Investigation of Rapid-Prototyping Methods for 3D Printed Power Electronic Module Development

Investigation of Rapid-Prototyping Methods for 3D Printed Power Electronic Module Development Haotao Ke, Adam Morgan, Ronald Aman, Douglas C Hopkins, Fellow Laboratory for Packaging Research in Electronic Energy Systems (PREES) Future Renewable Electric Energy Delivery and Management Systems Center (FREEDM) Department of Electrical and Computer Engineering North Carolina State University Raleigh, NC 27695-7911 Tele: 919-513-5929 Fax: 919-513-0405 hke@ncsu.edu, ajmorga4@ncsu.edu, rlaman@ncsu.edu, dchopkins@ncsu.edu Abstract The recent research in wide-bandgap (WBG) power electronic semiconductors has produced a wide variety of device and combinational topologies, such as HFETS, MOSHFETS, and the Cascode Pair. Each variation needs to be tested with certain package criteria (e.g. high voltage SiC devices up to 15kV, high current GaN devices up to 300A, or unprecedented high frequencies). Having a common package is costly and cannot provide an investigation of optimized performance. Hence, use of a rapid prototyping method to print power electronic packages and modules is needed. Also, the continual move to higher frequencies will require greater integration of packaging into the end application, as is presently done with point-of-load converters. The future modules will take on more functional integration, including more mechanical features, which further supports use of printed fabrication technologies. It is not reasonable to assume that a complete module can be directly printed, though most would be; some assembly is required. This paper discusses partitioning of a module process, and identifying key elements that can be combined for optimum power package production. To select the best process, or combination, for rapid-prototype printing of power modules current, Additive Manufacturing (AM) methods are evaluated, such as Stereolithography (SLA), Selective Laser sintering (SLS), and Fused Deposition Manufacturing (FDM). Several modules were fabricated to demonstrate mechanical resolutions in the packaging. A thermoplastic printer, specifically the MakerBot, which is a high end consumer 3D printer, produces packages with 100 micron resolution. The build object can have surface texture enhancement with post chemical treatment. Today, this is finding a home and proving useful in low volume rapid prototyping in small electronics companies. The ABS plastics are typically rated for <105⁰C applications. Another printed module to be reported uses a high-end commercial machine with <20 microns in resolution (Stratasys Objet) using standard UV curable polymers. This provides a slightly higher temperature range with greater mechanical integrity. Materials for >250 ⁰C that use both UV and thermal sintering are available, but not evaluated in this paper. Functional integration can include electrical, mechanical, and thermal appendages and sub-systems. Electrical sub-systems, such as gate drivers and sensors, can impact process partitioning, by requiring low power circuit fabrication processes integrated with those for high power. This paper demonstrates a printed polymer substrate process for functional integration of a signal-circuit. Since nearly all AM processes were developed initially for mechanical systems, many processed materials have not been electrically characterized, though the basic material compositions may have suitable electrical characteristics. This paper categorizes several materials for their potential suitability for power packaging. The evaluation is based on the electrical, mechanical, and thermal parameters, along with precision, surface texture (affecting electric field contours) and process times. Cost and performance will be of main concern. Key words Additive manufacturing; Power electronic; Printed electronics; Rapid prototyping 1

I. Introduction The next generation of power electronic semiconductor devices is making its way into broader markets, such as the smart grid, and this trend will only continue [1, 2]. Different applications have different design requirements, for example, electrical and physical isolation requirements pertaining to interconnection layouts and packaging terminations for medium and high power applications. Working voltages and environmental pollution levels dictate minimum clearance and creepage distances to prevent harmful breakdown in open-air structures [3]. Power packaging is trending more towards an integrated module design approach, instead of discrete semiconductor device packages. This will in turn foster the development of newer electrophysical topologies for applications, such as electric vehicles and photovoltaics to increase electrical and physical densities [2, 4]. During the module prototype development period, having a common power package can prove to be costly and may not necessarily provide optimized electrical, magnetic, mechanical, and thermal performance. Therefore, a rapid prototyping method, namely 3D printing, could be utilized when manufacturing power electronic packages. II. Investigation of AM Methods and Material The basic power module includes seven elements: the power semiconductor device(s), direct bonded copper (DBC) substrate, base plate, interconnects, housing, encapsulate, and terminations. Furthermore, by adding control circuitry, the module is converted into an intelligent power module (IPM). Each element may have an optimum AM material and process for its manufacture. However, designing for manufacturability, reliability, and