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JULY 2021 I SMT007 MAGAZINE 53 ere are several benefits to cleaning the low residue no-clean flux. One of the benefits is to ensure that any underfill or replacement conformal coating will be properly adhered to the PCB. Any residue le behind may out- gas and impact adhesion. e right defluxing process and saponifier is used to facilitate bet- ter inspection and enhanced cosmetic appear- ance. A defluxing process is a cleaning process in that everything is cleaned. Contaminant species from board fabrication (like feeder tape residue), component fabrication residues, assembly residues, and human handling resi- dues all are cleaned off in a cleaning process. All of these may cause performance issues and their resultant removal mitigates this risk. While the cleaning of the flux holds some distinct advantages, there are several disadvan- tages to cleaning a no-clean flux aer rework. Since DI water alone cannot be used to clean off the residue, a saponifier chemistry will need to be used and, therefore, extra expense added in terms of chemicals, cleaning equip- ment (if not already on site) as well as chem- ical disposal. By not properly cleaning all of this off, you can end up with active flux on the board and be in a worse position with respect to the reliability of the assembly. ese and other drawbacks need to be considered when contemplating the necessity for a post-rework cleaning process. e choice of the cleaning agent is impor- tant. It needs to match the residue and clean- ing process. Dissolution of the residue at a fast rate is critical. Equally critical is the cleaning method's ability to deliver the cleaning agent to the no-clean residues. Reliable hardware is more challenging to build due to component size miniaturization, bottom termination devices, shortened dis- tance between conductors, and higher pin- out devices in a small footprint. ese factors require that there is no entrapped flux residue and that the board is properly cleaned post rework. SMT007 Bob Wettermann is the prin- cipal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact To read past columns or contact Wettermann, click here. Cosmic dawn, when stars formed for the first time, occurred 250 million to 350 million years after the beginning of the universe, according to a new study led by researchers from the University of Cam- bridge and University College London (UCL). The study, published in the Monthly Notices of the Royal Astronomical Society, suggests that the NASA James Webb Space Telescope (JWST), scheduled to launch in November 2021, will be sensitive enough to observe the birth of galaxies directly. The UK-led research team examined six of the most distant galaxies currently known, whose light has taken most of the universe's lifetime to reach us. They found that the distance of these galax- ies away from Earth corresponded to a "look back" time of more than 13 billion years ago, when the uni- verse was only 550 million years old. Analysing images from the Hubble and Spitzer Space Telescopes, the researchers calculated the age of these galaxies as ranging from 200 to 300 million years, allowing an estimate of when their stars first formed. "Theorists speculate that the universe was a dark place for the first few hundred million years, before the first stars and galaxies formed," said lead author Dr Nicolas Laporte from Cambridge's Institute of Astronomy. "Witnessing the moment when the uni- verse was first bathed in starlight is a major quest in astronomy. "Our observations indicate that cosmic dawn occurred between 250 and 350 million years after the beginning of the universe, and, at the time of their formation, galaxies such as the ones we stud- ied would have been sufficiently luminous to be seen with the James Webb Space Telescope." (Source: University of Cambridge) Astronomers Pinpoint When Cosmic Dawn Occurred

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