The Use of Thick Print Copper and Silver Conductors for Power Applications PDF document - DocSlides

The Use of Thick Print Copper and Silver Conductors for Power Applications PDF document - DocSlides

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David K Anderson John Oleksyn Martin Batson John Cocker DuPont i Technologies 14 Alexander Dr Research Triangle Park NC 27709 Tel 919 2485403 Fax 919 2485041 Email davidkandersonusadupontcom DuPont UK Limited Coldharbour Lane Frenchay Bristol BS16 ID: 20813

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The Use of Thick Print Copper and Silver Conductors for Power Applications. David K. Anderson*, John Oleksyn**, Martin Batson** & John Cocker**. *DuPont i Technologies, 14 Alexander Dr. Research Triangle Park, NC 27709 Tel: (919) 248-5403 Fax: (919) 248-5041 E-mail: david.k.anderson@usa.dupont.com **DuPont (UK) Limited, Coldharbour Lane, Frenchay, Bristol BS16 1QD, England. Abstract The market for efficient power devices in applications such as motor control, DC:DC converters and power modules is continuing to increase. Metallized substrates su ch as thick film copper or silver conductors on alumina, Insulated Metal Substrates (IMS), Direct Bonded Copper (DBC) etc. are able to offer a variety of technical and commercial solutions to current handling, thermal management, circuit density and packaging. Thick printing Cu and Ag conductors ha ve been developed specifically for us e in power applications where excellent printing, thermal, electrical, wire bonding and solderin g properties are prerequisite. Efficient thermal management can require fired films on alumina in excess of 150 m and often printed in large areas. In some designs the thickness of mounting pads alone for bare silicon dies may need to be built up locally. This approach enables a single substrate to comprise both thinner printed, dense circuitry for signal control and thick device mounting pads for efficient thermal management. The flexibility of thickn ess control, through hole connections and the ability to incorporate printed resistors using standard thick film processing can offer solutions which complement the other substrate technologies in many applications. This paper describes the advancements made in optimiz ing the performance of thick printing copper and silver conductors designed for use in power applications and their role in this demanding technology. The features of the materials, process guidelines, and performance characteristics will be discussed. Key words: power, copper, silver, thick pr int, thick film, thermal management, conductor Introduction. The market for ener gy efficient power conversion circuitry in telecommunication, IT, industrial and commercial applications continue to grow worldwide. Typical applications for power devices include power semiconductor modules, DC:DC converters, lighting ballast, motor drive controllers, automotive control systems, etc. The functional requirements of these circuits are highly varied with current ratings ranging from a few amps to many hundreds or even thousands. Consequently the demands placed on the circuit design, operating environment, and materials used in their construction are equally diverse. A basic requirement of power devices is some form of metalized substrate to carry current throughout the circuit with low loss, to provide good thermal management of active and passive components and to do so in a cost effective and reliable manner. Improvements in efficiency and the reduction in size of all devices are major driving forces that significantly influence the type of metalized substrates used. Finally, in today’s ecologically conscious climate, the processes and
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materials used to manufacture the substrate and completed device should be environmentally friendly. Three substrate metalization technologies which are familiar to power circuit designers and engineers are Insulated Metal Substrates (IMS), Direct Bonded Copper (DBC) and Thick Film. All are able to offer commercial solutions to power applications and are widely used. IMS uses a ceramic filled organic dielectric film approximately 75 m thick to separate an etched copper metalization layer, 35 – 140 m thick, from an aluminium base plate. The organic layer naturally limits the upper operating temperature to less than 125°C. DBC is comprised of a ceramic substrate with copper foil 200 m – 500 m thick, bonded and etched on both sides making it suitable for high temperature and very high current applications. Thick film also applies a metalization layer fired at high temperature on ceramic. Table 1 lists the thermal conductivity of materials used in the three substrate technologies. As well as having excellent el ectrical insulation, the very good thermal conductivity of alumina is well known. Note that the material with the lowest thermal conductivity in the structure will have a significant effect on thermal management. Table 1 Thermal Conducti vity Of Materials. Thermal Conductivity (W/m K) Conductors: Bulk Ag Bulk Cu Bulk Al Insulators: 96% Alumina IMS Dielectric Glass 420 385 201 25 Application Of Thick Film Metalization. [1] Thick film conductors printed and fired on 96% alumina (Al ) substrates are being used in volume applications. Dupont has worked with suppliers of metalized substrates and power circuit manufacturers to further develop silver and copper conductor pastes which are able to offer a complementary solution to IMS or DBC in some applications. (1) Traditionally, thick film conductors are used to print closely spaced tracks a few microns thick, to meet the demands of miniaturisation, increasing functionality and density of multilayer and Green Tape circuits. Examples of this kind of complex circuitry abound in automotive applications where safety and reliability are paramount (such as ECU and ABS units, DC:DC and frequency converters, etc.). Power circuitry