New Materials for High Temperature Packaging

Transcription

1 Minapad 2014, May 21 22th, Grenoble; France New Materials for High Temperature Packaging Jean-Luc Diot 1, Emilien Feuillet 1&2, Renaud de Langlade 1 and Jean-Francois Silvain 2 1 NOVAPACK sas, 14 rue des Glairaux, Saint Egrève, FRANCE 2 ICMCB-CNRS, 87 avenue du docteur Schweitzer, Pessac Cedex, FRANCE Phone : ; Fax : & jl.diot@novapack.fr Abstract Current trends for microelectronics are showing continuous needs for increased power dissipation and high temperature materials. Taking mainly the examples of power packages and modules, we will propose revisited architectures of these types of packages in particular for wide gap semiconductors market (SiC and GaN), in order to be compatible with high junction temperature (T j > 300 C). The current architecture of power packages and modules manufactured in mass production is the starting point of the discussion and the temperature limitations of current materials are highlighted. In particular, as usage of over-molded thermo-set resins is limited to about 175 C, power package architecture would moved closer to power module architecture (or at least to current metallic packages architecture). Regarding joining technologies, usage of high lead alloys is a temporary solution valid up to about 200 C. TLPB (Transient Liquid Phase Bonding) is an alternative for high temperature joining and preliminary results are presented. Metal matrix composites (MMC) with matched CTE (Coefficient of Thermal Expansion) have been developed for heat-sink. Instead of traditional CuMo or CuW alloys, carbon fibers-metallic matrix composites will be used, as on top of their good thermal conductivity, they have low density and good machining properties, especially when compared to AlSi or Al/SiC. As a conclusion, power packages or modules compatible with a 250 C junction temperature are achievable but higher junction temperatures will need also to review dice interconnections. Key words: power packages, power modules, TLPB, Metal Matrix Composites Introduction Current trends for microelectronics are showing continuous needs for increased power dissipation and high temperature materials. Especially, functional demonstrators for wide gap semiconductors (SiC for power components and GaN for RF power components) with maximum junction temperature of more than 300 C have been already manufactured and theoretically these semiconductors could exhibit T j up to 1000 C! Such working temperatures are not compatible with current power packages and modules manufactured in mass production. Their architecture will be the starting point of the discussion and the temperature limitations of current materials will be highlighted. In particular, as usage of current over-molded thermo-set resins is limited to less than 200 C, power package architecture must be reviewed. We will focus on joining techniques and base materials for thermal heat-spreading. Soft solder need to be replaced for high temperatures applications and two main alternatives are existing: silver sintering and intermetallic joining. Ag sintering is a low temperature process ( 300 C) based either on Ag nano-particles or Ag microparticles with Ag organo-metallic to start low temperature sintering. Intermetallic (IMC) joining is also a low process temperature and is based on the formation of a liquid phase in a first step, enhancing solid-liquid diffusion and at the end of the joining process, only IMC, with melting temperatures higher than process temperature, are remaining. Initial experiments on tin-copper are presented. High temperature applications are also requesting materials with adapted coefficient of thermal expansion (CTE) in order to lower interfacial stresses between materials to achieve proper assembly reliability. In this perspective, copper heat-sink must be replaced. Metal matrix composites (MMC) are promising materials compared to others materials. Metal composites like copper-molybdenum or copper-tungsten that are traditionally used but machining, thermal conductivity and weight are their main drawbacks. AlSi is also a composite material recently developed but its main drawback is poor mechanical properties, especially toughness, when high silicon contents are considered. Al/SiC is obtained by infiltration by molten aluminum of a silicon carbide particles (SiC) rigid preform. Its main drawback is that net shaping

