GB2207440A - Thick film ink - Google Patents

Abstract

An inorganic binder such as a hypophosphite or a polyphosphate is used in thick film inks, to avoid the problems associated with organic binders.

Description

THICK FILM INK FIELD OF THE INVENTION The present invention relates to thick film ink systems.

BACKGROUND OF THE INVENTION Screen printing of special inks is widely used for the fabrication of electronic circuits known as thick film circuits. Screen printing consists of forcing the ink which it is desired to deposit on a substrate, through the fine mesh holes of a screen some of which are blocked by a special polymer, the free holes being a precise pattern of the circuit to be reproduced. Wet printed patterns so produced are dried by forced convection and applied heat. So that dried prints can be handled without disrupting the precisely defined pattern, an organic binder such as ethyl cellulose is incorporated into the ink vehicle which provides green strength by forming a matrix adherent to the substrate and 'permanent' solids content.By permanent is meant the particulate matter of conductor, dielectric or a mixture of both which give the fired inks the characteristic electrical properties of conductor, dielectric or resistor. The dried prints are then fired (600-1000 C), during which process the organic binder constituent is allowed to burn off prior to the sintering and densification stages. Several layers of circuitry can be built up on a substrate by consecutive print, dry and firing of conductor and dielectric layers. However, substantial difficulty in processing occurs because of the reluctance of organic binders to completely and cleanly leave the printed patterns during the 'burnout' stage.

Processing difficulties arise as a result of retained carbonaceous material, leading to the reduction of certain metal oxide species typically found in thick film ink system. Metal oxide reduction by carbon to give carbon monoxide or carbon dioxide can lead to blistering and poor adhesion in multilayer circuits. Also volatilisation and redeposition of low melting point metals formed by reduction can occur in unwanted areas.

These processing problems are exacerbated when processing base metal ink systems which must be fired in a neutral atmosphere. Copper ink systems are the major commercial product of this type. Much reliance is placed on pyrolysis for the removal of the organic binder but, for clean burn-out, it is often necessary to incorporate inorganic oxygen release agents into the inks, to keep the organic binder content low, to keep the loading of the firing furnace belt low, or to use so-called 'dual' atmosphere firing in which oxygen is bled into the burn-out zone. Further assistance in solving the problem of clean burn-out is given by belt furnaces in which high nitrogen flow rates and complex gas input and output manifolds are applied in the burn-out zone.However, many of these techniques to facilitate burn-out of organics in a nitrogen atmosphere represent significant cost penalties and complexities in processing.

SUMMARY OF THE INVENTION A composition according to the invention (which for convenience may be termed a "thick film ink") has the characteristics of a thick film ink and includes an inorganic binder. The invention thus avoids the use of organic binders, avoids the drawbacks described above, whilst retaining the rheological characteristics necessary for screen printing, dried print strength and fired film properties.

DESCRIPTION OF THE INVENTION A thick film ink of the invention comprises a diluent which is water and/or a volatile organic compound. The contents of these components are usually 1-50% by weight binder solids and 5-50% by weight respectively. The composition is substantially free of organic components (as binder). It is often appropriate to include other components such as a 'permanent' component, in the ink, e.g. a conductive compound such as pure copper powder (to give.a conducting ink) or a dielectric compound such as a fine powder frit of lead borosilicate glass. In such a case, the amounts of various constituents may be, for example, 1-45% by weight binder solids, 5-45% by weight diluent and 50-94% by weight of the further component such as copper powder.

The binder is preferably a hypophosphite or a phosphate, silicate, borate, polyphosphate, polysilicate or polyborate, usually of an alkali or alkaline earth metal, or a polyacid of a transition metal. Most of these materials are polymeric in nature to some extent.

The term "polyacid" includes heteropolyacids such as tungstosilicic acid.

Especially preferred inorganic binders are potassium or aluminium hypophosphite, potassium polyphosphate and aluminium acid phosphate. In an aqueous or organic system, such binders provide the necessary rheological characteristics and, when dried, the required 'green strength' for screen-printed circuit manufacture.

