Multilayer ceramic technology stems from the late ‘50th as the development of RCA Corporation. Later IBM has stroked up researches and has evolved the first multilayer ceramic substrate with the following parameters: surface 9 cm2, 33 layers, has 100 flipchips on its surface as LSI component. The substrate was fired at the temperature of 1600 °C, Mo, Mo-Mn, and W were used as conductive component.

From the middle ’80th the development of the computers has geared, so the aim was the increase of density of conductive network on substrate. More and more thinner wirings were used, but hereby the resistance value has increased. There was a need of conductive materials with lower resistance value. These are Cu, Au, Ag, etc. The substrates had to be good thermal conductor. The high speed of the signal propagation required low permittivity of ceramic.

In the beginning of the ’90s a substrate was developed by American and Japanese electronic and ceramic firms, which satisfied the demands above. IBM and Fujitsu were the first companies, who applied ceramic with low permittivity and Cu conductive layer on it in their products. In the second part of decade the interest leveled at wireless applications. The high frequency Bluetooth és Wireless applications required the low expansion coefficient and low permittivity.

In two tables are delineated the parameters of LTCC substrate of some relevant firm. The first table introduces the period from 1985 to 1990, the second table shows the products of current market marker. The conductive materials of it are Au, Ag, Pd, Cu and their alloys; the firing temperature is between 850 °C and 1000 °C. The second table shows loss tangent (tg d) too. The direction of advancement was the use of unleaded glass.

Firms dealing with glass-ceramic and parameters of their products between 1958 and 1990

Firms dealing with glass-ceramic and parameters of their products between 1990 and2007

The base materials of actually applied conductive pastes which can be burned in in air are Au-Ag, terner or binder of Au, Ag, Pt and Pd. The Au and Ag pastes have the lowest specific resistance, some Pt-Au pastes have the highest. The table below shoes the parameters of a few gold, silver and metal mixtures.

Adventages and disadventages of gold, silver and metal mixture

Nowadays various vendors have developed sundry conductive pastes that can be used in LTCC technology. These pastes show only little differences from the pastes used in thick-film technology.

Some properties of Au and Ag conductive pastes recommended for LTCC

The gold and silver conductive pastes have only a little difference from each other. So because of the high price of gold conductive pastes, silver conductive pastes gather ground increasingly in LTCC technology.

Ferro and DuPont LTCC conductive paste's parameters

It is a very difficult task to choose the optimal paste for the desired purpose because of the large conductive paste selection of LTCC paste manufacturers. The conductive pastes containing noble steel can be very different depending on their composition and with that their features, therefore the collective observance of price and quality can be very important. Primarily the gold conductive pastes are used because of the high price of gold at producing of high reliability and microwave circuits.

Additional companies dealing with conductive pastes: Coors, Murata, Heraeus, Electro-Science.

More than 50% of the embedded passive components are resistors. In thick-film circuits the resistors are realized by carrying up the appropriate electrical properties layer with screenprinting, afterwards they are burned in. The following requirements are put up for resistive layers:

• Stick well to substrate and conductive layer,
• to have large square resistance,
• to have low temperature coefficient,
• film-technology have to be simple and fully controllable,
• values can be easy to trim.

The resistive compositions developed for the LTCC technology are produced in decadic scales of its sheet resistance (1 Ω, 10 Ω, …, 100 kΩ, 1 MΩ). Often there is a need of an intermediary value. This can be achieved by the appropriate mixture of the consecutive composite decades i.e. to achieve 200 ohm square resistance value the 100 ohm resistive paste has to be mixed with 1 kohm resistive paste in a ratio of 70/30 based on different diagrams. If the composition sequence is made by two different materials, the manufacturer has to tell the margin of the two materials to avoid the mixture of them.

The second important property of pastes is the temperature coefficient (TC) of resistors made from them. This value is in case of thick-film resistors with the best quality ±50..100 ppm/°C. To adjust temperature coefficient it is necessary to admixture (i.e. oxides of Ir; Zn; Mn; Rh; Bi; Ni). Forasmuch as the specific resistance is in inverse ratio to temperature coefficient, the adjustment of these properties requires expertise in a great extent. The strict TC rules make resistors more expensive and with increasing of square resistance usually the thermal stability decreases. If power-level is low and the aim is small size it is expedient to choose the paste which leads to the smallest geometric size.

Some resisitive paste by DuPont and Ferro to LTCC technology

Currently DuPont and Ferro deal with making resistive compositions for LTCC technology. DuPont has sundry pastes slightly differ to each other. The 87 serial of Ferro has high tolerance (±30 %) and TC (±450 .. ±200 ppm/°C) therefore in case of microwave applications it is worth to choose of DuPont pastes.

A plain capacitor is made up of three layers: dielectric layer and two non-intercommunicating conductive layers separated with dielectric layer. These layers are screenprinted and co-fired with the substrate.

Benefits of buried passive capacitors:

• It can be realized by printing conductive layers without subsidiary costs
• Low level of tolerance

• Low value of specific capacitance
• Large claim of place, specific capacitance can be increased by multilayer parallel plain capacitor structure

To realize capacitor values in an acceptable range different dielectric pastes have to be used. Materials of dielectric pastes are usually BaTiO3, BaTiO3-epoxi, polymer-ceramic, epoxy-glass mixture.

Polymer materials have a major role in thick-film technology of buried conductor. Their advantages are high permittivity and minimal leakage current. Predicition of its fashion is low temperature (230 °C) which is a benefit against other dielectric materials too. Polyacrylonitrile (PAN), Polynorbornene (PNB), and Polyimids are also used as dielectric. The range of embedded capacitors can vary between 77 pF/cm2 and 16.000 pF/cm2.

Parameters of some capacitor-dielectric

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