2 must be done during manufacturing due to very poor material machinability (SiC particles). As an alternative, we are developing composites with carbon reinforcement, either carbon fibers or diamond particles. Several techniques are existing to manufacture such MMC and we believe that hot pressing techniques, based on powder metallurgy, are the most suitable to control the interface between metal and reinforcements. Copper-carbon fibers composites has been already developed since years but aluminum/carbon fibers composites are more challenging due to the presence of alumina (Al 2 O 3 ) on aluminum powder. Packages architecture Although some design variations and some improvements have been made, power modules architecture is remaining almost the same till decades (refer to figure 1). In such module configuration, attachments (die and insulator) are usually made with soft solder. RoHS (Restriction of Hazardous Substances, directive 2011/65/EU) has excluded the usage of eutectic SnPb as attachment material but not, for the moment, usage of high lead content solder alloys (such as Pb90Sn10 or Pb95Ag2.5Sn2.5). In such kind of modules, insulation is generally made by DBC (Direct Bonding Copper). DBC is a very flexible substrate, as double sided copper layers can be etched upon customer demand. Due to DBC manufacturing process, copper is fully annealed and from CTE variation Vs copper thickness (ref [8]), we have derivated a low Young modulus for copper in DBC in the range of GPa, equivalent to a soft solder! This is explaining the good temperature cycling usually observed for such power modules. Figure 1: Typical power module architecture (adapted from [2]). For high temperatures application, module structure should remain similar. Dielectric gels compatible with temperature higher than 250 C are already available. Cases are usually molded in thermoplastic resins. PEEK (Poly-Ether-Ether- Ketone) can withstand working temperatures of 250 C and for higher temperatures; they can be replaced by LCP (Liquid Cristal Polymers). DBC on Al 2 O 3 (alumina) is limited for high temperature applications due to the development of a conchoidal (interfacial) fracture between copper and Al 2 O 3, even with some design improvement like introduction of dimples (see for example ref [8]). Silicon nitride (Si 3 N 4 ) has a higher stiffness than alumina, reducing the risk of interfacial fracture (ref [9]); then DBC on Si 3 N 4 is the most promising insulating material for high temperature applications. High volume power packages are usually manufactured with a thermoset resin over-molding a metallic lead-frame, generally made in a high conductivity copper alloy (refer to figure 2). Current thermoset resins are only compatible with temperature in the range of 175 C maximum. Then, for high temperature applications, power package design must be reviewed and we suggest adapting a structure similar to power module with a pre-molded cavity in high temperature thermoplastic like PEEK or LCP (refer to figure 2). Figure 2: power packages (left: current architecture, right: high temperature architecture). Attachment materials Although soft solders present many advantages, this type of attachment material is not suitable for high temperature applications (> 250 C). As soft solders are subjected to thermal fatigue due to the difference of CTE (Coefficient of Thermal Expansion) between the parts of a package stack-up (for example, CTE difference between DBC and heat-sink), on the practical point of view, one generally admits that thermal fatigue is limiting usage temperature of soft solders to T f (where T f is melting temperature of the soft solder, refer to [2]). Even to replace high lead solder alloys (with a melting temperature around 300 C), there are not reliable replacement solutions: Heraeus (ref [4]), among others, has tested more than 100 compositions without success. Nowadays, for high temperature applications, two types of solutions are emerging (refer to figure 3): low temperature silver sintering (discussed during MinaPad 2014 by other authors) and TLPB (Transient Liquid Phase Bonding). Figure 3: Figure of merit of various die attachment techniques (adapted from [8]).

3 TLPB Transient Liquid Phase Bonding consists of having a liquid phase during the first step of the bonding process, coming from low melting temperature interlayers, such as for example tin (Sn), indium (In) or eutectic InSn. Under low pressures (less than 5 MPa), formation of intermetallic (IMC) is enhanced by liquid-solid inter-diffusion. At last, growth of the most stable phase occurs in solid state. In a first step, we focused on very common metals used in microelectronics: copper and tin. On Cu-Sn phase diagram (refer to figure 4), we can illustrate the expected process: formation and growth of Cu 6 Sn 5 in liquid phase, then complete transformation in stable Cu 3 Sn phase which has a melting temperature of 676 C. In order to characterize these joints, shear test experiments have been conducted. Due to the expected high shear values, double notched shear test has been performed. Test conditions have been optimized through F.E.A. (Finite Element Analysis) in order to develop only shear stresses during tests (ref [10]. Experimental results are shown on figure 6. TLBP based on Cu-Sn system is giving shear values higher than 100 MPa: plastic deformation of copper substrates is occurring before the failure of the joint! Figure 6: Results of double notch shear test of various joints Figure 4: Cu-Sn phase diagram For this preliminary work, 1 µm of Sn has been sputtered on copper substrates. Experimental joining trials have been performed under low pressure (3 MPa), under vacuum atmosphere with induction heating (allowing high heating rates). Typical results are represented on figure 5: joining only occurs when Sn layers have been melted. In a first step, corresponding to 240 C peak temperature, 2 phases are identified: AES (Auger Electron Spectroscopy) confirms that centre of the joint is Cu 6 Sn 5 and external layers are Cu 3 Sn. After a 10 minutes annealing at 260 C, only one phase is visible, identified by AES as Cu 3 Sn: Cu 6 Sn 5 IMC have been entirely transformed into this more stable intermetallic. Metal matrix composites For high temperature applications, management of CTE differences between 2 materials to be assembled will be a key issue to obtain reliable packages or modules. In this perspective, the main drawback of copper heat-sink is its high CTE of 16.9 ppm/ C. Then, it will be necessary to adapt the CTE of heat-sink material, keeping of course good thermal performance. As an example, figure 7 presents a FEA on the effect of the CTE of heat-sink for a power module assembled with TLPB. Figure 7: FEA simulation of the maximum shear stress induced during TLPB joining at 260 C in the module stack-up Vs CTE of heat-sink Figure 5: Microstructure of experimental joints (SEM) Metal matrix composites (MMC) based on chopped carbon fibers are promising materials for this application. Carbon fibers have anisotropic properties with a CTE of -1 ppm/ C along their axis. CTE of the composite is tailored by the amount of carbon fibers and by their orientation; as it has been shown that during hot pressing process C fibers tend to be aligned perpendicularly to pressing direction.

