LTCC: Low Temperature Cofired Ceramics

Technology and Equipment by Haiku Tech

The Low Temperature Cofired Ceramic (LTCC) technology can be defined as a way to produce compact multilayer electronic circuits with embedded passive companents. LTCC's are produced with the help of readily available ceramic single tapes, with applied conductive, dielectric and / or resistive pastes. LTCC technology excels in miniaturization of extremely robust electronic circuits for harsh environments and space. It excellent dielectric properties, thermal expansion coefficient and thermal conductivity make it the technology of choice for microwave and high power multi-chip modules.

The single ceramic layers are laminated together and fired in one step. This saves time, money and reduces circuits’ dimensions. Another great advantage is that every single layer can be inspected and in the case of inaccuracy or damage replaced before firing; this prevents the need of manufacturing a whole new circuit. Because of the low firing temperature of about 850°C it is possible to use the low resistive materials silver and gold instead of molybdenum and tungsten (which have to be used in conjunction with HTCC materials like alumina or aluminium nitride).​

Haiku Tech integrates lab scale lines as well as fully automatic LTCC lines for processing green tapes from KEKO Equipment and Schmid Thermal Systems-Sierratherm. Haiku Tech engineers also provide prototyping services and training on-site.

1. Tape Casting

The foundation of an LTCC product is a ceramic tape that is cast with a doctor-blade tape caster, dried by air and/or under web heating. The tape is wound onto a roll for further processing. Haiku Tech offers the CAM-series tape caster from Keko-Equipment.

2. Tape Blanking

The tape is unrolled and cut into individual pieces. It can process either a ceramic tape cast onto a carrier PET film or tapes without carrier film. Edge trimming and or inspection and marking are possible as options. Haiku Tech offers the SC-series sheet blanker from Keko Equipment.

3. Via Punching

Vias are punched into the green tape by a mechanical punching machine in order to lay the foundation for a connection between the ceramic layers of the LTCC stack. Mechanical punching offers the highest accuracy and best hole quality. Haiku Tech offers the Keko Equipment PAM-series punching machine.

4. Via Filling

Vias are filled with metal paste in an advanced screen printer. The tape is placed on a sheet of paper that lies on a porous vacuum plate. A mask is required; this mask should be made of a 60-200 mm thick stainless steel or nickel foil by etching or laser punching. Alternatively a (Mylar-) foil can be used on which the tape is usually applied. Haiku Tech offers the Keko Equipment P-200 series screen printer.

5. Screen Printing

Co-fireable conductors are printed on the green sheet using a thick film screen printer. Standard emulsion (325 – 600 mesh) type thick film screens are being used. After the printing process the vias and conductors are dried in an oven at 80° C for 5 to 30 minutes (depending on the material). Some pastes need to level at room temperature for a few minutes before drying. Haiku Tech offers Keko Equipment P-200 series stand alone and integrated fully automatic printing and drying systems.

6. Stacking

Sheets will be accurately collated and stacked one by one with the help of CCD vision alignment cameras to form the LTCC green stack.Crucial is the accruracy of the alignment and pre-inspection of the sheets by AOI. Haiku tech offer the Keko Equipment state of the art SB, ST and SW series stackers.

7. Lamination

There are two possibilities of laminating the tapes in the process of LTCC production.Uniaxial lamination, a process in which the tapes are pressed between heated plates at 70°C, 200 bar for 10 minutes (standard values). A more suitable method to avoid high shrinkage tolerances is the isostatic lamination process. The stacked tapes will be sealed under vacuum in a foil pouch to be applied to a pressure of up to 350 bar in heated water.Haiku Tech offer Keko Equipment ILS-series isostatic presses.

8. Cutting

After the laminating process the stacks are cut into individual pieces of different sizes by a guillotine hot knife. The green ceramic bars are placed onto a vacuum table and can be cut up to a thickness of 10 mm. Advanced vision systems position the green ceramic accurately for cuts with excellent dimension control. Haiku Tech offer Keko Equipment CM-series green cutters.

9. Cofiring

Finally the modules are fired in one step in a belt furnace. A firing profile under controlled conditions is needed in a multiple zone, programmable belt furnace. A typical profile shows a slow rising temperature (about 2-5°C per minute) up to 450°C with a dwell time of one to two hours, for the binder burnout. Then the temperature will rise up to between 850 and 875°C with a dwell time of about 10 to 15 minutes. The whole firing cycle lasts between three and eight hours. Haiku Tech offer Sierratherm elevator and belt furnaces the preferred choice for LTCC firing as recommended by DuPont and Ferro Electronic Materials.

Schmid 8500 series