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August 2016 • SMT Magazine 75 be approximately 2 mils in thickness. For most cleaning systems, this is just not enough of a standoff to get the cleaning agents underneath. A bumped device would be at the 4 mil height, thereby allowing the cleaning agents a better chance to do their job. Best Chance to Minimize Voids Another advantage of the bumping re- work process allows for the outgassing of the flux volatiles thereby presenting a nearly void- free device post rework. When the device has been selectively solder paste printed and is sent through a reflow profile, the volatiles have a chance to escape as the device is in an open environment. Contrast this to the case where the site location is printed and the volatiles can be entrapped creating solder voids. A typical bumped part x-ray is shown in Figure 3 dem- onstrates this. Conclusion When a leadless device such as a QFN, LGA or LCC needs to be taken off and then replaced, the bumping technique is one in which the de- vice can be routinely replaced without a high degree of skill required, one that offers a greater standoff height for more complete under pack- age cleaning and most importantly one that reduces the amount of voiding on placement. This is a well-proven IPC rework procedure and is gaining wide acceptance in small-medium re- work jobs. SMT Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. BUMPING OF QFNS/LGAS AND OTHER LEADLESS DEVICES The old rules don't necessarily apply when building electronic components out of two-dimen- sional materials, according to scientists at Rice University. The Rice lab of theoretical physicist Boris Yakobson analyzed hybrids that put 2-D mate- rials like graphene and boron nitride side by side to see what happens at the border. They found that the electronic characteristics of such "co-planar" hybrids differ from bulkier components. Shrinking electronics means shrinking their com- ponents. Academic labs and industries are studying how materials like graphene may enable the ultimate in thin devices by building all the necessary circuits into an atom-thick layer. The researchers led by Rice graduate student Henry Yu built computer simulations that analyze charge transfer between atom-thick materials. Yakobson said 3-D materials have a narrow re - gion for charge transfer at the positive and nega- tive (or p/n) junction. But the researchers found that 2-D interfaces created "a highly nonlocalized charge transfer" — and an electric field along with it — that greatly increased the junction size. That could give them an advantage in photovol- taic applications like solar cells. The lab built a simulation of a hybrid of graphene and molyb- denum disulfide and also con- sidered graphene-boron nitride and graphene in which half was doped to create a p/n junction. Their calculations predicted the pres- ence of an electric field should make 2-D Schottky (one-way) devices like transistors and diodes more tunable based on the size of the device itself. Yakobson said the principles put forth by the new paper will apply to patterned hybrids of two or more 2-D patches. "There's no reason we can't build 2-D rectifiers, transistors or memory elements," he said. "They'll be the same as we use routinely in devices now. But unless we develop a proper fundamental knowledge of the physics, they may fail to do what we design or plan." Rice postdoctoral research associate Alex Kutana is a co-author of the paper. Yakobson is the Karl F. Hasselmann Professor of Materials Science and Na - noEngineering and a professor of chemistry. The Office of Naval Research supported the research. Ultra-flat Circuits Will Have Unique Properties

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