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.