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104 SMT Magazine • July 2017 What properties should paste flux have? When using a hot air or IR rework system where the reflow cycle is mim- icking the original SMT reflow profile, and when using flux for re-attach- ment, paste flux otherwise known as gel or tacky flux is the correct type of flux to use. The extend - ed time above the solder liqui- dus temperature causes liquid flux to expire and not be effec- tive during the complete reflow cycle. Paste flux, on the oth- er hand, due to its high viscos- ity and tack properties, prevents components from being moved during hot-air reflow while be - ing able to remain active for the complete time-temperature pe- riod. This prevents disturbed solder joints, which are a defect for all three classes of product, from forming (IPC-A-610F). It also prevents ultra-miniature, low-mass parts from being blown off the board if using a hot air rework source. Typical - ly, the rule of thumb for the re- flow cycles is to make sure the tin-lead solders are 30–45 sec- onds above liquidus whereas the lead-free solder alloys are 60–90 seconds above liqui- dus. Tacky fluxes are designed to be active for this length of time. Using the properly-for- mulated lead-free or tin-lead version will ensure that the flux remain active during the entire reflow profile. In addition to the property of being able to remain active during an extend- ed rework cycle time, the flux chemistry is also important. No clean flux residues are intended to be benign after reflow on a PCB when processed properly and are intended to NOT require any post ap - plication cleaning. Conversely, wa- ter-soluble paste fluxes are gener- ally more active and are designed such that the residues need to be cleaned off the PCB post rework. While water soluble flux - es are aggressive, giving the re- work technician a wider process window, it often bonds with the PCB itself or other metal surfac - es, and as a result will require thorough cleaning. It is impor- tant to get the cleaning right es- pecially when conformal coating needs to be applied to the PCB. It is also important to clean off the residues when probe testabil - ity is important-that is the elec- trical probe needs to make good electrical contact with the pads. One of the other advantag- es of using gel flux is the abili- ty to control the application and hence the area requiring clean- ing after soldering. When using liquid fluxes, the material, due to its low viscosity , can spread onto multiple areas of a PCB. This becomes a problem when the flux spreads out from the soldering location and does not experience the full lo - calized reflow tempera- ture cycle. These contam- by Bob Wettermann BEST INC. Using Paste Flux for Rework KNOCKING DOWN THE BONE PILE