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ScienceDirect Procedia CIRP 37 (2015) 59 – 64
CIRPe 2015 - Understanding the life cycle implications of manufacturing
Investigations in the Optimization of Power Electronics Packaging through Additive Plasma Technology Joerg Frankea, Aarief Syed-Khajaa,*, Rene Schramma, Raimund Ochsa a
Friedrich-Alexander-Universität Erlangen-Nürnberg, Institute for Factory Automation and Production Systems (FAPS), Fuerther Str. 246b, 90429 Nuremberg, Germany
*Corresponding author. Tel.: +49-911-5302-9079; fax: +49-911-5302-9070. E-mail address:
[email protected]
Abstract Due to thermomechanical stresses at high temperatures and harsh environments, the interconnections between the substrate and the components in power electronics exhibit failure modes at temperatures >130 °C. This contribution introduces the concept of additive plasma metallization technology for the optimization of interconnections in electronics for reliable usage up temperatures >300 °C. Through the formation of intermetallic phases, diffusion soldered interconnections were realized by melting of solder paste with plasma based copper powder coating. The integration of the Plasmadust® technology into standard production process delivers highly reliable interconnections with better resource management. The interconnections were characterized by building power electronic modules to demonstrate the advantages compared to stateof-the-art technologies. The potential of the Plasmadust® process for power electronic production and further chances in component modification are discussed.
© © 2015 2015 The The Authors. Authors. Published Published by by Elsevier Elsevier B.V. B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the International Scientific Committee of the “4th CIRP Global Web Conference” in the person of the Conference Chair of Dr.the John Ahmet committee Erkoyuncu.of CIRPe 2015 - Understanding the life cycle implications of manufacturing Peer-review under responsibility organizing Keywords: Additive manufacturing; High temperature capability; Intermetallics; Plasmadust; Power electronics
1. Introduction Megatrends such as renewable energy, electro-mobility and healthcare offer enormous business potential and catalyze creative innovations. The optimization and technological modifications are particularly crucial, when the present stateof-the-art solutions do not satisfy the increasing requirements of new materials, applications, and energy efficient production. One of the key and dominant technologies in these mega trends is the power electronics, when it comes to conversion, transmission and use of electrical energy. The progressive trends towards miniaturization despite higher power densities and switching frequencies, better energy efficiency and performance are the drivers in search of newer technologies. In addition to the demanding technical specifications in operation, the reliability and lifetime requirements are also progressively increasing according to the application.
Nomenclature d k k0 Q R t T
thickness of the interlayer [m] diffusion parameter [m2/s] diffusion constant [m2/s] activation energy [kJ] gas constant [8,314 J/mol*K] time of thermal treatment [s] temperature [K]
In Power electronics, previous manufacturing solutions in field of assembly and packaging technologies for compact power modules reach their limits due to high technological requirements. Figure 1 shows the cross-section of a typical power module, where the power semiconductor component is connected to the top of direct bonded copper (DBC) substrate
2212-8271 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of CIRPe 2015 - Understanding the life cycle implications of manufacturing doi:10.1016/j.procir.2015.08.005
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through solder joints and wire bonds at chip-level. The bottom of DBC substrate is soldered to the ground-plate and heat sink for thermal management at system-level. The electrical and thermal circuits in the whole assembly are separated by the ceramic isolation in DBC substrate. Due to low melting tin (Sn) component in the solder layer and mismatch of coefficient of thermal expansion in the consecutive layers, the interconnections at chip-level exhibit failures such as delaminations and/ or solder degradation upon high thermomechanical stresses. This qualifies them as the weakest points in the complete module due to direct contact with chip at high temperatures >150°C during operation. These demand optimization of solder joints for high temperature operation and durability for longer lifetime of the product.
Fig. 2. Cross-sectional images of the standard Sn-based solder [1] and diffusion soldered joints with Cu6Sn5 IMPs[2].
Fig. 3. Production line for power module assembly with process times.
Fig. 1. Cross-section of a typical power electronic module.
The usage of expensive alternate compositions comprising silver (Ag) and gold (Au) instead of Sn result in high module costs and requires modification of production equipment with hydraulic presses. One promising and economical technology is the Transient Liquid Phase Soldering (TLPS), a variant of diffusion soldering technology, which makes use of conventional production equipment and standard solder materials in producing higher remelting interconnections with intermetallic phases (IMPs). 1.1. Diffusion soldering in power electronics production Diffusion soldering combines the features of diffusion brazing and conventional soldering process, where a low melting metal is used as an interlayer in bonding two highly melting metals. [1] The variants such as Transient Liquid Phase Bonding (TLPB) uses components with specific bottom-side metallizations (Sn and/ or Cu) and TLPS uses solder material (usually Sn and Cu based) as interlayer. [2][3] TLPB requires mechanical pressures up to 30 MPa during thermal treatment, which is not compatible with standard production equipment, but TLPS can be realized on any production line flexibly with just fewer modifications in the temperature profiles without any machine related modifications or new investments. The diffusion soldered joints from standard Sn-based soft solders form upon inward diffusion of Cu atoms into liquid Sn forming IMPs. These do not melt unless the assembly is heated up to the melting point of the formed phases i.e., 415 °C for Cu6Sn5 (η phase) and 676 °C for Cu3Sn (ε phase) in Sn-Cu bimetallic system. Depending on the morphology and distribution of the phases in the layer, these IMPs formed through isothermal solidification produce distinct improvements in mechanical and thermal properties.
