In the electronics the quality, reliability, operational speed, device density and cost of circuits are fundamentally determined by carriers. By the development of the manufacturing technology multichip modules have appeared, which are circuits containing several microchips. There are usually a few high-complexity microchips in these modules. These complex integrated circuits containing many inputs and outputs (I/O) need multilayer structures. There was a growing need of a technology, wherewith more than two wiring-layers are realizable.

Multilayer PWB carriers came out. The modern carrier is a multilayer printed wiring board, and passive devices are surface mounted on their terminals as discrete elements. To place an SMD element two solder bondings are necessary. With a new, modern technology it is possible to integrate passive elements (R; C) into inner layers. These passive elements (embedded resistors and capacitors) are created by film technology. The name of this technology is Embedded Passive Technology (EPT).

Passive elements

The advantages of the circuits containing embedded passive elements against SMD devices:

• As the solder bonding are reducing, the reliability is growing,
• Less elements have to be placed and attached,
• Size of circuits are reduced,
• Speed of the signal propagation is grown,
• Better electromagnetic immunity,
• Lower prime cost.

If it is necessary to use better material than plastic carrier, it has to be made of ceramics or glass-ceramics. There are two types of raw ceramics to manufacture Multi-Layer Ceramic (MLC) substrate:

• Ceramics fired at high temperature (T ≥ 1500 °C): High Temperature Cofired Ceramic (HTCC),

• Ceramics fired at low temperature (T ≤ 1000 °C): Low Temperature Cofired Ceramic (LTCC).

The base material of HTCC is usually Al2O3. HTCC substrates are row ceramic sheets. Because of the high firing temperature of Al2O3 the material of the embedded layers can only be high melting temperature metals: wolfram, molybdenum or manganese. The substrate is unsuitable to bury passive elements, although it is possible to produce thick-film networks and circuits on the surface of HTCC ceramic.

HTCC substrates could be only manufactured by companies possessing ceramic technology. The breakthrough for electronics industry was when – mixing glass to slurry – the firing temperature of ceramic-glass substrate was reduced 850 °C, so the equipment for conventional thick-film process could be used. LTCC technology evolved from HTCC technology combined the advantageous features of thick-film technology. Because of the low firing temperature (850 °C) the same materials are used for producing buried and surface wiring and resistive layers as thick-film hybrid IC (i.e. Au, Ag, PdAg or Cu wiring RuO2 based resistive layers). It can be fired in an oxygen-rich environment unlike HTCC boards, where reduced atmosphere is used.

The components of the glass are chosen the way that it crystallizes at the temperature of 850 °C, it has high bend strength and good electrical parameters. During co-firing the glass melts, the conductive and ceramic particles are sintered. Figure below shows the distribution of glass particles in LTCC glass-ceramic before and after firing.

Evolving binding at melting of glass in LTCC glass-ceramic structure

On the surface of LTCC substrates hybrid integrated circuits can be realized. Passive elements can be buried into the substrate, and it is possible to place semiconductor chips in a cavity. In some application (i.e. microfluidics applications) embedded channels are shaped. The number of layers can be as high as 40. Figure shows the structure of a complex circuit realized with the technology.

Complex LTCC circuit structure

The LTCC technology offers a couple of benefits:

• More economical manufacturing process compared with the conventional thick-film technology;
• Mass production methods can be really applied (several processing steps can be automated);
• Fabrication techniques are relatively simple and inexpensive;
• Tapes of different compositions can be manufactured with desired layer properties;
• Electronic circuits can be integrated,
• Design and manufacture 3-dimensional circuits;
• Possibility of cutting the tape / substrate into different shapes;
• Because of the possibility to bury passive components within the substrate, it reduces the size of circuits (down to about 50 percent in comparison to the PCB);
• Number of signal layers almost unlimited;
• Ability to perform at frequencies over 30GHz;
• High resistance against ambient working temperatures (up to 350°C);
• Good thermal conductivity compared to PCBs;
• Compatible with the equipment of the conventional thick-film technology;
• The electrical parameters of useable conductive materials are excellent (gold, silver, etc.).