Articulo Electronika 1

Development and Testing of Cold gas Sprayed Circuit Boards for Power Electronics Applications Eugen Rastjagaev, Jrgen Wilde University of Freiburg, Institute for Microsystem Technology (IMTEK) Department for Assembly and Packaging Technology Georges-Koehler-Allee 103, 79110 Freiburg, [email protected] Bernhard Wielage, Thomas Grund, Sabine Kmmel Technical University of Chemnitz, Institute for Materials Science Department of composite materials Erfenschlager Strae 73, 09125 Chemnitz Abstract Within the framework of the presented project, new mounting concepts for power electronics were implemented by ap-plying Al/Cu coat systems to insulators by means of cold gas spraying (CGS). The layers sprayed with cold gas were investigated and good material properties are observed. The applied metallizations show an extremely small porosity, no thermally induced phase conversions or process-related oxides and a high adhesive strength. Also the processing of assemblies with standard processes such as a soldering, adhesive bonding and heavy wire bonding is possible. The tem-perature shock stability of the new substrates was determined in temperature cycle experiments and it was compared with the temperature shock stability of the DCB substrates. The substrates were tested actively but also passively in bi-cameral temperature shock chamber and in power cycling experiments at power electronic function samples. The ac-complished reliability investigations showed that the characteristic lifetime of the cold gas sprayed substrates is approximately 80% of the characteristic lifetime of the DCB substrates. These demonstrate that the substrates produced in cold-gas technology can be used for power electronic applications.

1 Introduction Metallised Al2O3 and AlN ceramics which must exhibit strictly defined physical properties and satisfy reliability requirements are normally utilised for power electronics assemblies. Previously, these requirements could only be used with the DCB-substrate technology (Direct Copper Bonding). Mechatronics integration there is a search for simple, flexible and cost-favourable mounting technologies for mass applications in which partially reduced properties can be accepted. In principle, the thermal spraying processes can be utilised in order to achieve flexible metallised coats on various surfaces. Even on ceramic-metallic surfaces, cold gas spraying can produce firmly adherent coats with physical properties which are equivalent to those of the solid material. Previous applications of the process concentrated on the coating of ductile substrates with ductile spraying consumables. In the present paper, Al2O3 ceramic plates are coated with a multilayer aluminium/copper composite. In which aluminium acts as the adhesion promoter between the ceramic and the copper coat [1]. The regions to be selectively coated with conductor structures are defined by laser-cut, 1 mm thick steel masks. In this respect, sixteen individual modules are produced in one pass during the coating operation (Fig. 1). In analogy to the copper layers in DCB substrates, the thickness of the cold-gas-sprayed Al/Cu coat systems is 300 m.

Figure 1 Cold-gas sprayed Al/Cu metallization on alumina substrate with dimensions of 138 190 0.63 mm3

1.1 Power electronics manufacture of circuit carriers

Power electronics substrates manufactured by means of DCB technology are widespread in these applications. Quality characteristics of DCB substrates are a high ther-mal conductivity, a high temperature stability and a high electric insulating capacity of the ceramic. The copper metallisation will provide a high conductivity and current-carrying capacity as well as good heat transfer to the ce-ramic. At the same time, the overall system must have a coefficient of thermal expansion which is adapted to the

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devices integrated on the substrate. Figure 2 shows typical cross section of a power electronic module.

Figure 2 Cross section of a typical power module

1.2 Characteristics of thermal spraying processes and of coats produced with them

Cold gas spraying is a high-velocity thermal spraying processes towards lower thermal energy proportions and higher kinetic energy proportions in the process. A heated carrier gas is brought to a supersonic velocity by corresponding nozzle systems. The consumable in the form of fine sprayed particles is accelerated towards the surface to be coated which the sprayed particles strike, on which they flatten out and to which they adhere. In this respect, the particles are subjected to hardly any thermal loads but strike the surface to be coated at a high velocity. In this way, a firmly adherent coat arises in layers. The coated substrate is not melted completely during this operation. The coat adhesion is based on the effect that the abrasive action of a particle impacting on a surface changes into the adhesion of the particle as from a material-dependent specific velocity [3]. Figure 3 represents the cold gas coating system which was used in this study.

