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January 2017 • SMT Magazine 69 2. L. T. Manzione, "Plastic Packaging of Mi- croelectronic Devices," Van Nostran Reinhold, New York (1990). 3. JESD22-B112A, Package Warpage Mea- surement of Surface Mount Integrated Circuits at Elevated Temperature," October 2009. 4. N. Ranadive, "Use of Combination 2D and CT Scan X-ray Imaging for Solder Joint In- spection," SMTAI 2011. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. stencil. Sometimes limiting collapse height of the solder balls at the corners can be accom- plished through the use of standoffs (at the cor- ners which can be removed) after fellow. There are numerous other ways to mitigate the problem of warpage on BGA rework in ad- dition to the above noted. The solder paste for- mulation, the type of flux, and its activation level will determine how forgiving the BGA re- work profile will be. Changes in tooling, sol- dering methods, and preheating of component methods can affect the impact and degree of the BGA rework process yield warpage. SMT References 1. Garrett, David, "Elevated Temperature Measurements of Warpage of BGA Packages," SMTAI 2013. REDUCING WARPAGE ON BGAS DURING REWORK An interdisciplinary team of scientists at the U.S. Na- val Research Laboratory (NRL) have reported the first dem- onstration of metallic spin fil- tering at room temperature using ferromagnet-graphene- ferromagnet thin film junction devices. "The spin filtering had been theoretically predicted and previously seen only for high-resistance structures at cryogenic tempera- tures," said Dr. Enrique Cobas, principal investiga- tor, NRL Materials Science and Technology Divi- sion. "The new results confirm the effect works at room temperature with very low resistance in ar- rays of multiple devices." The thin film junctions demonstrated low re- sistance, and the magnetoresistance characteristic of a spin filter interface from cryogenic tempera- tures to room temperature. The research team also developed a device model to incorporate the pre- dicted spin filtering by explicitly treating a metal- lic minority spin channel with spin current conver- sion, and determined that the spin polarization was at least 80 percent in the graphene layer. "Graphene is famous for its extraordinary in-plane proper- ties, but we wanted to look at conductivity between stacked graphene sheets and how they interact with other mate- rials," said Cobas. To do so, NRL researchers developed a recipe to grow large multi-layer graphene films directly on a smooth, crystalline nickel alloy film while retaining that film's magnetic properties, then patterned the film into arrays of cross-bar junctions. "There is room for improvement as theory sug- gests the effect can be increased by an order of magnitude by fine-tuning the number of graphene layers," said Dr. Olaf van 't Erve, research scien- tist, NRL Materials Science and Technology Divi- sion. "However, current models do not include the spin-conversion that happens inside the ferromag- netic contacts. Once we account for those effects, we're already close to the ideal case of 100 per- cent spin polarization in the graphene layer, en- abling us to revise our device geometry and mate- rials to maximize the effect." Spin Filtering at Room Temperature with Graphene