CERAMIC ON METAL SUBSTRATES PRODUCED BY PLASMA SPRAYING FOR THICK FILM TECHNOLOGY SZKGOLOra

The arc plasma spraying process was applied to obtain ceramic coatings on stainless steel substrates. The outer coatings were formed from pure alumina or alumina


INTRODUCTION
The plasma spraying of dielectric coatings onto metallic plates offers the possibility of realization of the substrate for thick film technology.The dielectric coating is the insula- tor between the metal plate and the thick film circuit and could be also used as the capa- citor element.Prepared in this manner substrates have some advantages in comparison with the standard alumina substrates.Smyth et.aL showed that in the production of 50,000 substrates the cost of a typical alumina substrate 96% A12 Oa, size 25 x 25 x 0.65 mm is in the range of US $ 0.08 for each substrate.The preliminary cost of produc- ing 25 x 25 mm sprayed alumina on kovar substrates could be estimated as US $0.02.The production rate of plasma sprayed substrates reaching several meteres an hour is another important advantage.
The heat conductivity of such substrates can be expressed as: where: ?heat conductivity, l-dielectric coatings thickness, d metal plate thickness, s, 1, 2 parameters attributed to substrate, dielectric coating, metal plate.
It is obvious from eq. ( 1) that the application of a thick plate of good heat conducting metal will result in a good heat conducting substrate.Mackay and Muller 2 have deter- mined the sprayed alumina heat conductivity 18-28 W/(m K) as dependent upon the temperature.Taking from their paper X1 20 W/(m K)and assuming l 0.1 mm, d 0.5 mm and ?2 400 W/(m K) as for copper one can estimate the ?s value as nearly 100 W/(m K).The value is similar to the heat conductivity of very expensive beryllia ceramics.The sprayed coatings surface flatness is also better than for conventional sintered alumina substrates.This parameter is important from the screen printing point of view.
Up to now a similar subject was taken up by, apart from the both mentioned above papers, Braguier et.al. a This paper described sprayed alumina coating on copper plates as a dielectric in micro-strips lines.The idea of sprayed ceramic on metal substrates was shown also in a patent. 4r over four years porcelain enamelled substrates have been developed for hybrid circuits applications.A lot of papers on this subject have been published at ISHM con- ferences from 1978.These kind of substrates can be produced following Stein et. al. by: dipping, slushing or spraying of a water-based slurry or slip, electrophoretic deposition of glass, electrostatic spraying of dry powder.However, these technologies have limits for some application e.g. the firing temperature of the substrate is limited to 700C.Only Kuzel 6 has shown the possibility of heating the substrates up to 860C.Also the heat conductivity of typical enamels is several times lower than that of alumina (Lebailly 7 had reported the value of 5.15 W/(m K)).Finally one can refer to "brown plague" the term which describes the shortages between con- ductor and steel core reported as often appearing in enamelled substrates, by e.g.
Allington and Cotea or Schabecker. 9asma spraying technology offers an opportunity to avoid the above mentioned problems and to be tried for hybrid microelectronic substrate production.

MATERIALS
The powders applied in the investigations are characterized in Table I.Powder OD is spherical in shape (Figure 1 b).This shape provides facilities for feeding the powder to the plasma gun.The powder 01 is shown in Figure la.The distributions of used powder diameters, determined by the sedimentation method, are shown in Figure 2.
Stainless steel, type 1H18N9T, was used as the metal plates.The steel was cut into 20 x 30 x 2 mm plates.

TECHNOLOGY OF THE SUBSTRATE PRODUCTION
The steel plates were grit blasted prior to the spraying process.Silica sand with sizes lower than 0.4 mm was used for blasting.The pressure of air, in this process, was equal to 0.6 MPa.The sprayed plates were ftxed to the suction cup (Figure 3).This solution was chosen to minimize the residual stresses which could appear in the coating after spraying.
The suction cup was mounted in a swivel.The plasma spraying was conducted using a  plasma installation.The operational parameters are shown in Table II.
During the spraying process air cooling was used.The NiCr (80/20) subcoat was first deposited and then the alumina coating was applied.