Figure 3 Cold-gas-spray-coating-system at Technical University of Chemnitz The mechanical, physical and chemical processes which lead to this adhesion have not yet been described conclu-sively [4]. Investigations on ductile substrate materials and powders showed metallurgically joined regions at the particle/substrate interfaces. These may be attributed to high temperature peaks due to extreme internal friction and quasi-adiabatic conditions during impact of the parti-cles [5, 6]. The produced coatings exhibit a low defect density in comparison with conventional spray coats. They have low residual stresses after heat treatment and

can be manufactured in an approximately pore free form. The gas and oxide contents of the layers roughly corre-spond to the contents of the utilised original powders. Op-timally deposited and heat-treated layers achieve the same properties as those of the respective solid materials. Table 1 shows the comparison between the material properties of solid and cold-gas sprayed materials.

Table 1 Material properties of cold-gas sprayed materials compared to solid material

2 Investigation results

2.1 Substrate manufacture 300 m thick Al/Cu coat systems with porosities under 1 % were produced on 630 m thick Al2O3 ceramics by means of cold gas spraying (Fig. 4).

Figure 4 300 microns thick cold gas sprayed copper on alumina with Al adhesive layer The portion of the bonding promoting aluminium layer in the total coat thickness is approx. 20 %. Figure 5 shows the scanning electron micrograph of a ceramic coated on both sides.



Figure 5 SEM-image of cold gas spraying ceramic coated on both sides with Al/Cu- composite The minimum track widths and clearances attainable with the CGS technology were established in a design rule check. The coated substrate with the structures produced for the checking of the design rules is shown on Figure 6.

Figure 6 Alumina ceramics with structured Al/Cu layer to determine the design rules Accordingly, the minimum track widths and track distances are 250 m and 360 m respectively. The average adhesive strength of cold-gas-sprayed aluminium coats on the Al2O3 ceramics is 31 +/- 4 MPa. After a subsequent heat treatment at 300C for 10 h, the adhesive strength was doubled to 58 +/- 6 MPa on average.

2.2 Utilisation in power electronics

2.2.1 Electrical conductivity of cold gas sprayed layers

The electric conductivities of the cold-gas-sprayed copper and of the sprayed aluminium reach approx. 60 % and 20% of the theoretical electric conductivities of the re-spective solid materials. These relatively low electric conductivities are attributed not only to an increased inter-facial density in CGS coats especially in the case of the

CGS aluminium but also to the natural degree of oxida-tion of the utilised aluminium powder which is transferred into the coats in the form of oxide skins and creates addi-tional interfaces. As expected, the thermal after-treatment of the cold-gas-sprayed coats led to a lasting improvement in the electric properties [7, 8]. The thermal treatment caused in cold-formed materials diffusion, defects reduc-tion or recrystallization [9]. The listed processes depend on the length and temperature of treatment. Figure 7a shows the relative improvement in electrical conductivity as a function of treatment temperature. The cold-gas-sprayed materials were annealed for 2 hours in nitrogen atmosphere. Due to a heat treatment at 350C for 2 h, the specific electric resistance of the sprayed copper dropped by approx. 25%, that of the sprayed aluminium by approx. 12 % and that of the Al/Cu composite by approx. 36 %. This resulted in an average specific electric conduc-tivity of the CGS Al/Cu metallisation of approx. 80 % in comparison with DCB metallised copper coats. The ob-servation of the electrical conductivity changes at differ-ent annealing times and temperatures shows that a few minutes of annealing already improve the electrical con-ductivity of Al /Cu composite significantly (Fig. 7b).

Figure 7 Change of electrical conductivity of the CGS metallizations as a function of treatment temperature (a) and as a function of annealing time at different curing temperatures (b) Figure 7b shows, that after a thermal treatment at 305 C and 355 C and within 10 minutes change the electrical conductivity of the CGS-Al/Cu composite around 30%. In addition, after annealing of the CGS metallization at temperatures above 300 C and longer than 10 minutes, no significant improvement in electrical conductivity was observed.

2.2.2 Soldered interconnections and wire bonds on CGS-Metallisations

On the Al/Cu metallisations which were cold-gas-sprayed onto the Al2O3-substrates, the solderability and the wire bondability were tested. Figure 8 makes clear that standard power components like IGBTs and diodes can be soldered onto the CGS substrates directly in the as sprayed state or alternatively after thermal and mechanical treatments.



Figure 8 IGBT soldered onto the CGS substrates using the vapour-phase soldering process; left: as sprayed sub-strate; right: after thermal and mechanical treatments Examples of the produced heavy wire bond interconnec-tions (300 m aluminium) on grinded and tempered CGS-copper laers are exhibited in Fig. 9.

Figure 9 left: Scanning electron micrograph; right: Macrograph of the bonded joints (300 m Al wire) These wire-bond interconnections were produced with an automated Orthodyne 360A wire bonder with standard bond parameters used also for DCB substrates. The qual-ity of the bonded interconnections was tested by means of shear tests. In Figure 10 the shear strength of wire bonds is exhibited which were produced with 400 m thick bond wires on CGS metal.

