Isotropic conductive adhesives curing

D. Klosterman, L. Li, and J.E. Morris, Materials Characterization, Conduction Development, and Curing Effects on Reliability of Isotropically Conductive Adhesives, IEEE Transactions on Components Packaging and Manufacturing Technology Part A, Vol 21, 1998, p 23-31... 

Anisotropic adhesives are more sensitive than isotropic conductive pastes to the coplanarity of the connecting pads and leads of the substrate and the device because many of them are based on the single-particle bridging concept. The typical properties of a screen-printable isotropic conductive adhesive paste are a silver content of 70-75% by weight, a viscosity of about 80 Pa s, and a curing time of 15 s at 175°C. This fast cure cycle produces an adhesive layer with Tg... 

The electrical conductivity of isotropic conductive adhesives is established in all directions by metal particles, generally silver flakes. These silver flakes are embedded in a matrix consisting of a base polymer, generally epoxy resin, and hardeners such as amines or carboxylic acids. It is the hardeners that ensure the mechanical strength of the joint. The choice of these two components can influence reaction speed and curing temperature. Broadly speaking, any chemical reaction will be fast at a high temperature and slow at a low temperature. 

Polymer matrices of isotropic conductive adhesives are similar to anisotropically conductive adhesives. An ideal matrix for ICAs should exhibit a long shelf hfe (good room temperature latency), fast cure, relatively high glass transition temperature (Tg), low moisture pickup, and... 

There are general classifications of conductive adhesive materials based on the method by which conduction is achieved. Isotropic adhesives are the most common, and have been used in flexible connections. They normally contain about 70 wt. % Ag particles, but other metals are used as well [6]. The quantity of metal loading is sufficient to assure that the adhesive is electrically conductive in all directions upon curing, as illustrated in Fig. 4, hence the term isotropic. This is in contrast to adhesives with much lower metal filling, such that there are an insufficient number of particle-to-particle contacts to render the adhesive conductive, as depicted in Fig. 5a. However, if the adhesive is sandwiched and compressed between a pair of terminal pads, the particles between the... 

Adhesives used in connector applications include Loctite s 3887 isotropic epoxy conductive grade. This two part product comes as a thick paste which is cured by exposure to heat and which is designed for enhanced adhesion to gold plated devices in electronic interconnect applications. The product formulation is reported to absorb stress associated with extreme thermal mismatch between dissimilar substrates. 

---