Epoxy: Conductive and Non-Conductive Paste
There are four leading alternatives for flip chip mounting of gold-bumped die: non-conductive epoxy, conductive epoxy, thermocompression attach, and thermosonic compression.
Non-conductive Epoxy Process. This approach calls for an adhesive material to fill the void between the die and the package and around the gold bumps. This adhesive or epoxy will shrink as it cures, and provide the forces needed to hold the die and package together. The connection is made with a physical metal-to-metal contact between the gold ball and opposing bond pad on the package. In this case, the ball is typically a coined shape to maximize the surface area in contact.
It is common in this process to use an anisotropic conductive adhesive (ACA). This refers to a particular type of adhesive that becomes conductive only in the Z-direction or, more precisely, in the direction that it is being compressed. The compressed adhesive is that which is trapped between the bump and package. When this is compressed, conductive particles inside the epoxy will align themselves and create a conductive path between the die and package. Note that there is no conductive path in the X and Y directions, which would create a shorting path between the bumps.
Conductive Epoxy. The conductive epoxy solution calls for the application of a small dot of conductive epoxy to the top of each gold stud. As a variation of this solution, the epoxy can be placed on the opposing pad in the package.
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