Thick Film Heaters Made From Dielectric Tape BondedStainless Steel Substrates

S.J. Stein, R. Wahlers, M. Heinz, M.A. Stein - Electro Science Laboratories Inc. (USA)

R. Tait, R. Humphries - Agmet Ltd. (UK)Presented at IMAPS, 1995

Abstract

Low temperature tape and thick film materials for use on stainless steel substrates in domesticappliance applications were developed. The insulating glass-ceramic coatings were applied tostainless steel bases using screen printing and tape processing. Heaters and heat regulators usingPTC and other 850íC firing thick film materials were designed and built. Properties of the coatedmetal substrates and the heater devices are presented including initial and aged data demonstratingproperties and reliability in low profile heater applications.

Key words: Heaters, Coated stainless steel, Thick film, Tape, Dielectric.

Introduction

Ever growing pressure for low cost, smallsize and high performance in hostile environmentshas led to the use of a wide variety of substratesfor electronic circuitry. Insulated metals have at-tracted attention because of their mechanicalstrength, good thermal properties, large size/com-plex shape capability, shielding character and lowcost potential. Aluminum core (IMS) and steel core(porcelain enamel), in particular, have been suc-cessfully used in a number of different applications.These include under the hood electronics wheretemperature and mechanical stress do not permitconventional packaging, SMT and hybrid circuitboards and as bases for low profile heaters [1-6].They have also been used as part of the housing toreduce size and cost and in some applications toprovide EMI shielding [4,7,8]. In another instancethe ferromagnetic character of the core was usedto advantage in making a compact direct drivemotor [8].

Widespread usage of the IMS and porcelainenamel materials, however, has been limited by cost,use temperature restrictions, wet chemical process-

ing requirements and complicated fabrication pro-cesses [1,9]. This paper discusses insulated metalsubstrates made from high temperature, corrosionresistant stainless steel and refractory migrationresistant dielectric coatings designed to overcomethese limitations.

Procedure

Metal preparation

High chromium content ferritic stainless steelswere chosen for the metal base. These materialsprovide good corrosion resistance, adherent oxidefilms, high temperature firing capability and lowTCE values (as compared to the austenitic forms).The data in this paper were taken from samplesprepared from 430S17 stainless steel (selected be-cause of its ready availability) but good results havealso been obtained with German StandardNo.1.4762 which contains a slightly higher chro-mium content and about one percent aluminum.

Metal preparation starts with removal of anydirt, oil or grease. The surface is then roughenedand oxidized to promote dielectric wetting and

adherence. Air abrasion and/or abrasive belt wereused to achieve the desired roughening while theoxidation was done in belt furnaces at peak tem-peratures between 850º and 930ºC on 45 minutecycles. Test parts were made using 50 mm x 50mm and 90 mm diameter 430S17 metal bases whichwere 1.6 mm thick.

Coating application

Dielectric coatings have been developed witha wide range of dielectric constants (K-values) andcompatibility with a number of different substrates[10,11]. This paper will confine itself primarily toscreen printable pastes and low firing dielectric tapematerials which provide a K value of about 10 andare compatible with stainless steel substrates.

Screen printingThis application technique consisted of depos-

iting 3 layers of dielectric paste onto metal platesusing a 165 mesh screen and separately firing thelayers in a standard profile 850ºC belt furnace.

TapeFlexible sheets of low temperature tape, 125

to 175 microns thick, were laminated to the pre-pared metal bases using pressures of 250 to 500psi at 75ºC. After the lamination of each tape layerthe parts were fired in a standard thick film beltfurnace with a peak temperature of 850ºC. Thethicknesses of one and two fired dielectric tape lay-ers were typically 90µm and 160µm respectively.

Results

Coated metal substrate properties

Thick filmPrepared 430S17 metal bases were coated

with D-4913-A fired on at 850ºC. The D-4913-A,a thick film multilayer dielectric, was selected forits high initial insulation resistance and low metalion solubility achieved through the use of lead freeand alkali free dielectric materials and a high Tgvitreous phase.