RESULTS
The introductory researches were aimed at finding an alumina powder which when sprayed on to a nichrome substrate maintains adhesion up to a temperature of 800C.In these researches the A12 O3 powder, Metco type 105 SF, was eliminated.
4.1 Physical and Structural Properties of the Alumina Coatings.
The X-ray investigations were carried out using Cu Kct radiation.The bias cross-section of sprayed samples were examined using metallographical and Vickers microhardness tests.
The tensile bond strength of the sprayed coatings was determined according to the norm. 1 The results of these investigations are shown in Table III.
PLASMA SPRAYING FOR CERAMIC/METAL SUBSTRATES The sprayed alumina contains the metastable phase i-AlOs which considerably lowers the coating density (p 4000 kg/m3) for corundum.The microhardness, deter- mined under the load 2N, is also lower than for sintered carborundum (HV---20000 MPa).
The coatings sprayed using OD powder have higher corundum content.Therefore they conduct the heat better.The sprayed alumina coatings surface (Figure 4)shows that the sprayed powder grains were molten during impact with the substrate.They had melted on to the surface.The small grains, appearing after large grains had formed on the sub- strate, are also visible.There were no visible fractures on the coatings surface.The coating sprayed using 01 powder has a smoother surface than that of OD.The roughness of the first, determined using a ZEISS-ME 10 apparatus, is equal to 3.1 #m AA.The metallographical cross-section shown in Figure 5 exhibits some pores and discon- tinuities in the coating.  .2 Electrical Properties of the Alumina Coatings.
Two different types of ceramic coatings using OD and 01 alumina powders have been tested.Dielectric properties of the ceramic layers were tested in a capacitor system (Electrode area, S cm2), with stainless steel and thick film electrodes.A thick f'tim, P 202 (PdAg), electrode was fired at 800C peak temperature for 10 minutes.Surface resistivity was determined for a resistor 105 mm length and 0.5 mm width.The test pattern used is shown in Figure 6.Electrical parameters of the ceramic layers examined are summarized in Table IV.The insulation properties of sprayed OD powder coatings were much better than that of 01 powder.Electrical parameter differences were in 2 or 3 orders of magnitude in favour of the OD powder coatings.aCapacity measured between the metal plate and the thick f'tlm electrode, S 1 em coatings thickness: OD 50 tzm, 01 20 m.The properties of a thick Film resistor system made on the tested substrates was determined.The conductor ink, P 202 (PdAg), and the resistor ink, Du Pont DP 1321, were evaluated.The resistors and conductors were fired separately.All firings were carried out "1%1 5 0 -5 FIGURE 7   Multiple firing behaviour of sprayed coatings: OD powder "x", 01 powder "+"; 96% alumina substrate "o".Multiple firing behaviour of sprayed coatings: OD powder "x", 01 powder "+"; 96% alumina substrate "o".at 800C peak temperature for 10 minutes on a profile used for DP 1321 ink on alumina substrates.The tested resistors were 5 x 5 mm in dimension.

I ' l i IIIIIIIIIIII
The measured sheet resistivities and hot TCR were in agreement with the manufac- turer's data for DP 1321 on the alumina substrates.The sheet resistivities, 70 2/sq.and 75 2/sq., were obtained for OD and 01 powder coatings respectively.Hot TCR, 250 ppm/C and 190 ppm/C, were determined.The effect of retiring on the resistance and TCR is shown in Figures 7 and 8.For comparison the behaviour of the same resistors on 96% alumina substrates are also shown.The resistors made on OD powder coating sub- strates had better retiring stability than the resistors on 01 powder coating substrates.The stabilities of the resistors on 96% alumina substrates and OD powder coatings were similar.TCR changes during retiring were comparable for all types of substrates.

CONCLUSION
The resistors printed on stainless steel NiCr/A12 Oa/(OD) substrates show sufficient resis- tance stability and sheet resistivity comparable to those printed on conventional alumina substrates and a TCR that is only slightly higher.The sheet resistivity and dissipation factor of sprayed ceramics type OD are better than sprayed 01 ceramics but significantly worse than sintered alumina substrates.Therefore, the technology presented permits sub- strates to be obtained with electrical parameters comparable to conventional sintered alumina ceramics.

FIGURE 1
FIGURE 1The shape of alumina powders used, a 01, b OD.

FIGURE 2
FIGURE 2The distributions of the used powder diameters.
PLASMA SPRAYING FOR CERAMIC/METAL SUBSTRATES 149 4.3 Electrical Properties of the Thick Film Resistors.

TABLE II
TABLE III Some physical properties of plasma sprayed alumina coatings