4 Due to native alumina on Al powder, sintering will not occur during hot pressing process. To enhance the reactivity, AlSi11.3% powder alloy is added to the mixture in order to create a liquid phase during the densification process (refer to figure 8): process temperature is then between 585 C (melting temperature of AlSi11.3%) and 660 C (melting temperature of aluminum). Figure 8: Al-Si phase diagram In a first step, we have established that the influence of AlSi on the metal thermal conductivity is almost negligible and that a 5% percentage of AlSi is corresponding to densification optimum. Then, we have studied the influence of C fiber amount on the two main usage properties (refer to figure 9): vertical thermal conductivity (perpendicular to hot pressing direction) and inplane CTE (parallel to hot pressing direction). Figure 9: influence of amount of C fiber on (a) inplane CTE & (b) vertical thermal conductivity These materials properties are comparable with composites obtained by other routes and these variations can be explained theoretically (refer to ref [5]). Composite microstructure has been studied: TEM (Transmission Electron Microscopy) allows studying C fiber-aluminum interface (figure 10). Needle shape aluminum carbide (Al 4 C 3 ) is observed at the Al-C interface. These carbides contribute to effective mechanical and thermal transfers between metal matrix and carbon fiber. In particular, larger amount of Al 4 C 3 at tip end of C fiber (ref B on figure 10) will enhance mechanical interlocking effect. Those carbides can also explain the good correlation between experimental data and theoretical modeling (ref [5]). Figure 10: TEM micrograph of Al/C interface (A: side of C fiber & B: tip end of C fiber). Conclusion & perspectives High temperature electronics request to revisit packaging architecture, materials and joining technologies. Due to limitations of thermoset resins, packages structure is expected to move to premolded cavity packages. As well, for temperature higher than 250 C, dice front side interconnection through bond wires will no more be reliable and new technologies are emerging (e.g. [3], showing some developments made by Semikron, based on flex). Two types of joining technologies are emerging for high temperatures applications: silver sintering and TLPB (Transient Liquid Phase Bonding). We have presented preliminary fundamental work on TLPB on Cu-Sn system. TLPB allows obtaining, at low process temperature, thin joints without porosity, with high thermal stability and with high shear strengths. In a second step, we will concentrate on potential drawbacks of TLPB, such as low joint thickness and high stiffness of IMC, in view of application perspectives. High temperature electronics will also request materials with controlled CTE to reduce stresses in joints during device operation. MMC (Metal Matrix Composites) are good candidates as CTE can be adapted through the amount of reinforcement. MMC have also good mechanical properties and they can easily be machined. Initial developments of C fiberaluminum MMC have been presented and interest of power metallurgy route to control interfacial properties has been highlighted. More complete applicative properties of C fiber-aluminum MMC will be studied during the next steps of this development. Acknowledgements The authors would like to thank their partners in SatPack project (Euripides 2 EUR project), DGCIS and Euripides 2 organization for their constant support. References [1] D. Planson, SiC : State-of-the-Art, Components, Performances, Technologies and Medium Term Perspectives, GREPES, Brussels, March 19 th, 2009 [2] B. Allard & C. Buttay, Packaging Constraints and Solutions for High Temperature Power Electronic, Electronique Mag, March/April 2010

5 [3] P. Beckedahl & al, Performances Comparison of Traditional Packaging Technologies to a Novel Bong Wireless Sintered Power Module, PCIM 2011, May 17-19, 2011 [4] A. Grusd, Lead Free Solders in Microelectronics, Heraeus literature, 2009 [5] JF Silvain & al, Aluminum-Carbon Composites Fabricated by Semi-Liquid Route: an Overview, IMAPS La Rochelle, February 5-6, 2014 [6] R. Khazaka & al, a Review of Nano-Silver Interconnection, IMAPS packaging workshop, Tours, November 2013 [7] W. Martin & al, Thermal Resistance and Temperature Cycling Endurance of DBC Substrates, Hybrid Circuits, n 22, May 1990 [8] U. Voeller et al, Silicon Nitride Substrates for Power Electronics, IMAPS La Rochelle, February 6-7, 2013 [9] K. Guth & al, New Assembly and Interconnects beyond Sintering, PCIM 2010, May 4-6, 2010 [10] E. Feuillet & al, Cu-Sn Inter-Diffusion Bonding, IMAPS La Rochelle, February 5-6, 2014