The combination of aluminum (Al) and silver (Ag) for wire bonding is long established: aluminum (Al) heavy wire bonding on silver (Ag) metallization, such as pastes consisting of Ag as main component or comprising Ag alloys, was first established in the 1970s and was widely discussed (particularly in terms of reliability under high temperature and relative humidity) until the 1990s. Recently, discussion of the Ag/Al material system has reemerged in the literature, albeit within the context of a new bonding approach – thermosonic (TS) ball/wedge (B/W) bonding with Ag or Ag alloy wires, instead of gold (Au) or copper (Cu) wires, on Al-metallized chips. Several forms of Ag wire are currently being evaluated, including pure Ag, Ag alloys and a preliminary version of a palladium (Pd) -coated Ag. The present paper bridges the gap in the literature between the two bonding approaches by reviewing reliability studies for both Ag-on-Al and Al-on-Ag bonding.

The reliability challenges of inverter modules with insulated-gate bipolar transistors (IGBTs) and diodes silver sintered to DBC substrates and copper wire bonded were investigated in a project called "ProPower". It was one aim to find out possible failures when the modules are subjected to different kinds of power cycling. Coupled finite element (FE-) analyses are reported to understand the thermo-mechanical stresses that cause these failures. Initial parametric FE-studies on passive thermal cycles revealed that missing decoupling between the mismatched stacked materials, which previously was guaranteed by the soft solder, leads to coupling of thermo-mechanical stresses not only dependent on stack geometry but also on mounting conditions. Hence, a system view on the stress situation is required and was realized in FEA by including the full assembly with heatsink mounting, bonding wires, by including intrinsic stress due to processing and by different types of active loading, and in particular also of combined temperature and power cycling. Different parametric variations were studied, e.g. the effect of overmolding. Power cycling induced failure modes from testing are related to critical stresses and proposals for "physics of failure" based lifetime modeling are made.

Thermal management and especially cooling attracts more and more attention during phases of conception and fabrication of power modules to increase their field operation time and/or power density. Indeed thermal power losses impact the reliability of power systems, which are in a same time converging towards always higher power density and performances. Since standard solder die attach and wire bonding are reaching their limits, a novel concept of assembly has been realized with double sided cooling (DSC), as fully integrated part of the power module. Half-bridge modules either based on IGBTs and silicon diodes or based on SiC- devices have been assembled by combining Transient Liquid Phase Bonding (TLPB) and Transient Liquid Phase Soldering (TLPS). SiC and silicon wafers were prepared by deposition of a high density area array of Cu posts. In order to switch 1200V / 25A copper posts were electroplated 80

The three different die attach technologies soldering, silver sintering and transient liquid phase bonding (TLPB) were compared conducting active power cycling tests. The active power cycling method used was designed such that the temperature swing of the semiconductor die was held constant over the whole testing time. The TLPB interconnects consisting of the intermetallic phases Cu6Sn5 and Cu3Sn showed the highest life time followed by Ag sinter joints made applying a sintering pressure of 30 MPa showing almost the same life time. Even the pressure less sintered samples showed a much higher life time than the soldered interconnects made of SnAg3.5 solder. While the soldered samples failed due to solder fatigue, the silver sintered and TLPB samples showed failures like delaminations and cracks in the substrate rather than in the interconnects themselves.

A new lifetime model for aluminum wire bonds, which is capable of describing the physical influence of material properties, thermal loading condition and geometry, has been used to investigate the reliability of modified wire bonds. The model is based on finite element simulation and the description of crack propagation in the region near the Al-Si interface (wedge crack) through a modified Paris law. To improve the lifetime of wire bonds, the mechanical stiffness of the wedge can be reduced through geometric alteration; this can be done e.g. through laser cutting. The impact of the modified geometry on crack propagation rate and thus on wire bond lifetime is calculated using the new modelling technique.

This Paper presents reliability results of aluminum heavy wire bonds during power cycling on MOSFET modules. In order to investigate the effect of the wire material on the reliability different wire alloys like Al-H11, Al-CR, Al-R and AlX were used. Furthermore, the influence of the wire diameter on the lifetime of the power modules was also tested using 125 micrometer, 300 micrometer and 400 micrometer Al-H11 wire diameters. The bonding parameters were optimized for all samples so that a shear code 3 (area of the shear residues between 50 % and 75 %) and shear code 4 (area of the shear residues higher than 75 %) could be set. For lifetime testing active power cycling with different temperature swings (DeltaT = 75 K, 85 K, 95 K and 105 K) were performed. The samples were characterized by shear testing to evaluate the quality of the bonding zone in the initial state and after different intervals. The normalized shear force was plotted as a function of the cycle number. It was shown that the wire diameter has a significant influence on the reliability compared to the standard Al heavy wire material. Only the AlX wires show a distinct increase in lifetime compared to Al-H11, Al-R and Al-CR. A significant higher life time could be achieved using smaller wire dimeters. Microstructure analyzes using electron backscatter diffraction (EBSD) show a district rotated cube (RC) texture with same grain sizes slightly above the chip metallization for all 3 pure Al wire materials.

LED luminaires are already beyond retrofit systems, which are limited in heat dissipation due to the old fitting standards. Actual LED luminaries are based on new LED packages and modules. Heat dissipation through the first and second level interconnect is a key issue for a successful LED package. Therefore the impact of known bonding technologies as gluing and soldering and new technologies like sintering and transient liquid phase soldering were analyzed and compared. A realized hermetic high power LED package will be shown as example. The used new techniques result in a module extremely stable against further assembly processes and harsh operating conditions.

