Flip chip on paper assembly utilizing anisotropic conductive adhesive

Motorola BiStatix is a radio frequency identification (RFID) tag that utilizes a small RF silicon chip embedded in a carbon ink antenna. Unlike bar codes, which are passive tags, BiStatix identification tags can transmit and receive two-way information to help identify, track and route packages remotely. The BiStatix tag is inexpensive and made from environmentally preferred, low cost materials and manufacturing processes. The concept of flipping or inverting a silicon chip to be mounted on a paper substrate offers distinct advantages and enables meeting the cost and performance goals of this new product technology. The unbumped (bare) flip chip - without a dielectric layer and conductive polymer bumps - is aligned and placed on the paper substrate with compressive force. A thin layer of anisotropic conductive adhesive (ACA) is used to attach the IC chip to the conductive ink antenna on the paper substrate. The conductive adhesive underfills and cures in a few seconds. Advantages of this process are the elimination of additional curing processes and reduced equipment requirements. Another advantage is the capability of the adhesive to fill the narrow gap under the chip, enabling a lower interconnect height and a reduced total IC packaging thickness. Significant process development and reliability assessment was completed to achieve a flip chip on a paper substrate assembly process with high throughput and low cost. This paper discusses the assembly reliability results for this low cost flip chip on paper technology utilized in the BiStatix RFID tag.