Face-Down TAB for Hybrids

Bell Laboratories has for some time been developing tape automated bonding (TAB) techniques for hybrid applications involving medium- and large-scale integrated (MSI/LSI) chips. The present paper describes a face-down TAB approach which results in a structure that resembles a beam lead structure. Process steps required for the fabrication of the face-down TAB structures and physical design requirements to control mechanical failure modes are presented. Detailed information is given in the fabrication of a test structure designed to these requirements. The structure uses gold plated copper tape leads to attach an MSI chip to a thin-film metalized ceramic substrate. The structure is environmentally protected with an room temperature vulcanizing (RTV) silicone rubber encapsulant using the same techniques that are employed for the encapsulation of Bell System hybrids with beam leaded chips. Accelerated temperature cycling tests were carried out using this structure over a range of temperature differentials. The results are encouraging from the standpoint of mechanical reliability for communication applications. The face-down TAB approach is compared to the conventional face-up TAB approach where the back side of the chip is bonded to ceramic substrate on the bases of 1) process steps (with implications for assembly cost), 2) hybrid substrate area requirements, and 3) junction-to-substrate thermal resistance. It was found that the facedown structure appears preferable for hybrid applications where cost and/or area considerations dominate. The face-up structure appears preferable when higher thermal conductivity and/or backside electrical contact are required. It is concluded that for many communication applications the face-down TAB approach can be used to attach MSI/LSI chips to multichip hybrids with size, cost, and reliability benefits analogous to those presently associated with beam leaded chips.