on alumina substrates however requires a significantly different approach in the design of conductor pastes. Highly conductive, non-alloyed silver or copper compositions have been specifically created to print very thick layers through relatively coarse screens. The dried prints are fired at elevated temperatures to remove the organic components, sinter the metallic powders and bond the structure to the substrate. The fired film must be dense, have particularly good thermal and electrical properties and preferably be heavy (10mil) aluminum wire bondable and solderable. Some pastes are designed to provide a fired conductor layer thickness up to 200 m, depending on circuit requirements, using three consecutive print/dry/fire (PDF) processing cycles. Advantages Of Thick Film. The advantages of using thick film substrates (Figure 1) in power applications can be summarized as follows: Figure 1: Cross Section Through Thick Film Metallized Substrate. The ability to vary the fired film thickness enables the combination of thinner, higher density signal tracks and thicker power circuitry to be printed on the same substrate. Semiconductor dies can be mounted directly on the fired film to improve the thermal dissipation. This approach lowers assembly costs, improves reliability by reducing circuit-to-circuit interconnections, and leads to a very flexible
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solution. By comparison, alternatives to thick film often involve separating the power devices and control circuitry on different substrates. Thick film technology allows screen printed resistors, which may be laser trimmed to tight tolerances, to be incorporated. Thick film multilayer circuitry can also be processed on the substrate where required. This improves density, decreases size and cost. The attachment of components and interconnecting techniques commonly used within the microelectronic industry are applicable including high temperature soldering of silicon dies and heavy aluminum wire bonding. Lead frames can be attached directly onto the substrate without the use of intermediate carriers. Through hole interconnection is possible enabling double sided circuitry to be constructed. Thermal vias can also be used to improve thermal transfer from th e power device to a heat sink. Thick film printing and firing is an additive process that leads to good material and environmental management without the production of aqueous waste. The reliability and durability of thick film circuitry in harsh operating conditions such as military, automotive and aerospace applications combined with flexibility of construction and design is proven. The use of thicker printing conductors is an extension of existing technology enabling hybrid circuit manufacturers to process these specially designed pastes with existing equipment and expertise. 96% Alumina is a cost effective substrate material which has a high thermal conductivity and excellent insulation resistance. The Thermal Coefficient of Expansion (TCE) match between bare silicon power devices and the substrates is good (approximately 7.3ppm/°C), which helps to reduce thermal stresses when mounting large dies. (The TCE of IMS substrates is considerably greater at 25 ppm/°C). Conductor Pastes Developed for Power Applications The pastes shown in Table 2 were formulated to print large areas thick. This will accommodate production requirements based on block and pad patterns covering up to 90% of a 178mm x 127mm substrate. The selection and combination of metal powders, organic vehicle systems and inorganic binder phases ensure the fired films have a dense structure. This will maximise their electrical and ther mal properties without compromising other physical characteristics. When printing large areas the pastes have been designed to release cleanly from the scr een mesh while retaining good cosmetics and print resolution. Table 2 Non-alloyed Silver & Copper Co nductors For Power Applications. Copper Silver Product 6004 7731 7732 7740 Peak Firing Temp. / Atmosphere 600 °C / N 900 °C / N 900 °C / N 850 °C / Air Min. Track Spacing (mm) 0.35 0.35 0.35 0.35 165 mesh / 20 m emulsion (fired film thickness m) 25 – 35 25 – 30 25 – 35 25 – 30 105 mesh / 70 m emulsion (fired film thickness m) 80 60 80 60 (1x165 +2 x 105) P/D/F Cycles (fired film thickness m) 200 150 200 170 Typical m /sq @ 30um fired. <1.4 <0.7 <0.8 <0.8 Solderable Yes Yes Yes Yes Heavy Al wire bondable N/A Yes Yes Application specific
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Compatible with air fired resistors Yes resistors resistors Yes 6004 copper is a low-temperature (600 C) firing conductor. It can be used for through-hole applications, or as a solderable conductor. This paste may be used in combination with 850°C air firing resistors (post terminated) allowing high density, double sided power hybrids to be constructed. The two ‘high temperature’ copper conductors, 7731 and 7732 produce very dense highly sintered fired film which has excellent thermal and electrical properties after firing at 900°C in N The selection of metal powders and binder phases was critical to optimize these characteristics. 7731 is recommended when the thickness of the metallization is 150 m or less (in three PDF cycles). 