Figure 2 shows cross-sectional images of the interconnections showing the rest tin in standard solder joint and completely transformed diffusion soldered joint. Figure 3 shows a typical power electronics production process chain. It involves the printing of solder paste on the carrier DBC substrate (Fig 4.i), placement of power electronic components and soldering process for the complete assembly. Specially formulated solder materials (Sn99.xCu1-x) used for the connection of the components to substrates melt at around 235 °C depending on the composition of the solder alloy defining the thermal profiles (X~10 min). The connections produced as discussed previously, remelt during operation due to presence of pure Sn as the module reaches temperatures >190 °C, (Fig 4.ii) making the joint vulnerable to failure due to loss of adhesion. In TLPS process, the unused Sn in the solder joint will be used completely for IMP formation. The diffusion process starts with the formation of IMPs between solder layer and base substrate and these grow in thickness with time through subsequent thermal treatment and aging (Fig 4.iii). [4] For pressure-less TLPS with thicker interlayers >50 μm, the IMP formation needs relatively long thermal handling time (X in hours), increasing the production costs.
Fig. 4. i. Printed solder layer on DBC, ii. Standard soft solder based interconnection, iii. TLPS interconnection with high remelting IMPs.
1.2. Plasmadust®-Technology Plasmadust® is a new thermal spray technology using cold active plasma (with temperatures as low as 100 °C) for additive metallization of different substrate materials. Generally it offers the possibility to generate metal circuits up to 200 μm in thickness with metal powders (particle sizes
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between 200 nm to 20 μm) on the substrates. In this present work, the Plamsadust® technique had been used to treat the solder paste with copper powder to achieve accelerated IMP formation in interconnection during the soldering process reducing the production time for diffusion soldering. The plasmadust system is integrated in closing cell as shown in Fig. 5.a with rotary disc for substrate transport and plasma nozzle for powder spray. The substrates can be handled by the 6-axis-robot under the 2kW plasma generator. Inside the nozzle as shown in Fig. 5.b., the plasma beam is generated as well as the copper particles in the powder are partially melted and fused with the plasma beam. Copper powder can also be deposited precisely without the activation of plasma arc. The velocity of the exiting gas-powder compound depends on the pressure of the gas. Due to the lateral injection of the metal powder the exit velocity changes. In the center of the nozzle the velocity can reach 45 to 70 m/s. The deflection of the powder particles due to the nozzle geometry generates spreading velocities between 20 and 80 m/s. [5] [6]
Fig. 5. a. Technical configuration of the Plasmadust®-system; b. Schematic representation of the Plasmadust®-nozzle.
The coating process is influenced by several parameters, which have each a different effect on the layer thickness, the adhesion and the quality of the generated structures. The investigations as described in [5] show that the distance between nozzle and substrate, the process speed and the powder pressure are key factors for the Plasmadust® process. In this paper, economical production by usage of already available manufacturing equipment together with plasmadust step is demonstrated.
decelerates the IMP formation. [3] As the interlayer height increases, the time required squares. The diffusion based growth velocity of the IMPs follow the following equation (1) [7]: (1) ݀ ൌ ݇ ή ඥݐௌௗ The diffusion parameter k is a material-specific parameter whose temperature dependence can be expressed through the following Arrhenius equation (2) [7]: ݇ሺܶሻ ൌ ݇ ή ሺെ
ொ ோή்
ሻ
(2)
The diffusion distance to be overcome is the solder gap between the substrate and the chip. In case of chips with specific Cu metallization, the diffusion distances drop to half. Given the material parameters k0 and Q, the theoretical process time can therefore be calculated as a function of solder height of pure tin. For a 20 μm TLPS joint, it takes a thermal treatment of approx. 25 min (for X in Fig. 3). [3][8] However it can be inferred that even without specific numerical example is clear at this point, that a homogeneous distribution of copper particles of small size in the solder deposit positively enhances the TLPS process in terms of diffusion distances and thus growth rates with it. The main goal for the integration of Plasmadust® technology was to create thicker TLPS joints by depositing copper powder on the printed solder paste. The copper powder deposited reacts with the Sn from the solder paste forming IMPs and at the same time supporting the IMP formation from substrate side. Two types of solder pastes Type-6 (particle sizes 5–15 μm ≥ 90 %) und Type-7 (particle sizes 2–11 μm ≥ 90 %) were tested to analyze the paste printing compatibility and IMP formation. As test specimens, power electronic assemblies had been constructed by integrating the Plasmadust® process step into the production line as shown in Fig. 6, where the printed solder paste was treated with copper particles. The modules were built with components with standard metallization (Ni-Ag) and special Cu metallization on bare DBC substrates as summarized in Table 1.
2. Plasmadust®-enhanced TLPS process In power electronics, the presence of voids in the solder joint extremely increases the thermomechanical stresses at high temperature operation leading to failure of the module. During the soldering process, the interconnections must be outgassed to reduce the formed voids from the solvent inside the solder material. For standard solder joints above 80 μm, by using the state-of-the-art soldering ovens, the voids can be reduced to
