Thick film substrate with highly thermally conductive metal base

Abstract: A hybrid thick film substrate (12) utilizes a highly thermally conductive base metal such as copper. The base metal (32) is passivated by a suitable technique to prevent oxidation. One or more dielectric layers (36) are then screened onto the passivation layer (34) and fired at temperatures upwards of 600°C. The surface of the substrate is then in a condition to accept formation of a conductive pattern (14) or the like.

Description: The present invention is directed towards thick film substrates. There are numerous electrical circuit designs which require that conductive connections be formed upon a base substrate. In the electrical industry, printed circuit boards are formed on a great variety of base materials. For the most part, the substrates are composed of material with a relatively low thermal conductivity such as epoxy-glass or ceramic. In the fabrication of printheads useful in thermal printing applications, ceramic substrates have been preferred because of their excellent thermal stability, reasonable costs, and good electrical properties. While these properties have proven satisfactory for many applications, they do not provide for optimum electrical and thermal performance for all systems, particularly for the thermal ink jet printing application. For this application, for example, the thermal conductivity requirements are generally too stringent to be met by the ceramic class of substrate materials. This is because of the temperature control standard required to maintain a heated and pulsed printhead at the required operating temperature. Another difficulty with the prior art substrates, particularly ceramic, is the difficulty of achieving close dimensional tolerances of the substrate. The above disadvantages can be overcome by using a metal as the base of the substrate. A metal, for example, copper, is 20 times as conductive as alumina ceramic and enables either maximum cooling or constant elevated (or reduced) temperature operation, a condition most desirable for thermal ink jet printing. A metal substrate can be blanked to close tolerances and is cost effective in high volume manufacturing operations. A further advantage of using metal in the substrate is that it can be used as a ground plane and for EMI shielding for those applications requiring such functions. The problems associated with using metal as a substrate have been the difficulty in overcoating the metal substrate with the thick film layers necessary to enable the electrical components to be mounted on the substrate, and the implementation of suitable passivation techniques to prevent oxidation of the base metal. There are some prior art examples of metal substrates used for various applications.. U.S. Patent No. 3,613,230 to Griff discloses a method for fabricating circuitry which utilizes a pure copper sheet as a base plate with a dielectric layer (such as epoxy), and a copper foil layer laminated on the copper base plate. The copper foil layer is then etched to form a desired pattern and a second dielectric layer and copper foil layer are laminated over the first layer. U.S. Patent No. 4,307,147 to Ohishi et al. discloses an electrically insulating substrate having good thermal conductivity, the substrate is used for the manufacture of a printed wiring board. The substrate may comprise a highly thermal conductive metal plate, such as aluminum, copper, silver or iron (see Col. 3, lines 28-30). A film composed of a dispersion of metal oxide particles is formed on the metal plate. U.S. Patent No. 4,221,925 to Finley et al. discloses a printed circuit board having a metal substrate with a fired dielectric layer coated thereon. The resultant coated substrate is then passed through standard metallization processes to form a desired electrical circuit pattern. While these prior art designs may be satisfactory for the circuits and functions disclosed therein, there are other applications which utilize more complex geometries and have more stringent operating requirements. It is an object of the invention to enable such demands to be met. The present invention provides a hybrid thick film substrate comprising a conductive metal base, a passivation layer overlying said metal base and at least one screened and fired dielectric layer overlying said passivation layer. A highly thermally conductive substrate in accordance with the invention can provide the support for a thermal ink jet printhead. However, the invention is not limited to any such particular application, but is applicable to any system which requires a substrate having high thermal conductivity and stability. By way of example, a substrate in accordance with the invention will be described with reference to the accompanying drawings in which: Figure 1 shows a top view of a printhead used in an ink jet printing application bonded to the substrate, and Figure 2 shows a cross-sectional view of the substrate drawn through line 2-2 of Figure 1. Figure 1 shows a partial view of a printhead 10 of the type used in a thermal ink jet printer such as that disclosed in U.S. Patent 4,601,777 whose contents are hereby incorporated by reference. Printhead 10 is mounted on an electrode board 12 which connects electrodes 14 to electrodes 16 of the printhead. Electrodes 16 are connected to individual heating resistors (not shown) within the printhead structure. Referring also to Figure 1, the printhead comprises an upper substrate 20 which has a plurality of grooves (not shown) formed in a bottom surface thereof. The grooves serve as a channel for ink which is supplied to fill hole 24 by an ink cartridge (not shown). The front end of the grooves form an array of nozzles. A lower electrically insulated substrate 30, has heating resistors and connecting electrodes 16 formed on the top surface. The surface of upper substrate 20 is bonded to lower substrate 30 so that the plurality of resistors (not shown) are positioned within each channel formed by the grooves. The ink at each nozzle forms a meniscus; upon addressing of the individual resistor, current pulses through the resistor vaporize the ink to produce a temporary vapor bubble which expels droplets of ink onto an adjacent recording member. Referring now to Figure 2, there is shown a cross-sectional view of substrate 12 taken through line 2-2 of Figure 1. In a preferred embodiment substrate 12 comprises a copper base 32. Base 32 is silver plated to form layer 34 which serves to protect the copper from oxidation during subsequent high temperature thick film processing. Other metals (such as nickel) may be plated prior to the silver in order to prevent copper from coming to the surface during high temperature processing. Dielectric layer 36 is formed on layer 34 by one or more screening depositions, and fired at temperatures of approximately 600° centigrade or higher. Suitable photoresist techniques are used to leave exposed areas which will be required for bonding of the thermal ink jet device and for thermal communication path between lower surface 12 and layer 34. A conductive pattern is then printed on surface 40 to form the electrode 14. The fabrication techniques discussed above can be extended to fabricate a wide variety of thick film circuits. Any metal that can withstand the processing temperatures may be suitable for use as a substrate material. Multiple layers of dielectrics, resistors and conductors may comprise the thick film circuit. Suitable passivation techniques to preclude oxidation include electroplating, electroless plating, and vacuum deposition of other metals. Alternatively, deposition of passive dielectrics could achieve oxidation resistance and electrical insulation in one processing step. Another approach to preventing oxidation of the substrate metal at thick film processing temperatures is firing the thick films in an inert or reducing atmosphere. This is commonly used in the industry to fire non noble inks such as copper.

