Compact power electronic modules realized by PCB embedding technology

Power electronics packaging with a high level of compactness, robustness and versatility has recently become the focus of many technology development projects. In Europe main drivers for such efforts are e-mobility and "green" energy harvesting, which are promoted by European Commission and national authorities. One promising approach to realize a substantial level of size reduction and process flexibility in the packaging of power electronic systems is the embedding into the build-up layers of printed circuit boards. Conventional printed circuit board embedding -for low power applications- has meanwhile reached a considerable level of maturity and the number of products shipped per month are in the tens of millions. Using embedding technology for power electronic devices, however, is still challenging in many aspects. First of all for power applications the embedded system layout has to account for high currents and/or voltages and at the same time has to provide means to facilitate the power dissipation from the embedded components. PCB-substrates and modules therefore contain conductor traces with large cross sections and/or massive copper and ceramic structures in order to enable the required heat spreading. Such constructions are composed of materials with different CTEs. As a result considerable stresses prevails in the build-up layers. Since contrary to conventional PCBs the build-up of power modules are non-symmetrical a careful layout of embedded modules is necessary in order to avoid warpage of the package. Embedded power electronic components are semiconductor dies on the basis of silicon, silicon carbide and gallium nitride. The full area silver on the drain contact is compliant with the embedding approach. The gate and source contacts have to be reinforced by ten to fifteen micrometers copper. The required robustness and conductance of the electrical contact between chip and embedded substrate is achieved by back contact sintering using silver nanoparticles. Chips are mounted in a two-step process: they are placed in a high accuracy die bonder and pre-bonded, subsequently the contacts are sintered in a stack-lamination press at higher temperature and pressures. Front contacts between chips and the subsequent wiring layer are established by micro-via technology. During the embedding process power components are enclosed into a matrix of glass fabric and epoxy resin (prepreg). The choice of materials strongly depends on the foreseen use case. Subsequent definition of the conductor lines is an established printed circuit board process. A large variety of embedded power electronic modules has been realized so far. Size and performance of the systems differ accordingly: from modules with lateral dimensions of a few square millimeters containing two embedded components for low voltage and currents of up to 10 Amperes, to complex assemblies with 12 embedded semiconductors and a module area of several square decimeters for an operating Voltage of 600 V and a total power of up to 50 kW. The present paper will focus on recent developments in power electronic embedding using PCB technologies.