172 Prakt. Met. Sonderband 46 (2014) 
with P [N] the maximum load at failure, t [mm] the plate thickness, and f a dimensionless 4.2. EFF) 
factor depending on the geometry specimen and Poisson's ratio of the tested material [7]. 
For the case investigated, fwas 1.96 + 0.01, valid for 1.0 <f< 1.1 mm and v=0.2 [5]. Figure 4 
Raman measurements of residual stress were performed with a HR-800 high-resolution stress 
Raman microprobe (Horiba Jobin Yvon, Villeneuve d’Ascq, France). Excitation was cobabilt 
provided with a 514.5 nm Ar-ion laser that was focused on the surface of LTCC specimens 4 served 
with a long-working distance 100x objective lens (NA=0.8). Fluorescence signal from cr follow a | 
impurities of the alumina phase was collected with the same objective and analysed by the with the 
spectrograph. The fluorescent signal was used as residual stress sensor, since the Although 
measured peak shift with respect to an unstressed condition is directly proportional to the Le om 
trace of the stress tensor [10]. In this work, the shift-stress relationship was obtained by ie, to 
applying a known stress state (in 4 point bending) on bar-shaped specimens made of this cann 
LTCC_T material. The values reported here were an average of 50 points measured on t 
the surface of each specimen using the shift of the R1 peak of alumina fluorescence. ed 
flexural x 
4. RESULTS AND DISCUSSION samples . 
9 0, 
4.1. Microstructural analysis and material properties 20% con 
ith 
Figure 3 shows Scanning Electron Micrographs of the microstructure of LTCC_E and boron, 
LTCC_T materials of study. To reveal the internal microstructure cross-sections were the TET 
performed with a Focus lon Beam technique. For the LTCC _E material (Fig. 3a), equiaxed combinati 
alumina grains (dark-grey) are (uniformly) embedded in the glass matrix (light-grey). For and LTC! 
the LTCC T (Fig. 3b), alumina platelets (dark-grey) are oriented in-plane, as a multilayer 
consequence of the fabrication process (i.e. tape casting). the strenc 
bulk mate 
80MPa, 
compress 
also in ag 
Table 1. 
TET_1 an 
LTCC 
Syste! 
um J HM | 
Figure 3. SEM micrographs showing the microstructure of a) LTCC_E and b) LTCC_T ' LTCC 
materials. The internal microstructure was exposed using a Focus lon Beam technique. i 
The coefficient of thermal expansion resulted in aitec g=5.9 ppm/K and LTCC 
artee. 1 = 5.3 ppm/K, measured between 20°C and 400°C. The difference of ca. 0.6 ppm/K 
causes residual stresses, being compressive in the LTCC_T layers and tensile in the TET 
LTCC_E layers. The magnitude of residual stresses for both TET_1 and TET _2 systems 
was calculated with Eq. (1) and is given in Table 1. TET