cost seek to minimize process steps, and variations in material and interfaces. Additive manufacturing is just starting to investigate substantial use of mixed materials and composites. Thus, the immediate research is to evaluate existing materials and suitability to print each element, or, preferred, to print multiple elements having functional integration. Materials are divided at the top level as electrically conductive and electrically isolating. Metal interconnection, for example use traditional processes to solder / wire bond interconnections between devices. However, there has also been positive research regarding printing copper as interconnection. Experiments were done for building up copper posts on a DBC substrate by an electron beam (Ebeam) process. The result is shown in Fig. 1. Copper patterns with 3mm height, 2mm spacing and feature size 750 microns, were formed on a double sided Direct Bonded Copper (DBC) substrate. A critical challenge to the fabrication was stress management within the DBC by controlling the time rate of change of the thermal gradient within the structure. The pattern is representative of a Vivaldi antenna array used in RF systems. The results demonstrate the possibility of a printed metal on ceramic system for more complex interconnections or intricate structures. Further work will optimize the printing process and electrically characterize the printed metal patterning for power packaging. The AM techniques could be brought in to print the traces and interconnects within the module. Also, the concept of an IPM could be applied by combining control circuitry within the housing. Fig. 1 E-beam printed Cu posts on DBC substrate As mentioned above, one of the goals of this work is to develop a fabrication process for future power module designs that enable greater integration of peripheral components and features. A main element of any power module is the housing. The housing provides the module with its overall strength, integrity, and electrical and environmental isolation, as well as contains all of the inner electronic components. The power module also interfaces with external circuitry, such as gate drivers and loads. III. Additive Manufacturing Methods One objective of the power module design process is to print an embedded control and gate-drive circuit within a housing, hence tightly coupling the signal and power processing components. Combining signal and power by AM methods requires re-evaluation of AM materials and processes. An selection processes and materials, based on ready availability (i.e. not custom formulations), were evaluated for potential use as either an electrical conductor or dielectric at the temperatures needed by power modules. The temperatures depend on application, and range from accommodate new wide bandgap power semiconductors, such as GaN and SiC. Technologies selected were Fuse Deposition Modeling (FDM), Stereolithography (SLA), Selective Laser Sintering (SLS), Inkjet 3D Printing (3DP), and Laminated Object Manufacturing (LOM). 2

Traditional power module housings, which provide the environmental protection for the device during operation, are typically cast with materials, such as thermoset allyls/epoxy, Diallyl phthalate (DAP), thermoplastic polyester, and Polybutylene terephthalates (PBT). These materials provide the housing with overall mechanical strength, electrical isolation, and sufficient thermal conductivity. Therefore, tensile strength, Young s modulus, electrical / thermal conductivity, and the coefficient of thermal expansion are considered as criteria when bringing in AM technology to the housing fabrication process. In Fig. 2, a comparison of AM Materials and Traditional Housing Material is given based on the Heat Deflection Temperature (HDT) rating versus Tensile Strength. These are the primary parameters when integrating thermal and mechanical features into the housing, particularly with a focus to higher operating temperatures of the WBG semiconductors. The tensile strength / heat deflection temperatures are measured following ASTM D-638 and ASTM D-648 standards. strength of the more commonly used epoxy, which is cured by chemical reaction, and has yet to have been reported for use in 3D printing of structures. The AM materials with more desirable properties, such as high dielectric strength, low water absorption, high chemical resistance, good flame retardance, and most importantly, low economic cost, are needed if AM materials are to become commonly used in power package housings. Another underlying problem is that most AM materials are not tested or properties given in data sheets for electrical performance. Conversely, properties show that thermoset plastics are worthy of being used as AM material for power packaging applications due to their similar performance with the DAP/PBT material, but processing is a challenge. To achieve the goal of integrating a signal-level control circuit into the housing, precision of printing methods needed evaluation. Power packaging housings require relatively fine-features compared to the vast majority of traditional 3D printed objects. (Though, those features are generally considered large features compared to processes used in the microelectronics industry). Results are given in Table 1 for the selected methods cited above. Fig.2 Material Property Comparison of AM Materials and Traditional Housing Material In Fig. 2 it can be seen that most of the AM materials lie between DAP and PBT when considering the heat deflection temperature. Two SLA materials show