Step Discussion

Tape Sizing
The tape is normally cut to size using a blanking although adequate sizing for most applications can be accomplished with scissors or a scribing knife. When blanking is used to size the tape, registration holes are formed in the tape at the same time using a compound die.

Hole Formation
Via and registration holes are formed in the tape using punches, drills, or laser. The punches may be fixed hard tooling or part of a CNC center where the location of the tool head is controlled by a computer. Drilled holes are produced by high speed drilling equipment of the kind used to form holes in printed circuit boards. A variety of different types of lasers have been used to create the desired holes. Via holes down to 5 mils have been made with punches and drills, and down to 1 mil and smaller using lasers.

Substrate
The transfer tape process for forming a multilayer structure can be applied to a wide variety of dielectric materials and substrates. The material has been formulated for use on standard 96% alumina substrates. Substrate flatness comparable with that needed for thick film deposition is required in order to achieve good conductor line definition and dielectric thickness control.

Conductor Deposition
The specially formulated conductor materials are screen printed on the alumina substrate (or fired dielectric) using standard thick film equipment and processing techniques developed for forming conductive interconnect lines and inter-layer vias. The printed conductors are dried for 10-15 minutes at 125° C and then fired in a thick film continuous belt furnace at a peak temperature of 850° C.

Lamination
Next the tape sheets are bonded to the substrate. Registration fixturing is used to maintain the required alignment between the substrate and tape sheet. A combination of heat and pressure applied with a uniaxial press produces the desired bonding action. Typical temperatures of 50° C to 65° C with pressures of 500 psi to 1000 psi are used. A rubber sheet interface acts to minimize cracking due to non uniformities or warpage.

Burnout & Firing
The carrier film is then removed and the tape layer densified. In order to achieve the optimum electrical properties, the organic binder must be fully removed prior to firing of the dielectric. This occurs in the 200° C to 450° C temperature range and is usually done in a separate furnace but has also been successfully combined with the firing step. Improper removal results in trapped carbon residue which Impedes the sintering and can produce pores and blisters.

Dielectric firing is most conveniently done in a continuous belt furnace. A ninety minute cycle with a 10 minute hold at the 850° C peak temperature is recommended.

Index