The LTCC processing is very similar to that of HTCC without the complex firing conditions, flattening fires and plating steps.
Mostly the greensheets are shipped on a roll; the tape has to be unrolled onto a clean, stainless steel table. The sheet is cut with a razor, laser or a punch into parts (these parts have to be a little larger than the blank size, if the material needs to be preconditioned). If a laser is used it is very necessary to control the power to avoid firing of the sheets.
Some tapes need to be preconditioned (e.g. Dupont GreenTape); that means the greensheet has to be baked for about half an hour at 120°C (depends on manufacturer and material). Normally the tapes are shipped with an applied foil / bake sheet, which has to be removed before lamination at the latest; some processors use this foil as a filling mask for the vias.
A blanking die is used to create orientation marks and lamination tooling holes (and the final working dimension in case of to be preconditioned tapes).
Note: It is preferred to rotate the single parts in turns 90° to compensate the different x/y-shrinking of the LTCC.
- Forming vias:
Vias may be punched or drilled with a laser (low power).
- Filling vias:
Vias can be filled with a conventional thick film screen printer or an extrusion via filler.
In the first case the tape has to be placed on a sheet of paper which lays on a porous stone; a vacuum pump holds the tape on his place and is used as an aid for via filling.
Note: The possibilities of this method are limited; the vias must have a larger diameter than tape thickness. The smallest possible size of vias to be filled also depends on the viscosity of the paste.
- The second possibility to fill the vias is to use a special extrusion via filler that works with pressures of about 4 to 4.5 bar.
Both methods need to have a mask; this mask should be made of a 150-200mm thick stainless steel. An alternative to that is to use the (Mylar-)foil the tape usually is applied on.
For the filling of blind vias it is advisable to form the holes concerned of the masks a little smaller than the diameter of the blind vias. Otherwise there could occur problems with the filling rate.
Cofireable conductors etc are printed on the green sheet using a conventional thick film screen printer. The screens are standard (250 325) emulsion type thick film screens. Just like the via printing process, a porous stone is used to hold the tape in place. Printing of the conductor tends to be easier and of higher resolution than standard thick film on alumina. This is due to the flattness and solvent absorption of the tape. After printing, the vias and conductors have to be dried in an oven at 80 to 120°C for 5 to 30 minutes (depends on material); some pastes need to level at room temperature for a few minutes before drying.
Note: Resistors may vary their value when terminated with different conductors. With the help of a Micro-Screen printer, it is possible to print conductors with a 50mm line resolution.
- Register for Lamination:
Each layer is placed in turns over tooling pins.
Some processors use a heat pliers to fix the sheets in turns one on top of the other.
There are two possibilities of laminating the tapes.
The first is named uniaxial lamination; the tapes are pressed between heated platens at 70°C, 200 bar for 10 minutes (typical values). This method requires a 180° rotation after half the time. The uniaxial lamination could cause problems with cavities / windows. This method causes higher shrinking tolerances than the isostatic lamination.
The main problem is the flowing of the tape; that results in high shrinkage tolerances (especially at the edge of the part) during the firing and varying thicknesses of single parts of each layer (causes hard problems on the high frequencies sector).
The second way is to use an isostatic press. The stacked tapes are vacuum sealed in a foil and pressed in hot water (temperature and time are just the same like using the uniaxial press). The pressure is about 210 bar.
Note: deep cavities and windows need to have an inlay during lamination.
Laminates are fired in one step on a smooth, flat setter tile. The firing should follow a specific firing profile, what causes the need of a programmable box kiln.
A typical profile shows a (slow) rising temperature (about 2-5°C per minute) up to about 450°C with a dwell time of about one to two hours, where the organic burnout (binder) takes place; then the temperature has to be rised up to 850 to 875°C with a dwell time of about 10 to 15 minutes. The hole firing cycle lasts between three and eight hours (depends on the material; large / thick parts cause the need of a modification of the firing profile).
Note: especially resistor pastes need to have defined firing conditions (temperatures);Otherwise they vary enormous in value.
Some materials need to be postfired; that means the paste is to be applied after firing the tape and has to be fired again. The postfiring conditions depend on the used material and vary in a wide range.
Note: especially resistor pastes need to have defined firing conditions (temperatures); Otherwise they vary enormous in value.
If the fired parts have to be cut into smaller pieces or other shapes, there are three different ways to realize.
The first is to use a post fire dicing saw, which is a common method and works very well for rectangular shapes; it holds tight outside dimensional tolerances and allows high quality edges.
The second possibility is to use an ultrasonic cutter; the final part shows low tolerances and may have unusually shapes. This process is very slow and expensive.
The third method uses a laser to cut the fired tape; the tolerances are tight, but the quality of the edges is very bad.
©2001-2014 IMST GmbH, Last update: 26-Oct.-2010