Potassium polyphosphate (KPO3)n is an example of a generic group of mono and mixed metal polyphosphates. It may be made by a variety of methods: a simple route applicable to laboratory production is the thermal dehydration of potassium dihydrocen orthophosphate (KH2PO4). After an appropriate time at a suitable temperature, the melt is quenched to give soluble long-chain polyphosphate.

Aluminium acid phosphate is also made by a variety of methods: the mixing of fine alumina powder with phosphoric acid is the preferred example. A molar ratio of P205 : A1203 of between 1 : 1.5 and 3 : 1 may be used.

However, preference is given to lower ratios.

Water and/or a volatile organic compound may be used as a diluent. Organic diluents, by their nature, are completely removed during the drying process.

The binder component, when in a suitable diluent provides the necessary viscosity and rheological properties for silk screen printing. On drying, it forms a rigid matrix binding the particulate permanent constituent (conductor, dielectric, resistor) together and to the substrate, thereby giving 'green strength' to dried prints. On firing, the inorganic binder constituent also acts as adhesion promoter between the substrate and sintered permanent constituent.

The vehicle solids content is preferably less than 20% by weight of the ink permanent component for conductor inks but may vary substantially for dielectric and resistor inks, the level being a function of the nature of the permanent constituent. The ink itself preferably has a viscosity of 100 to 500 Pa.s measured using a Brookfield viscometer with a No. 7 spindle at 10 rpm and at 250C. The ink should of course wet the substrate to which it is applied; it will therefore usually be appropriate to ensure that the surface tension of the ink allows wetting of alumina substrate.

Inks are formulated using conventional paint dispersion and blending processes such as three-roll milling, and inks may be screen-printed using conventional thick film screen-printing equipment.

Drying may be accomplished by desiccation, infra-red or belt-/batch-ovens in either air or inert/neutral atmospheres.

Firing of inks with vehicle systems containing an inorganic binder may be accomplished using conventional firing profiles, generally bell-shaped, with a peak temperature between 600 and 1100 0C depending on the sintering characteristics of the permanent phases.

Four significant advantages are found when firing base metal ink systems containing inorganic binders in neutral atmospheres: (1) The neutral gas flow rates need be sufficient only to maintain a non-oxidising atmosphere and so protect base metal constituents. There is no need for fast flow rates to remove organic burn-out products, as none exist.

(2) There is no limit to the area of the belt covered by dried prints to be fired, which for organic vehicle counterparts is restricted to less than 30%, to avoid reducing atmospheres.

(3) Multi-layer circuits may be built without fear of blister formulation or delamination.

(4) Simple furnace designs can be used requiring little process control.

Whilst the use of an inorganic binder in a vehicle for thick film inks finds greatest advantage for base metal inks system, this does not preclude their use in noble metal air firing inks where burn-out may still be a processing problem leading to disruption and failure of multi-layer circuits in a manner already described.

Water or a volatile organic diluent may be used in either noble metal or base metal ink systems.

In particular, the co-firing of multilayer air or neutral atmosphere-fired circuitry using inorganic binders in the vehicle of the ink may be realised without the problems of burn-out associated with organic binders.

The following Examples illustrate the invention.

EXAMPLE 1 A copper conductor ink was made using an aqueous solution of aluminium acid phosphate having a molar ratio P O : A1203 of 2:1. A quantity of this solution was mixed with 8 ssm copper powder to formulate an ink with rheological characteristics suitable for screen printing.

The ink comprised 2.5 g binder solids, 19 g copper, and 2 g water.

Two sets of three meander conductor tracks of nominal width 0.5 mm, 0.25 mm and 0.22 mm as well as 16 square adhesion pads of length 2.5 mm were screen-printed on to 96% alumina substrates using a 325 mesh stainless steel screen with 12 ssm emulsion. The wet prints were dried in an infra-red belt dryer using settings typical of those used for conventional commercial inks comprising an organic binder. Dried prints were resistant to scratch by Talysurf thickness measurement which gave dried print thicknesses of 20 + 2 ssm.

