result in increased contact re- sistance if boards are left unassembled too long. Both immersion silver and ENIG result in even surfaces that are much flatter than can be ob- tained with HASL; moreover, both are more electrically conductive. The overall thickness of the immersion silver finish can be held to tighter tolerance than that for ENIG, so silver is preferred if there are press-fit connectors in the design. However, silver tarnishes quickly and assembly must therefore be completed soon af- ter board fabrication. ENIG is the better choice for designs with very fine traces. ENIG is also better for thin boards because it is a relatively low-temperature process. Both electrolytic soft gold, which would be a choice for designs that involve wire bonds in assembly, and electrolytic hard gold, which also supports wirebonding and has advantages for sliding contacts, have downsides. To ac- complish either finish requires the addition of buss bars on panels to electrically interconnect the copper features during the plating process, which afterward must be severed from the PCB circuits. Moreover, copper can remain exposed on trace sidewalls. Electroless nickel, electroless palladium, im- mersion gold (ENEPIG) is a somewhat more ex- pensive surface finish that has nearly universal advantages. Nonetheless, my advice regarding surface finishes jibes with my advice regard- ing hybrid stackups: Consult your prospective manufacturer at the outset of your project, to make sure you don't become invested too deep- ly to improve your design decisions. PCBDESIGN Amit Bahl directs sales and marketing at sierra Circuits, a PCB manufacturer in sunnyvale, California. he can be reached by clicking here. design for manufacturing CONSuLT WITH YOuR FABRICATOR—ESPECIALLY FOR HYBRID DESIGNS continues nerve agents are among the world's most feared chemical weapons, but scientists at the national insti- tute of standards and Technology (nisT) have demon- strated a way to engineer carbon nanotubes to disman- tle the molecules of a major class of these chemicals. The team's experiments show that nanotubes can be combined with a copper-based catalyst able to break apart a key chemical bond in the class of nerve agents that includes sarin. The team developed a way to attach the catalyst molecule to the nanotubes and then tested the effectiveness of the tube-catalyst com- plex to break the bonds. To perform the test, the complex was deposited onto a small sheet of paper and put into a solution containing the mimic molecule. For com- parison, the catalyst without nanotubes was tested simultaneously in a different solution. Then it was a simple matter of stirring and watching chem- istry in action. Principal investigator Angela hight Walker says that several questions will need to be addressed before cat- alytic nanotubes start showing up in clothing, such as whether it is better to add the catalyst to the nano- tubes before or after they are woven into the fabric. nanoengineers at the university of California, san Diego, are asking what might be possible if semicon- ductor materials were flexible and stretchable without sacrificing electronic function. Today's flexible electronics are already enabling a new generation of wearable sensors and other mobile electronic devices. But these flexible elec- tronics, in which very thin semiconductor materials are applied to a thin, flexible sub- strate in wavy patterns and then applied to a deformable surface such as skin or fabric, are still built around hard composite mate- rials that limit their elasticity. Nanotube-Infused Clothing Protects Against Chemical Weapons

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