Ag sintering is an emerging interconnection technology, especially in the field of power electronics, where for some applications it is demanded to operate at temperatures higher than 150°C. At these temperatures soft solder joints are reaching their limits. In this case Ag sintered joints can serve as a substitute for solder joints. The aim of this study is to get a better understanding of the failure behaviour of Ag sintered joints during power cycling under harsh temperature conditions and to study which analyses methods are qualified to detect the failures. Therefore, Si IGBTs were mounted on DCB substrates by pressure and pressure-less sintering and were subjected to power cycling tests under temperature conditions from + 30°C up to +180°C. The samples were analysed initially and after the cycling using IR-thermography, ultrasonic microscopy and X-ray microscopy. Furthermore, cross-sections were made and analysed using light microscopy and scanning electron microscopy. Generally, the results show, that depending on the pressure applied and the initially printed Ag paste layer thickness, Ag sintered joints have a 10 to 100 times higher life time than SnAg3.5 solder joints. By replacing Sn-based solder by an Ag sintered joint the first level interconnect is no longer the bottle neck of the assembly since failures no longer emerge in the first level interconnect solely.

A new highly reliable die-attach technology for high operating temperature is called Transient Liquid Phase Bonding (TLPB). The TLPB is based on phase transformation into intermetallic phases using a low melting solder layer and a high melting bonding surface. Due to this fact the re-melting temperature of such a transformed joint increased up to the decomposition temperature of the intermetallic phase, which might be more the 200 K of the used soldering temperature. In active power cycling analysis between +30 deg C and 180 deg C (DeltaT = 150 K) the TLPB showed more than 100-times longer lifetime compared to eutectic SnAg-solder. Furthermore, other failure mechanisms were found compared to Sn-based solder joints such as fatigue cracks in the copper layers of the DCB-substrate and the chip interface as well as delamination between the Al2O3-layer and the bottom Cu-layer of the DCB.

New demands on the thermo-mechanical design of sintered silver interconnections emerge. Development of this inter-connection technology and both experimental and theoretical studies on their reliability were subjects of the project “PROPOWER”. The focus of this paper is on theoretical analysis of thermo-mechanical reliability risks of a project demonstrator, an insulated-gate bipolar transistor (IGBT) module, subjected to power cycling loadings. Coupled electro-thermal-mechanical analyses have been carried out using the finite element method (FEM). Introduction of a new interconnect material means at the same time introduction of a new constitutive behavior and new failure modes. As the material stiffness increases, the decoupling effect of compliant solder layers reduces and intrinsic mechanical stresses increase in the whole power stack. This leads on one hand to less low cycle fatigue in the interconnect, as plastic dissipation is reduced, but on the other hand to higher failure risks like brittle cracking and sub-critical crack growth. However, if early brittle failure can be avoided by appropriate designs, the new interconnection technology allows an increase in fatigue reliability of several hundred percent. Based on the complex theoretical framework simulation results are validated by testing in order to achieve trustworthy thermo-mechanical reliability predictions. Failures like chip metallization damage and the different damage mechanisms of the die bond if either solder or sinter silver is used are related to the different stress situations in the module.

This Paper reports an emerging lead-free joining technology for high temperature application, which can be used for operating temperatures above 200 °C. It is called: “Transient Liquid Phase Soldering (TLPS)”. The TLPS paste used contains a tin-copper powder mixture and is almost completely transformed into Cu6Sn5 and Cu3Sn intermetallic phases after soldering. Due to the reaction between the liquid tin and the copper powder a skeleton of intermetallic phases are formed immediately during soldering and prevents the paste from collapsing so that a lot of voids remain in the solder line. The challenge for this investigation was to understand the mechanism of the skeleton formation, describe them in detail and find possibilities to avoid the skeleton formation. In this paper a new TLPS paste and two processes are described as a means to manufacture an almost void-less joint. Furthermore, a model was developed that describes the TLPS process in detail. The activation of the TLPS joint is crucial and will be described. Temperature cycling results, failure mechanisms and conclusions to increase the lifetime as well as reliability of such TLPS interconnects will be presented in this paper.

The study focuses on a new variant of transient liquid phase soldering (TLPS) using tin based solder with copper powder. This technology may act as an alternative for lead free joining of semiconductor dies in power electronic applications at high operating temperature. Lead-free joining technologies currently used like gold-rich solders and silver sintering are well suited for high temperature applications. However, due to the high metal price they have a limited acceptance. Using a special soldering process it is feasible to produce an almost void-less solder joint, using a paste of tin-based solder powder (e.g. SAC305), copper powder and a solvent which is hardly activated. The resulting interconnection is characterized by an almost complete transformation into intermetallic phases of Cu6Sn5 and Cu3Sn. Thus the melting point of the transformed interconnect can be increased up to the decomposition temperature of the Cu6Sn5 intermetallic phase which is 415 °C. A two-step soldering process allows to eliminating the typical skeleton structure that forms as a result of the immediate reaction of the liquid tin-based solder with the higher melted copper powder to form the Cu6Sn5 and Cu3Sn intermetallic phases. An alternative way compared to the two-step-process is also explained in this study: Capillary forces let the solder flow into the gap filled with Cu spheres.