A simple nitrogen firing six-zone belt furnace with 90 minute profile and 10 minutes at peak of 9000C was used to fire the dried prints. Average resistivity values normalised to 25 Am fired print thickness of 2.4 + 0.4 mohm/square were found. Fair solderability was achieved after light burnishing. Adhesion failure 2 occurred at 70 + 15 kgf/cm2 for direct pull testing.

EXAMPLE 2 The procedure of Example 1 was repeated except that 1,2-ethanediol was used as the vehicle diluent. The ink comprised 0.8 g binder solids, 13 g copper, 0.6 g water (derived from preparation of the binder) and 3 g 1,2-ethanediol. Results-are similar in all respects except for excellent solderability for as-fired conductor tracks and pads.

EXAMPLE 3 The procedure of Example 1 was repeated, but using a firing profile of 30 minutes through-put with 3.5 minutes at peak of l0000C. The profile was bell-shaped. Results are in all respects similar to those of Example 1.

EXAMPLE 4 A copper conductor ink was made using potassium polyphosphate with 1,2-ethanediol as diluent and 8 ssm copper powder as the conductor solids content. The ink comprised 1.8 g binder solids, 20 g copper, about 0.6 g water and 2.8 g 1,2-ethanediol. A suitable thick film ink rheology was found at 15 wt% vehicle solids content.

The ink, when fired according to the profile of Example 1, gave results according to Example 2.

EXAMPLE 5 A copper conductor was made using sodium hypophosphite as the binder and a mixture of organic vehicle diluents. Sodium hypophosphite was found to be soluble in glycol to the extent of 33 g in 1000 cc of glycol. The resulting solution was miscible with both terpineol and butyl carbitol. A quantity of this liquid vehicle containing all four constituents was mixed with 8 #m copper power to formulate an ink with rheological properties suitable for screen printing. This ink was printed and fired in a procedure similar to that used for Example 1 to yield a well-defined pattern of a copper layer with a bright untarnished finish firmly bonded to the substrate.

Claims (15)

1. A thick film ink which comprises an inorganic binder.

2. A thick film ink according to claim 1, in which the inorganic binder is selected from hypophosphites, phosphates, silicates, borates, polyphosphates, polysilicates and polyborates.

3. A thick film ink according to claim 1, in which the inorganic binder is selected from phosphates, silicates, borates, polyphosphates, polysilicates and polyborates.

4. A thick film ink according to claim 1, in which the inorganic binder is a polyacid of a transition metal.

5. A thick film ink according to claim 1, in which the inorganic binder is selected from potassium or aluminium hypophospite, potassium polyphosphate and aluminium acid phosphate.

6. A thick film ink according to claim 1, in which the inorganic binder is selected from potassium polyphosphate and aluminium acid phosphate.

7. A thick film ink according to any preceding claim, which comprises water as a diluent.

8. A thick film ink according to any preceding claim, which comprises a volatile organic compound as a diluent.

9. A thick film ink according to claim 8, in which the volatile organic compound is selected from 1,2-ethanediol, terpineol and butycarbitol.

10. A thick film ink according to any preceding claim, which is a conducting ink in which the conductor is a metallic powder.

11. A thick film ink according to any preceding claim, in which the conductor is pure copper powder.

12. A thick film ink according to any of claims 1 to 9, which is a dielectric ink in which the 'permanent' constituent is an inorganic dielectric.

13. A thick film ink according to claim 12, which is a dielectric ink in which the 'permanent' constituent is a fine powder frit of lead borosilicate glass.

14. A thick film ink according to claim 12, which is a dielectric ink in which the 'permanent' constituent is a fine powder frit of lithium zinc silicate glass ceramic.

15. A thick film ink substantially as herein described.