Claims: A thermally conductive substrate comprising a conductive metal base (32), a passivation layer (34) overlying said metal base, said passivation layer being such as to protect said metal base from oxidation during high temperature thick film processing, and at least one screened and fired dielectric layer (36) overlying said passivation layer. A subsrate as claimed in Claim 1, wherein said metal base is copper and said passivation layer is silver (or silver over nickel) deposited by an electroplating process. A substrate as claimed in Claim 1 or Claim 2, including electrical circuit elements (14) on the surface of the dielectric layer. A printhead for a thermal ink jet printer, said printhead (10) comprising a plurality of ink channels, and being mounted on a substrate (12) having on its upper surface a plurality of conductive lines (14) which privide electrical interconnections to the printhead, said substrate being highly thermally conductive and comprising a conductive metal base (32), a passivation layer (34) overlying said metal base, said passivation layer being such as to protect said metal base from oxidation during high temperature thick film processing, and at least one screened and fired dielectric layer (36) overlying said passivation layer.. A method of fabricating a thermally conductive substrate comprising the steps of coating a highly thermally conductive metal (32) with a passivation layer (34), said passivation layer being such as to protect said metal base from oxidation during high temperature thick film processing, selectively screening at least one layer (36) of dielectric material onto said passivating coating and firing said dielectric layer at temperatures of at least 600°C. A method as claimed in Claim 5, wherein said firing step is performed in an inert atmosphere. A method as claimed in Claim 5 to Claim 6, wherein said passivation is accomplished by an electroplating process.

Inventors: BLESSINGTON DANIEL R, CAMPANELLI MICHAEL R, KNEEZEL GARY A, BLESSINGTON, DANIEL R., CAMPANELLI, MICHAEL R., KNEEZEL, GARY A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: H05K1
IPC codes: H05K1, H01L23, H01L21, B41J2