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24 The PCB Magazine • February 2015 DES Solder Fluxes At the University of Leicester a new type of solder flux has been developed based on a novel class of liquid called deep eutectic sol- vents (DES). These liquids are composed of or- ganic salts and small, polar organic molecules called "hydrogen bond donors". Because of the unique chemical makeup of these liquids they act as a highly coordinating environment for metal salts. It has been demonstrated that, even at relatively moderate temperatures, they have good solubility of metal oxides. [1, 2] The high metal salt solubility means that DES act as highly active solder fluxes for a wide variety of metal substrates. At the higher tem- peratures at which soldering takes place (typi- cally above 200°C) they can rapidly remove the oxide from a surface of a metal substrate enabling solder joint formation [3] . Figure 1(a) shows a solder wetting balance trace for oxi- dised copper wire coated with a DES flux and immersed to a depth of 5 mm into a SAC 305 solder bath at 260°C. This is then compared to an example wetting trace for the standard flux Actiec5 under the same conditions. Actiec5 is a rosin flux with added organic acids to im- prove solderability and is generally regarded as a highly active flux in the electronics and PCB industries. For all wetting traces a buoyancy re- sponse is seen as the copper wire is dipped into the solder pot. Where a DES flux was used a very rapid increase in the force is seen as the copper oxide is removed from the copper surface and a solder join starts to form. This is in contrast to Actiec5 which, while still wetting the surface of the copper, takes a much longer time to do so suggesting that the DES flux is much more ac- tive than Actiec5. As mentioned prior, the high activity of these DES solder fluxes means that they can be used to solder a wide variety of metallic surfac- es including all common PCB surface finishes: brass, nickel, mild steel, stainless steel and cast iron. Remarkably, they can also be used to sol- der electroless nickel as shown by the solder wetting trace in Figure 1(b) where an electro- less nickel surface was coated with a DES flux and immersed in SAC 305. The response is very similar to that seen for copper wire, where after an initial buoyancy force is overcome the force increases rapidly as the oxide is removed from the surface and the electroless nickel substrate can interact with the molten solder. The ability to solder electroless nickel is im- portant for two areas within the PCB industry: 1) The potential to sell electroless Ni PCBs, or indeed bare copper PCBs, without any additional protective layer, to end users, and 2) HASL coating of electroless nickel using DES fluxes. HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature Figure 1: repeated solder wetting balance measurements using a dES flux on (a) copper wire and (b) electroless nickel coated copper sheet with SAC 305 solder.

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