Heat cure conductive adhesives

The electrical properties, especially for the high-purity semiconductor-grade silicones, are excellent even at temperature extremes of —80 °C and 200 °C and in high humidity. Silicone adhesives are available as electrically conductive and as electrically insulative types. Typical electrical properties for moisture- and heat-cured silicone adhesives are given in Table 3.12. 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

Electromagnetic/radio-frequency interference shielding materials have to meet much lower demands in terms of overall electrical conductivity (typically 4-5 orders of magnitude lower than a silver-flake-filled adhesive). This means that cheaper conductive fillers can be employed, for example, silver-coated copper flake, nickel flake, and carbon black. Typically the adhesive has to form a compliant joint between two mating surfaces, and hence room temperature vulcanizing or heat-cure silicone is often a convenient choice of matrix material. 

Conductive adhesives cure at much lower temperatures than are required for solder flow and permit use in contact with heat-sensitive components. Two-component conductive epoxies cure at room temperature. 

For adhesive bonding, the induction is used for indirect heating of the adhesive. The inductor must be adapted to the shape and desired temperature profile. Likewise, the frequency has to be adapted to the size and conductivity of the component. The reproducibility of the heating depends on the uniformity of thickness of the adhesive film, which in turn depends on the allowable variation of a component and requires precise control of the temperature in the sensitive region for adhesive curing. 

Electrically conductive heat cure adhesives are required for a number of... 

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