better temperature withstand capability than epoxy. These materials are potentially useful in high temperature packaging. For the tensile strength, most AM materials are close to DAP/PBT; yet none of them exceed the tensile Table 1. Precision of AM Methods AM Methods Layer Tolerance Thickness (mm) (mm) Cost Fuse Deposition Modeling (Makerbot) 0.10 0.40 Low Stereolithography 0.05 (3D system) 0.25 High Selective Laser 0.08 0.76 Sintering (3D High system) Inkjet 3D Printing Laminated Object Manufacturing (SD300) Varies by system Varies by system Depends on material 0.16 1.00 Low The smallest component in the control circuit design is a 0805 (80mil x 50mil, 2.3mm x 1.27mm) surface mount resistor. The smallest geometry is the lead pitch of the gate drive chip (0.85mm). To mount the component in the printed lid, the resolution (layer thickness and tolerance) should be smaller than the component geometry. As in [5], FDM methods could not provide a surface with desirable 3

smoothness. These methods produce rough textures on the surface that make it difficult to dispense conductor traces and place components. A developing solution is to use an alcohol (e.g. acetone) surface treatment that dissolve and smooth the surfaces; although this diminishes previously printed fine geometries. The SLS process has the capability to achieve the accuracy needed for placing small 0805 components. However, the cost is high when compared to a similar housing printed by FDM or SLA methods, where the cost of the latter is 10 times lower. The LOM process uses a binder between printed layers, which affects the thermal stability and water absorption of the housing material making it not suitable for power packaging. Based on these drawbacks, SLA and 3DP methods are the preferred choice to produce AM housings. secured with a standard epoxy process. The lid is fitted onto the housing with the substrate leads protruding through the lid openings. The substrate leads are folded onto the lid. The gate lead and any sensor leads (from the substrate) are connected with Ag-loaded polymer. To provide greater connection strength, caverns can be patterned into the lid to allow a Ag-loaded epoxy to flow in and form a stronger stress relieved bond. The final step is to fill the housing through the lid with silicone, which is vacuum cured. SUBSTRATE PROCESS 1a. DBC Patterning & Solder Masking HOUSING PROCESS 2a. Housing Printing LID PROCESS 3a. Lid Printing W/ Ckt Pattern IV. Experimental Design and Process As previously stated, the main goal was to develop a module housing that contained an embedded gate driver circuit, as seen in Fig. 3, adhering to requirements and specifications utilized by the industry. The 3D printed power module housing, described in the sections below, was created by a Stratysys Objet350 Connex 3D printer using VeroClear-RGD810 and TangoPlus FLX930. The housing contains a gate driver circuit printed into the housing lid, which sets into the housing body that contains the power semiconductors. 1b. Die Placement, Attach & Wire Bonding 1c. Terminal Placement & Solder Attach 4. Housing Attachment to Substrate 3b. Component Placement 3c. Conductive Printed Polymer Interconnect & Terminal Attach 5. Lid Attach to Housing 6. Silicone Encapsulate Fill, Vacuum Cure, & Test Fig. 4. Design process flow chart Fig.3 Power Module Circuit Schematic Production of the full module is summarized in Fig. 4. The DBC substrate is processed with traditional power module assembly processes, such as a two-level solder hierarchy. The substrate could have been secured to a baseplate also.) The lid is printed with an embedded circuit pattern; components placed lead-side upward, and a Ag-loaded polymer is dispensed into/onto the component leads to form the traces and interconnection. Also, terminations are connected and secured to the lid traces with the conductive polymer. The housing is fitted over the substrate and For the material and equipment used in the experiment, a FAN3122TMX gate driver (SOIC8 package) is used in the embedded circuit. A Si power MOSFET (IRFC3205, 55V/110A) and diode (IR135DM12CCB, 1200V/8A) are used as the controlled devices. The following materials and equipment are used for their respective applications listed: Orthodyne M20 wedge bonder with 4mil Al wire interconnection, EFD solder paste S10D502B2 (Sn10/Pb88/Ag2) and 63NCLR (Sn63/Pb37) for die attachment and terminal attachment, Wacker SEMICOSIL 921 for silicone encapsulation, DuPont 5025 silver conductor for the conductive traces in the control circuit, and Nordson EFD dispensing system for trace dispensing. 4

The dimensions of DBC substrate conform to standardized dimensions developed for rapid prototyping of projects at the University s packaging facility (PREES). The dimensions are shown in Fig. 5, along with a picture of one of the standardized layouts. Interconnect paths were created by trenches 1mm deep and 0.5mm wide, and filled with dispensed conductive silver paste (DuPont 5025). The circuit was cured at 120 ⁰C for 10 to 15 minutes. A test circuit is shown in Fig.7. Fig.5. Standard DBC Substrate (mm) This housing design was chosen to mimic a Vicor HDC300B Converter Brick Module to show compatibility with current power module technology. SolidWorks was used to draft STL (stereolithography) file required by many 3D printers, including the Objet printer. Fig.7. (a) Test sample for silver paste curing. (b) Test sample with mounted components The FAN3122TMX gate driver IC (8 pin SOIC), is mounted upside down in its respective receptacle. Passive components, such as the capacitors and resistors, are mounted on top of pedestals within the connecting trenches to inhibit the flow of conductive silver paste underneath the components, which may lead to short circuits. A side view of the connecting trench is shown in Figure 8. 