Insulated, Passivated & Adhesively-Promoted Bond Wire Using All-in-One Al2O3 Coating
Authors: Park Soojae , Lee Jonghyun , Cho Chulhyung , Kim Namhoon , Lee Yongje , Seo Sichun , Kim Manho , Yoon Youngkwon , Min Eulgi , Choi Kyujung , Lee Sang-Hoon , Nam Hong-Sik , Cho Monghyun , Moon Jeongtak 
Published: May 2023

Wire bonding technology has been dominant over soldering joint method for electronic packages especially for low I/O density due to its assembly simplicity. Bond wires are composed , at most, of a combination of two different metals, which are Pd-coated Cu and Au-coated Ag wires. Those wires possess conductive metal surfaces, which do not allow electrical insulation, corrosion resistance and strong adhesion to polymeric encapsulation materials. Cu wires is most susceptible to oxidation, but HAZ (Heat-Affected Zone) of Ag wires is also subject to oxidation at elevated temperatures. Those wire metals do not bond well to epoxy-based encapsulation adhesive, which is exposed to reliability issues in heat and moisture environments. Demands for excellent adhesion of bond wires and encapsulation recently increased for the growing applications of automotive and advanced LED packages. Both applications are subject to higher temperature and moisture levels than conventional packages, some of which are placed in temperature and moisture-controlled environments. Similarly to CTE (Coefficient of Thermal Expansion), the coefficient of moisture expansion can induce the hygral strain at the same magnitude of thermal expansion [1]. The interfacial crack between bond wires and encapsulation grows along the axis of bond wires until bond wires may rupture into two pieces. Despite the large mechanical strengths of metals, the large volume of encapsulation surrounding the wires can exert extreme stresses induced by CTE and CME mismatches. Demands for insulated bond wires recently soared since electrical shorting is inevitable due to decreased wire diameter coupled with high bandwidth requirements, and insulated wire was identified as a potential solution to enable 25µm pitch wire bonding [4]. Insulated wires allow wire-bonded wires to sweep, touch and cross, especially for long-wire loops, since electrical short can be prevented. An organic coating was applied on bond wire metals for electrical insulation, but it suffered poor stitch or 2nd bonding since organic materials do not form strong bonding to wire metals except for Al metals. The electrical conductivity of Al metals falls short of noble metals and Cu metals. Except for Au bond wires, Ag, Pd-coated Cu and bare Cu wires are susceptible to accelerated oxidation during ball formation since they require formic gas to prevent oxidation at elevated temperatures [5]. Similar to HAZ after ball formation is completed, it is TBMZ (To-Be-Melted Zone) that is exposed to oxidation at elevated temperature as molten balls above its melting temperature approach to TBMZ. TBMZ eventually melts to be a part of growing molten balls, but it experiences high-temperature oxidation immediately before melting. The surface of TBMZ transforms to its metal oxide in the neighborhood of growing ball even during a short period of time, and thus preventing gradual melting of metals, which is responsible for tilted balls as one of bonding requirements. Thick oxide surfaces immediately before melting are responsible for off-centered ball as it creates unsymmetrical breakdown of oxide layers on the surface of the TBMZ region neighboring molten balls. It is the formic gas that prevents metal oxidation of TBMZ. Note that this TBMZ is a wire region to be melted until the completion of ball formation, but not HAZ (Heat-Affected Zone) after molten balls solidify. This TBMZ region eventually melts to be a part of molten balls. It is bare Cu wire that needs passivation for longer shelf life. The surface of bare Cu wire experiences oxidation even at ambient environment so that its shelf life may be as short as 7 days. In this work, we introduce Al2O3–coated insulated wires that provide excellent insulation and bonding strength for Au and Ag bond wires, using ceramic coating instead of metal coating. Al2O3 material is also well known for excellent electrical insulations for decades. Due to its ceramic properties, Al2O3 coating on conventional bond wires can produce strong/reliable wire bonding by far better than organic insulations, especially for the 2nd bonding. In addition, our ceramic passivation does not change IMC (Inter-Metallic Compound) types since the ceramic materials are not coupled with molten metals. Thus there is no reliability degradation induced by Al2O3 insulation at bonded joints [6, 7]. Contrarily, metal coating on conventional bond wires such as Au-coated Ag wires and Pd-coated Cu wires, are severely susceptible to IMC changes, and often responsible for premature failures. Adhesion between Al2O3–coated insulated bond wire with epoxy-based EMC and adhesives is greatly increased since the adhesion of Al2O3 to epoxy resin is by far greater (two orders of magnitude) than that of Cu to epoxy [8-10]. Conventional bond wire metals do not adhere well to epoxy while Al2O3 is well known for excellent adhesion to epoxy. We describe our innovative idea incubation, original ALD (Atomic Layer Deposition) coating processes for ultra-thin Al2O3 deposition on wire metals, and multitudes of required experimental data associated with Au and Ag bond wires. We present cost-effective ALD coating methods even for <10nm-thin Al2O3 insulation on 18µm-thin bond wires.