7732 was developed to extend this thickness to 200 m in three cycles using a 105 mesh screen throughout. Both copper conductors are solderable and have excellent heavy (10 mil) aluminum wire bond performance. Initial bond adhesions are 500g and aged adhesions after 1000 hours at 150°C are 450g. The non-alloyed silver conductor, 7740, is air fired using a standard 850 °C profile. It is compatible with many Dupont-supplied air fired thick film resistors. This is allows power circuit manufacturers, to use the same conductor composition, substrate, and process to print heat- spreading pads, bus bars, control circuitry and resistor terminations. Processing Guidelines for Thick Printing Conductors [2] While printing the conductors thick is similar to standard thick film processing, consideration must be give n to the mechanics of the operation. A 165 stainless steel mesh, with a 20 emulsion can be used to print the first conductor layer if signal tracks are requir ed. This will provide a fired metallization layer around 25 - 35 m thick depending on the actual composition used. Areas where increased thickness is needed are overprinted with subsequent layers. A coarser screen is used with the selection of mesh depending on the resolution required. A 105 mesh screen with a 70 m emulsion is recommended to maximize thickness. After screen selection, the key to printing thick is to use a hard squeegee (80 durometer or greater) and minimal squeegee pressure. It is also advisable to ‘step-in’ the dimensions of overprinted layers from the edge of the underlying layer to maximize the surface planarity and to simplify component mounting. It is recommended that the film thickness should be built up using sequential print/dry/fire processing, since cofiring of metal layers may be detrimental to the density of the fired film. Table 2, shows that combinations of screens can be used to create the optimum pattern resolution and thickness. The TCE difference between a thick metal layer and alumina may lead to some bowing when large area substrates are printed on one side only. The degree of bowing is greater with thinner substrates, thicker metallization or larger printed areas. The application of a thin rear face conductor can be used to control the degree of bowing. If metallization on the backside is required, it should be printed after firing the first top conductor layer. Screen selection depends on the film thickness required. Thermal Performance of Thick Film. An important performance characteristic of the fired film is thermal dissipation because efficient transfer of heat from power devices is imperative. This is governed by the composition and structure of the fired film and by its printed thickness. A thicker layer allows lateral heat-spreading to occur. It is therefore important to stud y the combined effect of substrate and fired film thickness in actual applications. To determine those effects, Dupont ran a study jointly with one of its end-use customers. Two alumina substrat e thickness (0.38mm and 0.63mm) were each printed with two thickness of 7731 copper (25 m and 150 m) to provide four material combinations. A 25 m layer of 7731 was printed on the back side of each part. A series of silicon power devices, with increasing area, were soldered directly to the top conductor layer. Each assembly in turn was soldered onto a heat sink drilled to accept a thermocouple. Cu rrent was applied to the silicon devices and electrical and temperature measurements were made to investigate the thermal transfer between the silicon junction and the heat sink. The temperature of the silicon and substrate during the measurements were 140°C and 100°C respectively. This repres ents typical applications. Figure 2 shows the calculated thermal resistivity (Rthjs) between the silicon junction and the heat sink. A comparison is made with IMS and DBC substrates under the same conditions. The IMS
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was 3kV rated material with 105 m of copper and a 2mm thick aluminum base plate. The DBC Figure 2: Thermal Performa nce Of 7731 Cu Conductor. Junction To Heat sink Thermal Resistance (Rthjs) 0 0.1 0.2 0.3 0.4 0.5 Silicon Die Area (cm Rthjs (°C/Watt) 150um Cu / 0.38mm alumina 25um Cu / 0.38mm alumina 150um Cu / 0.63mm alumina 25um Cu / 0.63mm alumina IMS Substrate DBC Substrate was comprised of 200 m of copper on each side of a 0.38mm alumina substrate. The data in figure 2 shows the improvement in thermal conductivity (i.e. a reduction in Rthjs) as the thickness of the copper print increases. This is due to increased lateral h eat spreading efficiency of the thermally conductive c opper. In all cases, the thick film copper performance is better than the IMS substrate. The 150 m thick layer of 7731 copper is very close to that of 200 m of DBC on the same 0.38mm alumina substrate. This result in particular emphasizes the significance of the thermal properties of the alumina. Although the copper thick film is 50 m less than the DBC metallization, on the front face and 175 m on the rear, it is the thermal conductivity of the substrate and solder films which dominate the results. Consequently 7731 metallization has a similar performance to the bulk (DBC) copper. A secondary effect shown in figure 2 is that while alumina subs trates are good thermal conductors, a thinner subs trate will actually transfer heat to the heat sink more efficiently. Conclusions: Thick film metallization on alumina substrates is proving to be a viable option in power applications. Thick printing copper and silver conductors have been developed specifically for use in power circuits where ex cellent printing, thermal, electrical, wire bonding, an d soldering properties are prerequisite. Print thickness can be built up locally allowing flexibility in design and cost management. This approach enables a single substrate to comprise both thinner printed, dense circuitry for signal control and thick printed device mounting pads for effective thermal management. The flexibility of thickness control, through hole connections and the ability to incorporate printed resistors using standard thick film processing offers solutions which complement the other substrate technologies today and in the future. References: 1. Very Thick Printing Conductors For Power Applications; Roger Parr, DuPont, ISHM Nord ic Proceedings, September 1996. 2. Printing Guidelines For Thick Film Conductors In Power Applications;
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DuPont Photopolymer & Electronic Materials, UK, Guide L-12024.

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