Methoden und Technologien zur Optimierung der Entwärmung aktiver und passiver Komponenten auf keramischen Mehrlagensubstraten

Die Entwicklung der Elektronik schreitet stetig voran. Durch die steigende Funktionsdichte einzelner Baugruppen und die damit einhergehende Miniaturisierung gewinnt das thermische Management zunehmend an Bedeutung. Die hierdurch wachsenden Anforderungen an den Schaltungsträger hinsichtlich der thermischen Performance erfordern die Verbesserung von bestehenden und die Entwicklung von neuen Entwärmungskonzepten. Diese Arbeit beschäftigt sich mit der Optimierung der Entwärmung von Komponenten auf keramischen Schaltungsträgern. Die Arbeit umfasst dabei drei Schwerpunkte: Chipmontage, Wärmespreizung und Wärmeableitung. Die thermische Performance von Chipmontageverfahren wie Silbersintern und reaktives Löten wird ermittelt. Neuartige Silberstrukturen werden im Keramiksubstrat integriert und die Wärmespreizfähigkeit dieser simulativ und messtechnisch qualifiziert. Zudem werden Flüssigkeitskühlkonzepte für keramische Schaltungsträger vorgestellt und mittels Simulation und Messung bewertet.This thesis deals with the thermal path in a multilayer module based on low temperature co-fired ceramic (LTCC). The thermal path describes the path of heat flowing from a heat generating component to a heat sink. The thesis focuses on new materials, technologies and methods, which improve the thermal performance of the multilayer module. The thermal path can be subdivided into three critical parts. The first part contains the joining between the chip and the substrate. So called thermal interface materials (TIM) are used as bonding agent to ensure a strong and thermally conducting bond between chip and substrate. The thesis addresses the pressure less silver sintering technology, which provides a high thermally conducting bond between chip and substrate. Moreover reactive soldering is investigated as potential chip and substrate bonding technology. This technology utilizes a reactive multilayer foil, which reacts fast exothermically after a short thermal pulse. The delivered heat can be used to join materials with different coefficient of thermal expansion (CTE), like copper and LTCC or silicon. The thermal resistance, the mechanical strength and reliability of bonds based on reactive solders and silver pastes are characterized and compared to conventional bonding agents like solders and adhesives. The second part addresses the entire ceramic substrate. To enhance the poor thermal conductivity of the ceramics metals in form of cylinders are integrated in the substrate. These so called thermal vias consist mostly of gold or silver based materials and are integrated below the chip bond area. The influence of the via geometry, via material and sinter process on the thermal performance and the hermeticity of the substrate are evaluated within the scope of this thesis. Furthermore new silver foils are integrated in the LTCC substrate during co-firing, to form a LTCC substrate with massive silver structures. Therefore, the LTCC is locally replaced by the silver foil, which forms areas with a very high thermal conductivity inside the substrate. The thermal performance of these silver structures inside LTCC substrates is investigated on the basis of two demonstrators. The third part of the thermal path deals with the heat sink. The possibility to integrated fluidic channels inside the LTCC substrate, which can be utilized for active cooling with a coolant, is discussed within the scope of this thesis. The investigations cover the design and the fabrication of the fluidic channels and the fluidic interfaces. Moreover the thermal performance of these cooling concepts is evaluated

Abstract Influence of substrate thickness on thermal impedance of microelectronic structures q

The thermal impedance Zth(jx) has been calculated numerically, using the boundary element method, for a silicon substrate with a uniform heat source on top. The key feature is that the dynamic thermal behaviour is calculated directly in the frequency domain. The calculations were performed for a wide range of values for the thickness of the substrate. By representing the thermal impedance in a Nyquist plot (i.e. Im[Zth(jx)] vs. Re[Zth(jx)] with x as parameter), mainly two circular arcs are observed. For the lower frequency arc, the impedance values as well as the frequency scale are found to be largely influenced by the substrate thickness. The arc corresponding to high frequencies on the other hand remains unchanged under thickness variations. Further analysis revealed an almost perfectly linear relationship between the thermal resistance Rth = Zth(jx = 0) and the substrate thickness, even when the heat source is not centred on the substrate. Both the slope and intersection value obtained from the curve fitting can be explained by a simple geometrical model including the fixed-angle heat spreading approximation, used since many years in the literature