Table 1 gives properties of the D-4913-Acoated metal substrates. An array of 16 mm x 6mm pads of 9695 Pd/Ag conductor was printedand fired on top of the dielectric coatings to allowmeasurements of insulation resistance (IR) and

breakdown voltage (Vbd) to be made. The Pd/Agconductor was fired at a peak temperature of 850ºC(45 min. cycle). The three layer dielectric coatedsubstrates had IR of >1012Ω at 25ºC whilst the Vbdwas >2500 Vac. Examples were subjected to 10refire cycles at 850ºC, and whilst the IR at 25ºCremained at >1012Ω, the 300ºC figure was reducedto 1011Ω. It must be noted that the 2500 Vac in theVbd tests was a limiting factor due to arcing be-tween adjacent conductor pads and not breakdownof the dielectric barrier to the steel. British Stan-dard 3456 requires a minimum breakdown voltagevalue of 1250 Vac for Class I appliances employ-ing heaters on insulated steel substrates.

TapeWhile changes in the dielectric composition

can give significant improvements in the results,properties like breakdown voltage still necessitatethe use of thick coatings. Tape processing providesa method of obtaining the required coating thick-ness without resorting to the multiple printing/fir-ing steps associated with thick film processing.

Properties of substrates prepared by thetape coating process are listed in Table 2. Insula-tion resistance and breakdown voltage of sampleswith one or two separately fired layers of tape on

430S17 metal bases are presented. Pieces weretested using D-41030, D-41020 (both being lead-and alkali-free tapes), and a combination of the two.

9695 conductor pads were used on all test pieces.Both single layers and the combination had excel-lent IR at both 25ºC and 300ºC. IR after 10 refirecycles remained at 1012Ω for D-41030, but was re-duced for the combination and for D-41020.Samples made from two layers of D-41020 tapeon stainless steel were fired in a box furnace at850ºC and tested at 400ºC. Initial and “after tenrefire” IR values were >109Ω.

Adhesion of the tape coating layers was char-acterized by thermal shock and drop tests. The droptest consisted of allowing the substrates to fall from2 meters onto a concrete floor ten times and exam-ining them for cracks. Thermal shock testing wasdone by immersing 700ºC furnace heated samplesin water. This water shock test was repeated fivetimes and then the parts were examined for crack-ing. Dye testing showed no cracking of the dielec-tric in either test.

Compatible thick film pastesConductive, dielectric and resistive thick

film pastes developed for firing on Al2O3 substratesat 850ºC were tested on the tape coated metal sub-strates. In this initial evaluation phase, no attemptwas made to optimize the pastes for the differentchemistry and TCE of the coated metals.

Table 3 provides sheet resistivity and conduc-tor adhesion data of silver based conductors onAl2O3 and tape coated stainless steel. Resistancecharacteristics are very similar for the two substratesbut the Pd/Ag materials appear to bond better toAl2O3 . Solderability was excellent on both sub-strates as noted in the table. In addition, a numberof base metal conductors are available and are be-ing studied.

Table 4 gives key dielectric characteristicsof D-4913A used as a multilayer/crossover dielec-tric. Properties are essentially identical on the twosubstrates.

Resistance value comparisons of the 3900thick film series on Al2O3 and tape coated metalsubstrates are shown in Figure 1. In general resis-tance values are close with the 1MΩ/sq 3916 pasteshowing a somewhat lower value on the coated

stainless steel. TCR values were generally under100 ppm with a few greater values noted for someof the pastes.

