Conductor pastes

The functions of thick-film conductors are so varied that no one composition is suitable for them all. Conductors play an important role in thick-film devices and the combination of properties is highly complex for both the paste composition and the fired films. Their conductivity, solderability, adhesion, and bonding capability must be excellent since they are the final connection between the circuit and the outside world.

Most of the characteristics of thick-film conductors depend on the composition of the functional phase in the paste, which consists of finely divided particles of:

9 precious metals, particularly Ag, Au, Pd, Pt.

9 base metals such as Al, Cu, Ni, Cr, W, Mo.

The particle size, size distribution, and particle shape have a significant influence on the electrical and other physical properties of the fired film conductor. Conductor based on Pt are

mainly used for heaters or resistive temperature detectors (RTDs). Some of the properties of the most commonly used conductor pastes are shown in Table III.4

Material Sheet resistance (MΩ/cm²)

Wire bonding Soldering Remarks

Ag 1.5-2 Thick Al wire All soft solders Ag migration

AgPt 3-5 Thick Al wire All soft solders Ag migration AgPd 10-60 Al wire All soft solders Good resistance

Au 3-8 Au or Al wire Low melting solders No metal migration AuPt 20-100 Au wire All soft solders No metal migration AuPdPt 10-100 Au wire All soft solders No metal migration

Cu 2-3 Au wire All soft solders High conductivity

Table III.4. Conductor pastes

To sum up, when choosing a conductor paste we have to be aware of the following paste parameters: electrical conductivity, solderability, solder leaching behaviour, bondability (Au or Al wire), electro-migration, adhesion in various working conditions (e.g. thermal aging, thermal cycling or shock, etc.), line definition (i.e. smallest printable line width and gap), compatibility with other pastes, working temperature, maximum allowable firing temperature and price.

Sheet resistance of fired conductors

With the exception of metal-organic conductors, the thickness of which is in the range of thousands of Angstroms, the usual thickness is between 10-20µm. It can be observed that the resistivity of films based on a single element is lower than that of films based on binary or ternary alloys, as happens in bulk metals. Moreover, the resistivities of thick films are usually larger than those of the parent bulk metals and may have quite a range of values, because the microstructure of thick-film conductors is not completely dense and includes voids and glass particles (Figure III.10).

Figure III.10. Simplified view of the cross section of a thick-film conductor

The resistivity of the fabricated layer is also controlled by:

9 the relative fraction of glass or other compounds included in the composition to bind the metal particles to one another and to the substrate.

9 the density of the resulting composite, i.e. the number of residual voids in the structure after the organic vehicle has been fired out.

9 the processing conditions, mainly temperature and time at peak temperature, which define not only the distribution and location of the binder and the number of the voids but also the interactions between metal and binder and between film and substrate.

Adhesion to the substrate

One of the most important factors that affects fired thick-film adhesion is the chemistry of the permanent binder. Three binder types are commonly used in thick-film conductors:

9 Glass (Pb/B2O3/SiO2 with Bi2O3 as fluxing agent). In glass-bonded systems (fritted conductors), the glass typically migrates to the substrate-metal interface during firing.

Intrusions of glass extend from the substrate into the metal film, and sometimes to the surface of the conductor, to form a mechanical bond. In excessively high firing temperature profiles, glass may "float" on the conductor surface and result in poor solderability, poor adhesion and wire bondability. The chemical and physical properties of the substrate play an important role in determining how the glass binder phase wets the substrate surface and is attached to it.

Variation in surface chemistry, grain size, and smoothness of the substrate can cause wide variation in thick-film conductor performances.

9 Oxide (fritless conductors). Small amounts (0,1-1 %) of chemically active oxides like CuO, CdO or NiO, added to the paste, react at high temperatures with alumina substrates to form oxides like CuAlO2, which provide adhesion.

9 Glass+oxide. A combination of glass and oxides is used but the amount of glass is much lower than in the fritted systems. Adhesion is believed to be related to a combination of the effects mentioned above.

To understand the functionality of Bi2O3 in greater detail, its chemistry was explored for typical conditions in actual use. These experiments suggest that molten solders are reducing agents that are sufficiently strong to reduce bismuth oxide to bismuth metal, according to the following chemical reactions:

3 Sn + Bi2O3 → 3SnO + 2Bi (III.7)

3 Sn+ Bi2O3 → 3SnO2+ 4Bi (III.8)

Since bismuth oxide is part of the permanent binder phase in a fired thick-film conductor, a reduction of Bi2O3 to Bi metal destroys the glass network and reduces adhesion.

Bondability of thick-film conductors

Gold conductors (and sometimes also Pt/Au and Pd/Au conductors) have several unique performance characteristics that make them essential in certain applications. In fact, semiconductor devices can be bonded directly to the gold conductors by the formation of a gold/silicon eutectic between the gold conductor surface and the back of the silicon chip. Gold conductors are also excellent for most types of wire bonding. They are ideal for applications in which semiconductors are attached by "chip-and-wire" techniques. Wire bonding with aluminium wire is also possible on Ag, Pt/Ag, and Pd/Ag with reliable results. To obtain good yields during wire-bonding, the surface of the conductor must be very uniform, without oxidations and glass.

Paste price

Table III.5 compares the price of the most commonly used conductive pastes. The price of a hypothetical Ag-based conductor is considered to be 1.

Metal/Alloy Relative price

Ag 1

Ag/Pt 1,8-3

Ag/Pt/Pd 2-4

Ag/Pd 1,6-4

Au/Pt 40

Au 30-40

Cu 2

Table III.5. Price of the most commonly used conductive pastes

The actual cost for the user depends not only on the type and amount but also on the coverage factor. This factor is defined as the area of the substrate covered by 1 gram of the paste, screen-printed according to the manufacturer's specifications.