Adhesive Flip Chip
Flip Chip Assembly Contents
Another form of flip chip mounting that is gaining favor in the portables world today is adhesive flip chip. There are several forms of this technology, depending on the requirements. A form of flip chip using non-conductive adhesive film to directly bond a stud-bumped IC to a fine line circuit board has been reported. (See Flip Chip Reference 3.) Because the adhesive film does not contain conductive particles, as in the case of conductive adhesives, it can be used for smaller pad pitches. The film also acts as a encapsulant, or underfill material, for thermal-mechanical management.
The other form of adhesive flip chip mounting utilizes an anisotropic conductive material, in either a film (ACF) or adhesive (ACA) form, as the electrical and mechanical joining agent. Anisotropic conductive film looks like paper and consists of thermosetting adhesive, conductive particles, and release film. (See Flip Chip Reference 4.) Figure 6 is a schematic representation of anisotropic conductive film flip chip technology. Anisotropic conductive adhesive looks like paste and consists of thermosetting adhesive and conductive particles.
Figure 6: Cross-section of adhesive flip chip.
Several advantages to adhesive flip chip mounting technologies are noted:
In general, the pad pitch of the die is finer than possible with solder flip chip. The cleaning step is not as rigorous as with solder based systems, it eliminates process steps such as coating the interconnect pads with solder and eliminates the use of lead. The bumping process may be simplified by the use of stud bumping, which utilized the same processes and equipment to form a bump as to wire bond to a IC pad. Thermocompression and ultrasonic energy are used to form a gold ball bond on the IC pad; the wire is cut at the top of the ball and leveled, creating a gold bump suitable for ACF bonding to a substrate.(See Flip Chip Reference 5.)