Positive temperature coefficient pastes werealso tested because of their advantages in heaterapplications for which the coated metal substratesare especially well suited. They can be used to fab-ricate discrete temperature sensing elements or as

the heating element itself providing a current limit-ing feature to the heater. Table 5 gives the resultsindicating resistance values on the tape coated stain-less steel are about double those on Al2O3 whileTCR values retain their desired highly positive val-ues.Applications

The tape coated stainless steel substrateswere evaluated for use in low profile heater appli-cations in which heating elements are printed di-rectly on a “heat distribution” plate. Thick filmmaterials have proven to be a reliable cost effec-tive choice for heating and heat sensing control el-ements in these applications [6]. Layout and ge-ometry are important design considerations in mak-

ing the devices and customized configurations aresometimes needed to achieve highly uniform tem-perature distributions [5]. The layout shown in Fig-ure 2 was used for initial evaluation purposes.

Low resistance (0.7Ω/square) 3900 series re-sistor paste was used for forming the meander path

heating element, 9695 (Pd /Ag) for the contact pads.These materials were printed and fired on D-41030/ D-41020 composite coated stainless steel sub-strates at 850ºC.

Testing to date has consisted of bias humidityand heat cycling. In the bias humidity test 50 voltswas applied between the stainless steel base andtop surface circuitry. These voltage biased plateswere put in a humidity chamber held at 85ºC and85% relative humidity. After 500 hours of such test-ing no change in insulation resistance occurred.

Heat cycle testing consisted of heating thesubstrates to 300ºC by application of power to thepart for 2 minutes, holding at 300ºC for 2 minutesand then shutting the power off for 2 minutes (whichallows it to cool to about 175ºC). After more than3500 of such cycles no short circuits, open circuitsor cracks in the tape coating have been detected.

A more practical test was devised in which aheater element, as shown in Figure 2, was attachedto the base of a flat bottomed stainless steel bowlusing a thermally conductive silicone adhesive, DowCorning Q1-9226. Connection was made to the Pd/Ag terminations by ultrasonically bonding 300µmaluminum wires. The two following experimentswere conducted:

1. With the bowl containing 300 ml of coldwater, 100 Vac was applied to the element(120W) and the temperatures of the heatingelement and the water were monitored (seeGraph 1).

2. With a fresh charge of 300 ml of coldwater, 230 Vac was applied (620W) and tem-peratures again monitored (Graph 1).

Graph 1 shows that with power level at 120Wthe heat loss from the test rig would not permit thewater temperature to rise further than 75ºC, theelement temperature stabilizing at 140ºC. At 620Wdissipation the water reached boiling point in a rela-tively short time (225 seconds), with the elementreaching 330ºC. These results indicate poor ther-

mal conduction between the steel plate and themetal bowl, due to the dished shape of the bottomof the bowl. Uncertainty of the performance of theadhesive at higher temperatures prevented takingthe tests further to the required power level of 2kW typically used in self-heating water kettles.

Figures 3a and 3b show thermal images of aheater element powered to 120W, at intervals of 5seconds after switch-on. Figure 3a shows theprinted side of the substrate with the shape of theresistive element clearly seen. Figure 3b shows thereverse side of the substrate, and the thermal lagand subsequent greater distribution of temperaturesare apparent.

Summary & Conclusions

Insulated metal substrates consisting ofglass-ceramic coating layers on high temperaturecorrosion resistant 430S17 stainless steel were pre-pared. The insulating coatings were applied usingscreen printing and transfer tape processing andsubsequently bonded to the metal base by firing at850ºC. Both techniques allow straight forward pro-cessing, selective coating and provide for metal baseaccess. The transfer tape process had the advan-tage of lower labor cost.

Breakdown voltage values greater than 2500

Figure 2. Heater Test Pattern

Graph 1. Performance of Water Heater

volts ac were achieved with both tape and thickfilm coated substrates having sufficient dielectricthickness. Insulation resistance also depended oncoating thickness but composition was found to bemore critical for maintaining high IR’s at elevatedtemperatures. Refractory coating compositionsgave IR’s over 1012Ω at 300ºC, retaining valuesgenerally greater than 1010Ω after ten refires at850ºC as a result of their diffusion resistant chem-istry.

