Active Solder… What-Why-How

What is meant by “active solder”?  The term evolves from active brazing; I assume that does not really help you…. But it is true that active brazing was the key technology that led to the development of active solders.

Most important in the brazing/soldering sense what does “active” really denote. By practice, active brazes and solders have elements added to their base compositions that are reactive with surface oxides and other compounds that form naturally and can, when thermally activated (heated to reaction temperatures), these reactive elements can interact and substitute themselves into the chemical structure of surface compounds (oxide layers,  solid oxides  or other ceramics) in a way that the new interface compounds form which are well bonded to the surface of a material being joined.

For example, elements such as titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb) and tantalum (Ta) have an electronic outer shell structure that enable them to more easily react with many compounds and can thus integrate themselves (when thermally activated) into many compounds found in surface films on metals, semiconductors and ceramics. Thirty years ago or so, with the advent of vacuum metallurgy and vacuum brazing, the integration of such “reactive” elements into brazing filler metals became commercially viable. Wesgo Metals (division of Morgan Advanced Ceramics) and Degussa (now BrazeTec) pioneered such reactive element additions to braze filler metals.  These reactive metal additions to braze fillers are “active” (typically in low concentrations, 0.1 – 2 w/o) since upon melting, the elements diffuse to the braze interface and “reduce” the local oxide (or ceramic) and enter into the bonding structure. Thus the reactive elements become a “active” participant in the chemical constituency of the interface compounds that the term “active braze” stuck and these braze filler metals have been termed “active braze alloys”. Mostly Ti, Hf, Zr or V are being commercially added to Cu-Ag, Au, and Ni alloys. However due to the “activity” of these braze fillers brazing the active braze process is exclusively done high vacuum in order to exclude any oxygen or nitrogen that might react the Ti, Hf, Zr or V elements, preventing sufficient reactive element activity to diffuse and react with surface compounds.  It has also been observed that elevated temperatures, normally over 800°C, have been required to thermally activate the substitution process of the reactive elements with interface compounds in brazed assemblies in order to effect a braze joint.

The extension of “active” brazing to soldering and to be active at soldering temperatures (below 450°C) was a goal of investigators at Euromat and S-Bond Technologies. In 1996 their first “active” solders were patented. Before this time the element indium was the only active commercially available solder. Note that Ti and Hf additions had been made to Sn-Ag base solders but the soldering required heating these filler metals to over 800°C in a vacuum to get the reactive element “Ti” to react with surface compounds and bond. The discovery by S-Bond Technologies of small additions of rare earth elements (e.g. Ce, La, and Lu) gallium and titanium together in solder filler bases  (Sn-Ag, Pb-Sn, Sn-Sb, Sn-Bi, etc.) enabled the “active” soldering phenomena to occur when soldering  most metals and many ceramics at solder melting temperatures (from 115°C to 420°C). The addition of rare earth elements and gallium enabled these Ti containing “active” molten solders to wet and adhere to many metal and ceramic surfaces with using flux. A key part of active solders is that they are “self fluxing” as they are melted and bond to a wide range of base materials without the need for added chemical fluxes or plating…. A key attribute of active solders (and/or brazes).

It was found; however, that the active soldering behavior (wetting and bonding without flux) required that “mechanical activation” be used in conjunction with the active solders patented by S-Bond and Euromat. This process is a means, again without chemical fluxes, to break up the stable solder oxide film that forms when the active solders melt. With the addition of Ti and rare earth elements into Sn-Ag and other solder bases, the rare earth element modifying the melting surfaces’ oxides forming a protective layer for the molten solder, but encasing the other active elements from coming into contact with the base materials. Once the thin oxide film is broken in a continuous way, Ti, Ce and Ga interact with the base materials’ surface compound and bonds to or breaks up these compound layers. See how active solders work on aluminum to provide metallurgical bonds with the need for aggressive chemical fluxes.

So for active solders…

The What: Solders that when melted can wet and adhere to metals, ceramics and glasses without the use of chemical fluxes

The Why: To join at soldering temperatures, without flux and eliminate contamination and entrapment associated with flux usage. Also is enables solder joining of metals, ceramics, carbon/carbides

The How: With Ti, rare earth and gallium additions, many solder bases can be made “active” provided the active solder compositions are used in conjunction with mechanical activation that effectively disrupts the oxide films that form constantly on molten active solders.

If you have further questions about active solders and would like to know more about how they could benefit the assembly of your components [flux free and no plating] then Contact Us.