Figure 10 Shear strength of 400 m aluminium wire bonds on CGS-copper as a function of the annealing temperature prior to bonding The treatment temperature was varied in a range from 150 C to 400 C for 2 hours. An optical inspection after the shear test reveals that for tempered CGS-Substrates the copper layer partly fails by cohesive failure in most cases (Fig. 11a). From annealing temperatures above 250 C on the bonds on CGS metal layers achieved bond strengths similar to DCB copper metallisations (Fig. 11b).

Figure 11 Damage image on an un-tempered CGS-layer a) after the shear test and b) after tempering at 305 C for 2 h (400 m Al wire) In Figure 12 a complete test circuit with soldered devices and wire-bonded devices on a CGS-substrate is exhibited.

Figure 12 Demonstration module with IGBT and diode after the chip soldering and wire bonding

2.2.3 Thermal characteristics of the CGS substrates

Special test modules were assembled in order to characterise the thermal management performance of the novel substrates. To that purpose chip diodes with an area of 5.6 mm 5.6 mm were soldered onto the carriers in order to produce the dissipated power Pth . The backside of the modules was directly cooled with DI water. In Figure 13 the scheme and the accomplished test assembly for the measurement of the thermal resistance are displayed.

Figure 13 Measurement of the thermal resistance; left: scheme; right: test device The thermal resistance Rth was taken as a measure of the thermal management capability of the substrates. It is computed from the difference between the diode tempera-ture Td and the temperature on the water-side of the mod-ule Ts and the dissipated power of the chip. Alternatively



th

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th

sdjsth P

TTRP

TTR )(;)( ,,

the Rth was computed with reference to the temperature of the cooling water Tw (Eq. 1). (1) The temperature of the power diode was taken up with a Pt-100 thermal resistor, which was mounted directly onto the power diode using a thermally conductive adhesive. The temperature on the backside of the substrate was measured with a CuNi/Cu-thermocouple which was formed by contacting a CuNi wire to the backside copper film of the substrate. The flow rate of the coolant is a variable factor which affects the thermal resistance in the direct cooling experiments. It was measured with a flow rate sensor and the Rth-values were taken up as a function of the flow rate. Fig. 14 shows that the Rth with a reference to the cooling water temperature Tw decreases with increasing flow-rate. When the reference point for Rth ist he backside of the module with a temperature Ts , the Rth will increase, because the heat spreading zone will shrink when the heat transfer coefficient at the substrate is increased by stronger coolant flow. Comparing the Rth-values of DCB- and CGS-modules directly (Fig. 14) it becomes evident that the thermal resistance Rht of CGS is just 13 % higher than for DCB-modules. This is valid at a flow rate of 3 l/min.

Figure 14 Comparison of the thermal resistances Rht of DCB- and CGS-modules as a function of the cooling water flow-rate

2.2.4 Reliability tests Passive temperature cycling tests were performed in a temperature chamber (VTSCH). The test conditions were temperature holding points at -40 C and +150 C with soak times of 20 min each. The temperature transi-tions were performed at average rates of 63 K/min. Sin-gle-side copper-coated CGS-substrates typically will fail after 100 temperature shock cycles due to de-lamination of the CGS-metal from the Al2O3-substrate. In order to prevent the de-lamination the design of the substrates was modified. The improved substrates were provided with an increased aluminium adhesion layer, onto which the smaller copper layer is deposited. So a step-wise transi-tion is achieved at the edges which will decrease local

thermal stresses. On these substrates cleavage between metal and ceramic material will be observed from ap-proximately 250 temperature shocks on. The second test of circuits produced with CGS- and DCB-substrates was active power cycling. These were controlled in a manner to achieve force temperature transitions between 35C and 135C at a cycling time of 1 min. The DCB-modules were actually operated with higher electrical power dissi-pation as the CGS-modules. The reason for the implemen-tation of tests at constant temperature was compliance to the thermo-mechanical failure criteria for assembly tech-niques, which are based on a constant temperature swing. This means that the solder and bond connections on CGS module may fail earlier by operation of this module with the same power dissipation as the DCB module. Due to practical reasons for the power-cycling test CGS-modules both with soldered as well as with adhesively bonded de-vices were used. Fig. 15 exhibits thermographic images of the CGS-modules during these tests.

