S-Bond® Solder Products: How They Work
S-Bond® products work with the addition of titanium and/or rare earth elements to conventional solder alloy bases. These active elements migrate to any interface and react with the opposing material surface to remove oxides and nitrides and transport them into the bulk of the solder as an inert material. This process occurs while the material is molten, and once the thin "skin" that forms on the surface of the molten solder is broken, it allows contact between the bulk solder and the substrate surface. The breaking of this skin is referred to as "activation" and is done by application of a low level of mechanical shearing action at the interface between the S-Bond material and the substrate. The level of shear required is small and can be delivered by brushing or scraping the surface, sliding the joining surfaces relative to one another, or application of high frequency vibration to the parts to be joined. The figures below show two types of mechanical agitation, one is brushing or peening and the other is ultrasonic agitation.
Once the skin layer has been disrupted, the bulk solder reacts almost instantaneously and in the case of a molecular bond, irreversibly with the substrate surfaces, creating a tightly held layer of solder on the substrate. This means that the resulting joint may be disassembled and reassembled simply by re-heating above the melting temperature of the SBT product and then re-joining the parts with some additional activation to insure reaction with the new solder. The bonded layer at the substrate surface will not be affected, so good interfacial bond strength is maintained and re-activation is not required.
The activation process for S-Bond products is not ultrasonic soldering, which is where a large amount of ultrasonic energy is directed against the surface of the substrate material to break up the oxide surface layer and allow the molten solder to circumvent and interpenetrate the layer to reach the base material. The oxide layer being broken in S-Bond® joining is very thin, is only on the solder material, requires very little energy to be disrupted, and does not remain as part of the bond.
A feature of S-Bond products is that they do not flow or wick into openings like conventional solders. Unless pushed, our materials stay where they are placed. This can be useful in situations where precision joining is required.
S-Bond adheres to surfaces utilizing one of two mechanisms that are shown to the left. the metallurgical mechanism operates with copper and aluminum surfaces when S-Bond joins at 250C. The active elements in S-Bond alloys enable the reaction of Sn-Ag-Ti with the underlying base metals to produce a metallurgically reacted joint. This process can also operate on many metals be activated at significantly elevated temperatures, normally as a metallurgical pre-treatment, to activate the surfaces for S-Bond joining.
The other joining mechanism that operates at regular joining temperatures (250°C) is one of adhesive joining or attraction of surfaces with opposite "electronic" charges. Since metals such as titanium and stainless steel have very thin 'dielectric' protective oxides such as TiO2 or Cr3O2, they provide an insulator layer that the S-Bond active elements and the elements in the base metals or ceramics attract across as in typical Van der Vaals attractive bonding, as the schematic above illustrates.