The coatings bonded well to the 430S17 stain-less steel resisting mechanical and thermal shock.No cracks were observed after five direct immer-sions in water from a 700ºC furnace. Likewise nonewere produced as a result of 10 drops from a heightof 2 meters to a concrete floor.

Compatible 850ºC firing dielectric, resistiveand conductive thick film pastes were found. Se-lected pastes were then fired on rectangular tapecoated stainless steel substrates to form “heat dis-tribution” plates. Testing of these plates showedinsulation resistance remained unchanged after 500hours of 85ºC, 85% relative humidity and 50 voltbias humidity testing. They performed well in 3500cycles of powered element heat cycle testing show-ing no cracks, shorts or opens. In additional test-ing circular heater plates were attached to stainlesssteel bowls and evaluated as “water heaters”. Ele-ment and water temperature were monitored ver-sus time and thermal images taken. The results were

the 1986 ISHM Conference, Atlanta, pp. 196-202.

2. S. J. Stein, C. Huang, and A. Gelb,“Comparison of Enameled Steel Substrate Prop-erties for Thick Film Use”, Proceedings 2ndEuropean Hybrid Microelectronics Conference,1979, Ghent, Belgium, pp. 525-538.

3. N. Yonemura & M. Fukuda, “A NewInsulated Metal Substrate (IMS) for Giant PowerHybrids” IMC 1994 Proceedings, Omiya, Japan,(April 1994), pp. 110-113.

4. C. Zardini, P. Sokomala, E. Woirgard &H. Fick, “Surface Mounting of Large Size Ca-pacitors on Insulated Metal Substrates”, IMC1994 Proceedings, Omiya, Japan, pp. 145-150.

5. P. Baumbach, R. Tait, J. Whitmarsh andM. A. Stein, “Materials Substrates and Designsfor Manufacturing Heaters Using Thick FilmTechnology”, Proceedings of the 1990 ISHMConference, Chicago, pp. 699-705.

6. R. Tait, R. Humphries, and J. Lorenz,“Thick Film Heater Elements and TemperatureSensors in Modern Domestic Appliances”, IEEETransactions on Industry Applications, Vol. 30(3), May/June, 1994, pp. 573-577.

7. S. Ishihara, K. Uchide & S. Toyecka,“Bendable IMST Substrate for High-DensityMiniaturized Hybrids”, IMC 1986 Proceedings,Kobe, Japan (May 1986) pp. 169-173.

8. N. Suzuki, K. Yajima, S. Murayama, M.Itoh and T. Yamaguchi, “Application of PorcelainSteel Substrate Thick Film Circuits in Japan”,Proceedings of the 1984 ISHM Conference,Dallas, pp. 105-110.

9. M. Budweit, “Noble Steel, A Substratefor Extended Application in Thick Film Technol-ogy”, Proceedings 5th European Hybrid Micro-electronics Conference, 1985, Stresa, Italy, pp.447-453.

encouraging but indicated the importance of opti-mizing heat transfer between the heater plate andwater container in such a design.

References

1. W. Teat, B. Marten, D. Blazej and R.Oboodi, “Properties of A New Selective Ce-ramic-Coated Metal Substrate”, Proceedings of

Figure 3a. Thermal Images of PrintedSide of Substrate

Figure 3b. Thermal Images of ReverseSide of Substrate

10.R. Wahlers, S. J. Stein & G. Sykora,“Tapes and Thick Films for High FrequencyPackaging,” Proceedings of the 40th ElectronicComponents and Technology Conference, 1990,Las Vegas, pp. 116-121.

11.R. Wahlers, D. Dychala, C. Huang, S. J.Stein, and P. Danner, “Low Firing TemperatureCapacitor Tape Materials”, Proceedings of the1993 ISHM Conference, Dallas, pp. 232-237.