Bonding to Non-Metallics

A unique property of indium is that it will wet and bond to certain non-metallics such as glass, glazed ceramics, mica, quartz and various metallic oxides. The bond is formed by the adhesion of the indium suboxide to the non-metallic, and in turn the metallic indium to its oxide. Since presence of the oxide is necessary for adhesion, no fluxes can be used since they would reduce the suboxide.

Recommended solders include Indalloy numbers 1E, 3, 4, and 290. Indalloy 1E and 4 exhibits the best wetting, while Indalloy 3 and 290 exhibit higher strength due to the hardening effect of the silver, although they do result in slightly decreased wettability. For non-metallic bonding, an indium applicator is recommended. An information booklet, included with the kit, describes how to use this applicator to create the indium sub oxide layer required for bonding, as well as when to use flux and when to avoid it.

In most cases, ultrasonic energy, like that generated by an ultrasonic soldering iron or pot, is effective in promoting wetting of the surface. Bond strengths of 400-700psi are typical of this bonding technique.

Procedures


Before bonding
  1. Thoroughly clean the non-metallic substrate with a strong alkaline cleaner.
  2. Rinse the substrate with distilled water.
  3. Rinse the substrate with electronics grade acetone or alcohol.
Pre-tinning
  1. In case of glass, quartz, or glazed ceramics, adhesion is enhanced by heating the material to about 350°C (to drive off any moisture that is present) then cooling it to about 200°C. At this temperature, rub indium into the heated non-metallic using an indium applicator to form the indium suboxide.
  2. Once the suboxide is formed, continue rubbing either indium or a high indium-containing alloy until the required amount is bonded to the suboxide.
To bond two non-metallic substrates
  1. Pre-tin each surface with indium or a high indium containing alloy (as described above).
  2. Bring the two pre-tinned substrates in contact with each other and reflow at 20-40°C over the liquidus temperature of the solder used to pre-tin.
To bond a non-metallic substrate to a metallic substrate
  1. Pre-tin the non-metallic surface with indium or a high indium containing alloy (as described above).
  2. Using an appropriate flux, pre-tin only the metallic surface with the same indium or high indium containing alloy as used on the non-metallic surface.
  3. After this pre-tinning, completely remove the flux residue.
  4. Bring the two cleaned, pre-tinned surfaces in contact with each other and reflow at 20-40°C above the liquidus temperature of the pre-tinning solder.

Also see Bonding Non-Metallic Materials in the technical section under application notes for more information.