Figure 15 Thermographic image of CGS-based modules with soldered devices during power-cycling: left: during cooling to 35 C; right: during thermal shock at 135 C In the power cycling test was an electrical failure of IGBT devices occurred. Assuming a Weibull-curve the characteristic lifetime of the soldered CGS-Modules was approximately 57.000 cycles and that of the adhesively bonded ones was 253.000 cycles. The soldered DCB-Modules which served as a benchmark tolerated 72.000 cycles up to 63 % failures (Fig. 16).

Figure 16 Weibull-diagram for the time-to-failure of CGS- and DCB-based modules in power-cycling between 35 and 135 C. So the characteristic lifetime of the high-power assem-blies on CGS reaches 80% of that of the DCB-based stan-



dard modules. As an interesting result we found that the lifetime of the adhesively bonded CGS-modules (Ag-epoxy) is higher that that of SAC-soldered devices by a factor of four.

3 Summary and conclusions A new process for the metallisation of power substrates with aluminium and copper has been developed using the cold-gas-spray technology. With mechanical masking, it is possible to generate metal patterns up to 500 m thick-ness and down to 250 m width on one or both substrate sides. Generalizing, the electrical, thermal and mechanical properties of the layers reach up to 80 % of the respective properties of Al2O3-based DCB. By working out appro-priate design rules, for the Al/Cu metal edges, the thermal cycling stability of the bare substrates was increased sig-nificantly. It was possible to solder IGBT and diode chips onto the substrates and to wire-bond these using standard processes which are established for DCB. After suitable surface treatment and annealing, the Al wire bonds were almost equivalent to DCB regarding the mechanical prop-erties. For the preliminary module testing with power cy-cling, CGS substrates with IGBTs and diodes were as-sembled using a highly conductivity adhesive. The tested substrates failed after 160.000 to 293.000 cycles with a T=100 K. This is equivalent to a relative characteristic lifetime of the soldered joints on CGS-substrats of 80 % of the number of cycles to (63 %) failure on DCBs. Also functional modules were built up using novel insulation systems like anodized aluminium and CGS.

4 Acknowledgement The IGF Project 15.441 B / DVS No. 10.050 of the re-search association "Forschungsvereinigung Schweien und verwandte Verfahren e. V. des DVS, Aachener Strae 172, 40223 Dsseldorf" was, on the basis of a resolution of the German Bundestag, promoted by the German Min-istry of Economic Affairs and Technology via AiF within the framework of the programme for the promotion of joint industrial research and development (IGF).

5 References [1] Marx, S.; Paul, A.; Kohler, A.; Huttl, G.: Cold spray-

ing: Innovative layers for new applications. In: Jour-nal of Thermal Spray Technology 15 (2006), Nr. 2, S. 177-183.

[2] Lutz, J.: Aufbau- und Verbindungstechnik von Leis-tungsbauelementen. In: Halbleiter-Leistungsbauel-emente. Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg (2006), S. 269-318.

[3] Schmidt, T.; Gartner, F.; Assadi, H.; Kreye, H.: De-velopment of a generalized parameter window for cold spray deposition. In: Acta Materialia 54 (2006), Nr. 3, S. 729-742.

[4] Wielage, B.; Grund, T.; Rupprecht, C.; Kummel, S.: New method for producing power electronic circuit boards by cold-gas spraying and investigation of adhesion mechanisms. In: Surface and Coatings Technology 205 (2010), Nr. 4, S. 1115-1118. [5] Borchers, C.; Gartner,F.; Stoltenhoff, T.; Kreye, H.:

Microstructural bonding features of cold sprayed face centered cubic metals. In: Journal of Applied Physics 96 (2004), Nr. 8, S. 4288-4292.

[6] Wielage, B.; Wank, A.; Podlesak, H.; Grund, T.: High-Resolutional Microstructural Investigations of Interfaces between Light Metal Alloy Substrates and Cold Gas Sprayed Coatings. In: Journal of Thermal Spray Technology 15 (2006), Nr. 2, S. 280-283.

[7] Hall, A. C.; Cook, D. J.; Neiser, R. A.; Roemer, T. J.; Hirschfeld, D. A.: The effect of a simple annealing heat treatment on the mechanical properties of cold-sprayed aluminium. In: Journal of Thermal Spray Technology 15 (2006), Nr. 2, S. 233-238.

[8] Calla, E.; McCartney, D. G.; Shipway, P. H.: Effect of heat treatment on the structure and properties of cold sprayed copper. In: International Thermal Spray Conference Proceedings (2005), S. 170-176.

[9] A.K. Sinha, Physical Metallurgy Handbook, McGraw-Hill, New York, 2003



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Articulo Electronika 1