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November 2014 • SMT Magazine 63 HigH-reLiabiLity, Pb-Free, HaLogen-Free soLDer continues arTiCle tronic assembly components. Simplistically, these materials reduce metal oxides to elemen- tal metal to allow the soldering process—namely solder wet- ting and complete joint co- alescence—to occur. The most common oxide-active chemi- cal groups used in the soldering process are carboxylic acids and halogen-containing molecules. Carboxylic acids de-protonate to form the carboxylate ion and a free proton; during the solder- ing process a metal carboxylate complex is formed with water as a bi-product. Typically orga- no-metallic salts are readily sol- uble in the resinous flux matrix and are inherently safe from the point of view of post-soldering electrical and electrochemical reliability. A similar reaction oc- curs when considering halogen- based active materials; ionic ha- lides also form metal salts and are 'neutralised' by the resinous system post-soldering. There are two main drivers focussing on the elimination of halogen from soldering products. The first is environmental due to the potential for residual halogen- producing poly-halogenated dioxins during PCB incineration. This driver is predom- inantly focused on polybrominated flame retar- dants but does impact solder flux formulations. The second is performance. Some early indica- tions show that trace halogens (and potentially halides) can contribute to increased potential for electrochemical migration and corrosion potential as PCBs transition to finer pitches and higher voltages. Flux Development changes When halogen is removed from a flux, it cannot simply be replaced with other metal ox- ide active molecules. The halogen-containing compounds are very stable within the flux pri- or to reflow because the halogen is covalently bonded. Therefore, simply replacing the halo- gen-containing compounds with other activa- tor types can often reduce stability and thus present challenges rebalancing the formula- tion. Moreover, halogens are exceptionally ef- ficient at oxide removal and offer significant reflow performance benefits. Figure 7 presents an example of the reflow performance of a stan- dard no-clean, halogen-containing reflowed solder joint in a typical aerobic profile as a di- rect comparison to the identical formulation with the halogens removed. The PCB used for this testing was a standard FR-4 PCB with OSP copper solderable pads. The profile used is a typical aerobic linear profile for SAC alloys. Fig- ure 8 presents the relative wetting speeds of the two pastes using an industry standard wetting balance tester onto oxidised copper substrates. It is evident that the halogen-containing ma- terial shows complete coalescence of the alloy and acceptable wetting onto the substrate. In Figure 7: Comparative reflow of halogen-containing solder paste (top) and an equivalent halogen-free (bottom) solder paste.