0.25mm Fig.6. SolidWorks Design for Power Module (a) Lid and (b) Housing 1.0mm Pedestal 0.5mm 0.25mm 0.5mm A housing lid was designed to cap off the internal power electronics, give mechanical strength to the gate, source, and drain terminals, and to incorporate an embedded gate driver circuit that would otherwise be stationed separately from the module, e.g. on an external PCB. Greater power densities and switching speeds are obtained by embedding such circuits into the power module. V. Housing Body and Lid Fabrication The most novel aspect of this power module housing design is the incorporation of the gate driver circuit into the lid itself. The drain, source, and gate terminal holes were labeled A, B, and C, respectively, using the Objet printer. A separate silk screen labeling step is not required. The gate driver circuit was routed on the lid s surface, in between terminal holes A, B, and C. Holes A and B are rectangular and aligned with the brass terminals on the DBC substrate. Adjacent to each of these are indents for the nuts and screws used to connect the drain and source terminals to an external circuit. The gate hole allows for a circular gate lead on the substrate to pass through, be bent over, and soldered into the gate driver circuit. Fig. 8a. Trench dimensions with Pedestal Fig.8b. Trench Profile The trench is set 0.25mm below the lid s surface to provide an overflow region for the conductive silver paste to contain possible surface over accumulation of printed silver paste. Also, included in the lid are three terminals for V DD, GND, and IN for the gate driver IC, as shown in Fig. 7b. In Fig. 9 are images of the lid and housing together (upper image), and the housing with substrate (lower image). New 5

designs, layouts, and structures can easily be produced making way for true 3D power modules that could include orthogonal components and structures. Objet 350 Connex and VeroClear-RGD810 was used to create an open housing that fits over the substrate. The Object also produced a lid for the module. The lid was patterned with embedded trenches for dispensed, silverloaded conductor material; indents for insertions of electrical components as part of a gate drive circuit, and recessed features for holding mechanical fastener hardware, e.g. nuts, for making metal terminations with the substrate. The module housing and lid are fitted together, and silicone is back-filled and vacuum cured to provide electrical isolation and attachment. Figure 10 Cost Comparison for Object 350 in Low Volume Situation Fig.9. Module Lid with Housing (top) and Housing with Substrate (bottom) VI. Cost Overview The work performed here is focused on rapid prototyping of power electronic modules that have similar net shaping and terminations as found in other high volume commercial products. Some cost analysis is enlightening to determine a break point when prototyping versus small volume manufacturing should be considered. In regards to cost efficiency, the Objet material is not currently well suited for mass production due to its high cost. The production of the housing body and lid cost approximately $200. The material portion of the cost was responsible for less than ten percent of the overall housing production cost. The majority of the cost is due to the machine time, as Fig. 10 shows. This machine cost is primarily composed of the maintenance contract and replacement cost. As the technology progresses the cost of material will likely decrease. VII. Results and Conclusion In this paper several AM methods, materials and assembly processes are evaluated for rapid prototyping a power electronics module. The overall module form factor mimicked a Vicor HDC300B Converter Brick Module. The module used a traditional DBC (Direct Bonded Copper) substrate with power semiconductors and high-current terminals solder attached to the patterned Cu substrate. The A set of standardized processes and subassemblies have been developed to provide quick prototyping of power modules that includes materials for future higher temperature wide bandgap (WBG) power semiconductor systems. References [1] M. A. Briere. The power electronics market and the status of GaN based power devices. Presented at Compound Semiconductor Integrated Circuit Symposium (CSICS), 2011 IEEE. 2011,. DOI: 10.1109/CSICS.2011.6062462. [2] P. Gammon. Silicon and the wide bandgap semiconductors, shaping the future power electronic device market. Presented at Ultimate Integration on Silicon (ULIS), 2013 14th International Conference On. 2013,. DOI: 10.1109/ULIS.2013.6523479. [3] Y. Liu, "Design rule for isolation," in Power Electronic Packaging: Design, Assembly Process, Reliability and Modeling, 1st ed. Anonymous New York: Springer, 2012, pp. 9-16. [4] S. Timothe, R. Nicolas, C. Jean-Christophe, G. Victor and I. Pheng. A novel power system in package with 3D chip on chip interconnections of the power transistor and its gate driver. Presented at Power Semiconductor Devices and ICs (ISPSD), 2011 IEEE 23rd International Symposium On. 2011,. DOI: 10.1109/ISPSD.2011.5890857. [5] H. Ke, Y. Xu, D. C. Hopkins, Conceptual Development Using 3D Printing Technologies for 8kV SiC Power Module Package, 46th International Conference and Exhibition on Microelectronics, Orlando, FL, Sep 30 03 Oct, 2013 6