LTCC can be defined as a multilayer circuit fabricated by laminating single greensheets (term for unfired tapes; Green TapeTM
, Dupont) with printed conductor lines etc. on the surface on top of each other and firing them all together in one step. This process is almost similar to that of HTCC, but the big advantage of the LTCC system is the possibility to use low resistivity conductors like silver, gold, copper and alloys with palladium and platinum instead of tungsten and molybdenum; the separation between this two (different) technologies is defined with the firing temperature: in excess of 1000°C for HTCC and below for LTCC (mostly 850 to 875°C, what makes it possible to use silver for conductor lines etc.). The hole process includes several steps which are described under "LTCC Process". As a disadvantage it has to be mentioned that these ceramics provide a bad thermal conductivity (about 2.5 to 4 W/mK), what can, if it’s necessary be compensated with the help of so called Thermal Vias which rise the thermal conductivity up to about 20 W/mK (70 W/mK; Source: DuPont).
It is possible to involve photo processes into the row of process steps of LTCC, but (nearly) all of the to be applied pastes are printed on the greensheet with the help of a conventional thick film technique. Standard thickfilm pastes must not be used (except for several postfire processes; depends on material); this is due to the shrinking the tape is subjected to during the firing. This shrinking is about 10 to 15 percent in x/y-axis (shrinkage of x- and y-axis may be different) and about 10 to 45 percent in z-axis.
It is also possible to integrate passive elements like resistors, capacitors and inductors into the substrate. Resistors are processed with the help of special pastes, which have to be printed on the tape just like conductor lines and have to be post- or cofired. The nowadays problem is the tolerance of buried resistors, which provide only values of about 30 percent. Capacitors and inductors are build only with the help of special forms of the conductor lines (plates, spoils); there are and there will be much more materials like ferrite pastes and high-k pastes, which offer a great range of possibilities to process such elements.
The LTCC technology avoids many disadvantages the others have and offers a couple of benefits:
- Economizes multiple steps of the manufacture process compared with the conventional thickfilm technology (parallel processing);
- 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;
- Thermo-physical properties can be modified;
- Possibility of auto-packaged devices fabrication;
- Electronic circuits can be integrated, using its hybrid nature;
- 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 (factor 10);
- Good match to semiconductor TCEs;
- Very good hermeticity of the substrate.