Many different forms of electronic devices employ printed circuit boards having electronically conductive ink compositions. Conductive ink compositions are screen printable and are used to form conductive elements in electronic applications. For example, conductive inks are utilized as screen-printed electronic circuitry in through hole connection, jumpers, printed board wiring and similar electronic applications to provide stable electrical interconnections. In addition, conductive inks may be used in the production of capacitors.
Most currently available conductive inks generally consist of phenolic resin. Some currently available conductive inks also contain an epoxy resin or a mixture of resins, such as acetylacetone with cyanic acid ester resin or acrylic resin with melamine resin. Such compositions also contain electrically conductive filler materials which are dispersed in the resins and which remain an integral part of the final composition after processing. For example, U.S. Pat. No. 6,322,620 discloses a conductive ink comprising at least one epoxy resin, methoxymethylmelamine (a crosslinking agent), a particulated electrically conductive material and an organic solvent.
There are many drawbacks to currently available conductive inks. One such drawback is that many conductive inks have very short shelf-lives at room temperature. These inks are very difficult to transport and store. Shipping, in particular, is quite difficult in that the inks must be maintained within a coolant, such as dry ice, for the duration of the transit. A further drawback of conductive inks is that they frequently require high drying and curing temperatures and relatively long drying and curing times. Consequently, it would be advantageous to develop a conductive ink which would exhibit long shelf life at room temperature. Furthermore, it would be advantageous to provide an ink that requires lower drying and curing temperatures at shorter drying and curing times.
Additional properties are also desired in a thermoset conductive ink. One such desired property is that the conductive ink be sufficiently conductive, or have low electrical resistivity, to carry an electric current when cured. Other desired properties are that the conductive ink have good abrasion and chemical resistance when cured so that it is not easily scratched or wiped off by solvent and that it have good adhesion to the substrate, commonly a copper substrate, when cured. Further, it is desired that the conductive ink provide appropriate rheology for good screen printability and be stable at relatively high temperatures on the substrate so that it remains unaffected by any heat treatment which may be necessary.
Accordingly, it would be advantageous to provide a conductive ink which has a long shelf life at room temperature, low drying and curing temperatures and short drying and curing times. It would be further advantageous to provide such a conductive ink having other desired benefits as well.