Co-fired ceramics

Updated March 11, 2004

Co-fired ceramics are hard substrate materials in which metalization and ceramic are fired together to form signal interconnects such as buried feedthroughs, microstrip, and stripline. There are two broad categories of co-fired ceramics, low-temperature co-fired ceramics (LTCC) and high-temperature co-fired ceramics.

Low-temperature co-fired ceramics

LTCC has been used for the past 15 years or so. This technology combines many thin layers of ceramic and conductors resulting in a versatile mix of microstrip, stripline and three-dimensional interconnects, making possible a whole mess of designs that are not practical on regular alumina or any soft substrate.

Prior to forming the layers, the ceramic/glass frit is held together with a binder and formed into a sheet which is delivered in a roll. In the "green" state, this material is known as "green tape". Check out Jim L.'s poem, "Ode to Green Tape"!

Holes are punched into the layers where vertical interconnects are required, and conductors are screened onto the layers to form horizontal interconnects such as groundplanes and striplines. This is very similar to the thick-film process. In some applications, resistors are formed. Resistors buried on internal layers cannot be laser trimmed, so their accuracy is on the order of 20%.

The tape layers are then stacked up on some alignment pins and compressed to drive out air pockets. Then the tape is fires in an oven. The temperature/time profile is very important in ensuring a quality product.

During firing, the binder is driven from the material and the glass frit melts and joins the layers. Because the resulting structure is part glass/part alumina, its relative dielectric constant is somewhere in between, often around 6.0. The process of firing the part shrinks all of its dimensions. One of the most critical parameters to using LTCC is the shrinkage tolerance, or how accurate and repeatable the parts shrink from one to the next. Shrinkage depends not on just the bulk properties of the substrate, but on how much metal you load it with as well.

The fired panel can go though one more metalization step if necessary. Then the panel is diced.

HTCC

Coming soon!