Online proceedings - EDA Publishing Association

V.Table 2: FE-Simulation test-matrixL1[mm]L2[mm]A zu[mm²]A AG[mm²]GAP(100µm)0 14 56 40 1 vacuum1 14 56 40 1 air3 14 14 40 1 air4 14 56 40 1 vacuum6 14 14 40 1 vacuum7 14 56 40 1 vacuum8 30 56 40 1 vacuum9 14 56 80 1 vacuum10 30 56 80 1 vacuum11 14 56 40 3 vacuum24-26 September improve 2008, the future Rome, interfacial Italy contact. To put the bond asexactly as possible between the position of the lower andupper temperature sensor, the position of the drilling wascopied to the other side.The applied silver powder is a flake powder with mostparticles smaller than 10 µm (see figure 9). For these firsttrials the powder is applied by dispensing and drying toachieve the envisaged final geometry of the bond of 1 mm x1 mm x 0.1 mm. The area of 1 mm² is produced by cutting.To improve the repeatability of the geometry we will aim atan application of the powder paste by stencil printing.So we have to do the experiments and further simulationsin vacuum. In the next step we have to avoid or minimize the(small but existent) flux caused by mechanical stabilization(secondary circuit). This can be done by using high L2values (see V4 and V6) or decreasing the adhesive-area.Increasing the Ag-bond area from 1mm² to 3mm² causes alower temperature gradient but also reduces the thermal fluxthrough the mechanical stabilizer from about 10 % down to2.5 %. At higher Ag-bond areas a mechanical stabilizer mayno longer be necessary. In further investigations one has tofind out the technological parameters for Ag-bonding toavoid mechanical stabilizations. This is described next.Table 3: Results of the FE-SimulationsP mech[W]P air[W]∆T AG[°C]V.P AG[W]0 8,7 0 0 15,51 7,3 0,49 3,4 13,13 7,6 0,91 2,2 13,74 8,3 0,81 0 14,86 8,2 1,01 0 14,67 8,3 0,81 0 14,88 5,8 0,56 0 10,39 10,6 1,26 0 18,910 8,3 0,98 0 14,611 22,9 0,52 0 8,9C. Sample Preparation, Set-up & Test MatrixFirst of all the copper blocks are prepared. Their outerdimensions are 25 mm x 10 mm x 4 mm. The silvermetallisation is deposited galvanically. Afterwards thedrilling for the temperature sensors is performed.After these preparing tasks the substrates are heated underreducing atmosphere to reduce potential oxides and toFig. 9: SEM picture of used silver powder.The maximum bond force of the equipment used is limitedto 50 kg. The two copper blocks with the silver powder inbetween are bonded under pressure and temperature.Standard parameters for silver are 25 MPa to 45 MPa and225 °C to 250 °C. Possible bonding times are from someseconds to minutes [3]. Due to the high pressure the bondingarea is limited to about 20 mm² (for 25 MPa). Theassemblies were done with slightly higher temperature andlower pressure.D. Material AnalysisThe sintered material is analyzed by focused ion beammicroscopy (FIB) (see figure 10). A fine grainmicrostructure has developed and original grains of theparticles are hard to identify. The microstructural grain sizehas been enlarged compared to the original powder size.Many twin boundaries have been formed in themicrostructure.Fig. 10: Analysis of the sintered material by FIBBy the microscopy software “analysis” it was possible toestimate the percentage of voids in the cross section done©EDA Publishing/THERMINIC 2008 116ISBN: 978-2-35500-008-9