Most conventional isotropic adhesives use relatively large (several micron) silver flake in a resin base, typically epoxy. Consequently, they have limitations in terms of conductivity, strength and moisture resistance. They typically cure in the 120-150⁰C range, much lower than the reflow temperature of solders.

There are many opportunities to use novel conductive adhesive systems to reduce processing temperatures, but issues of strength under physical shock and high moisture environments are not possible to solve using conventional fillers and polymer systems.

Nano sized fillers have a great advantage over conventional fillers which generally need to be in physical contact before conductivity is significant. They can be effective even if not directly in contact!

Conventional fillers require a relatively high loading of around 75% filler by weight, and preferably a polymer which shrinks on curing to give direct physical contact between filler particles and the matrix. Oxidation of the filler is an issue, as all polymers are relatively permeable to moisture. That is why silver flake is still the primary filler, as it does not cause (many) objectionable electrochemical effects and its oxide is conductive.

The penalty of this approach is that the high loading of metal required to raise the electrical conductivity (the percolation threshold) is very close to the level at which the physical properties of the polymer degrade due to excessive filler content.

The percolation properties of nano particles are quite different. The percolation threshold is reduced to as low as 2% and has been demonstrated in the case of indium-tin oxide as well as carbon black and carbon nanotubes. The comparative percolation thresholds are shown schematically in Figure 1. This phenomenon relies on electron tunneling to create some level of conductivity that can occur when conductive particles are separated by 10 nm or less.

What this gives us is the opportunity to lower the filler loading in adhesives and to create conductive materials that may be strong and in some cases translucent or transparent for displays or UV curable adhesives, with curing temperatures in many cases less than 130⁰C.

Figure 1. Percolation thresholds for nano sized and conventional conductive fillers (schematic).

Non-conductive adhesives such as underfills can be used in die attach and other interconnect applications. The shrinkage of the polymer pulls the bump towards the pad. Unfortunately, the conventional 500nm size fillers needed to modify the thermal expansion coefficient and other characteristics of the adhesive can interfere with the direct connection between bump and pad. Work by C.P. Wong and his group at Georgia Tech has shown that nano sized fillers are swept out of the way by fluid flow and do not interfere with joint formation (Figure 2).

Figure 2. Wafer bump and contact pad, showing interference by pigment size particles (upper) and non-interference by nano size particles (lower), after Shih and Wong.

Dr. Rae's entire paper, "Nanotechnology and Low Temperature Electronics Assembly," presented at SMTA Pan Pac 2005, is available from the SMTA Knowledge Base