SMT Magazine

SMT-Sept2017

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22 SMT Magazine • September 2017 Circuit boards are not always perfect after reflow or wave soldering. Scrapping boards with one or two defects is expensive, so rework hap- pens. A good concept for rework is less is more, especially less flux. Use as little rework flux as possible, as in the old Brylcreem ads, "Just a lit- tle dab'll do ya." Liquid flux for rework is often supplied in felt-tipped applicator pens that make applying a few microliters of flux to a solder joint easy. Small squeeze bottles with tiny needle tips also work well. Paste/gel flux can be supplied in small syringes; use a small applicator tip to en- sure that only a small dot of flux is applied to the rework site. Of course, using only no-clean, flux-cored solder wire without any assisting flux is the most electrochemically reliable way to do no-clean rework. The core flux in the wire will be properly heat deactivated because it will not flow until the wire is melted. Additionally, be- cause core flux is mostly rosin or resin and hard, any activator remaining will be trapped in the flux residue. No-clean core fluxes are usually light colored and glass-like in appearance, pro- viding nice cosmetics. Most low solids, no-clean soldering fluxes must be exposed to soldering temperatures to become deactivated. The activator systems of these fluxes contain organic acids that remove oxides and allow solder to wet. When these or- ganic acids are exposed to high enough tem- peratures for a long enough time, the activators volatilize and go away. If this doesn't happen, the unheated or under-heated flux residues can by Karen Tellefsen ALPHA ASSEMBLY SOLUTIONS Rework and Reliability: Less is More! FEATU RE COLU M N: ACROSS TH E BOARD

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