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20 SMT Magazine • September 2017 hot air repair machine, you can set up a prop- er profile, and know what the impact is to the rest of the board. Soldering iron for repair? If possible stay away from it. I know you can't al- ways. Also, I see people hanging boards over solder fountains for extended periods of time, I don't like that. I see people reballing compo- nents that aren't spec'ed for reballing. There are just so many areas in rework that are detrimen- tal to the reliability of the board." So how to ensure the reliability of the boards after rework or repair was done to them? "There are standards that you have to meet when it comes to rework," explains Price. "Just like if you're assembling a circuit board, there's an acceptability standard that you work to. Typically, when meeting those standards, then your rework activity can be considered reliable. There are also various inspection steps, such as X-ray and in-process visual inspection that are completed to ensure the rework steps are com- pleted and to confirm reliability. In regard to re- pair, within each repair procedure in the IPC- 7711/7721 that we follow there are tests and verification steps that need to be completed. For example, it would include a metering step to verify proper connection when repairing a missing surface mount pad." SMT A LOOK INTO PCBA REWORK AND REPAIR A team of engineers has developed stretchable fuel cells that extract energy from sweat and are capable of powering electronics, such as LEDs and Bluetooth radios. The biofuel cells generate 10 times more power per surface area than any existing wear - able biofuel cells. The devices could be used to pow- er a range of wearable devices. The epidermal biofuel cells are a major break- through in the field. Engineers from the Univer- sity of California, San Diego, were able to achieve this, thanks to a combination of clever chemistry, advanced materials and electronic interfaces. This allowed them to build a stretchable electronic foundation by using lithography and by using screen-printing to make 3D carbon nanotube-based cathode and anode arrays. The biofuel cells are equipped with an enzyme that oxidizes the lactic acid present in human sweat to generate current. This turns the sweat into a source of power. The engineers reported their results in the June issue of Energy & Environmental Science. In the pa - per, they describe how they connected the biofu- el cells to a custom-made circuit board and demon- strated the device was able to power an LED while a person wearing it exercised on a stationary bike. Professor Joseph Wang, who directs the Center for Wear- able Sensors at UC San Diego, led the re- search, in collaboration with electrical engi- neering professor and center co-director Pat- rick Mercier and nano- engineering professor Sheng Xu, both also at the Jacobs School of Engineering at UC San Diego. Stretchable Biofuel Cells Extract Energy from Sweat to Power Wearable Devices

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