3.1 Formation of mullite-cordierite composites granules by spray drying process
The sintering process is used to produce mullite-cordierite composites with high mechanical strengths, low TECs, and dense structures. In the sintering process, several variables, such as the particle size distribution and morphology of the powders, sintering temperature, and sintering aids, affect the properties of the sintered body. The particle size distribution of the powders greatly influences the reaction rate as well as the sintering behavior. For example, in coarse powders having a low initial density due to non-uniform size and high agglomeration, the particles are separated by a large distance, resulting in a low densification rate. Hence, it is necessary to control the particle size distribution of the starting powders.
The particle size distributions of the starting powders before and after attrition milling are shown in Fig. 2. As shown in Fig. 2(a), for the mullite powder, the peck of the particle size distribution curve (which has a unimodal distribution) moved toward the smaller particle size, and the D50 value decreased from 3.96 to 1.75 µm after attrition milling. As shown in Fig. 2(b), the particle size distribution of the cordierite powder shifted from a bimodal distribution to a unimodal distribution, and the D50 value decreased from 4.23 to 3.23 µm after attrition milling. The SEM images in Fig. 2 indicate that the initial powders with various particle sizes became homogeneous powders with uniform and small particle sizes following the milling process. It was expected that the starting powders with sizes over 10 µm would be crushed during the milling process.
Figure 3 shows the microstructure of the granules formed by spray drying the mixed powders of mullite and cordierite before and after attrition milling. It shows the dependence of the particle size distribution of the mullite and cordierite powders on the formation of granules. Granules formed using the initial starting powders before attrition milling were found to have different morphologies (Fig. 3(a)). In contrast, the granules formed using starting powders with homogeneous size distribution obtained through the attrition milling process were found to be spherical (Fig. 3(b)). Moreover, the D50 value of the unmilled mixed powders (147.3 µm) was larger than that of the attrition milled mixed powders (88.31 µm). The span defined as (D90 – D10) / D50, indicates the width of the particle size distribution. The unmilled mixed powders had a larger span. The span before the attrition milling was 1.22 and span after the attrition milling was 0.71. These results show that the particle size and the size distribution of the initial powder affects the morphology of granular powders obtained by spray drying. In a previous work, it was shown that the granules obtained from finer initial powders and powders with a narrow size distribution formed smoother and denser structures, when compared to the large initial powders. Furthermore, the spherical granules of the powders have good flowability, which is an important factor in the fabrication of dense and mechanically strong ceramics [21]. Therefore, the spherical granules obtained by spray drying the fine powders after attrition milling are suitable for fabricating the mullite-cordierite composite pellets and substrates.
3.2 Characterization of the sintered mullite-cordierite composite pellets produced by varying the cordierite content
It is important to determine the optimal composition ratio and sintering temperature to improve the mechanical and physical properties of the sintered mullite-cordierite composite pellets before fabricating a large-area composite ceramic substrate. First, the effect of the composition ratio on the sintered mullite-cordierite composite pellets was characterized. Figure 4 shows the SEM micrographs of the fractured surfaces of the mullite-cordierite composite pellets with various cordierite weight ratios (0, 10, 30, 50, 70, and 100 wt%) sintered at 1300℃. A porous structure was observed in the sample with 100 wt% cordierite, while the sample with 100 wt% mullite exhibited a more compact structure. For samples of pure cordierite and mullite, the density (cordierite = 2.55 g/cm3 and mullite = 3.02 g/cm3, respectively) was measured to be similar to the average theoretical density (cordierite = 2.53 g/cm3 and mullite = 3.17 g/cm3, respectively) [22], which is attributed to the structure of the samples, as shown in Fig. 4(a) and 4(f). As the mullite content in the composite pellets increased, the density noticeably increased as shown in Fig. 4(c)-(e), compared to Fig. 4(b). Thus, increasing the content of mullite not only accelerates densification during the sintering process, [23] but also increases the overall density of the samples as the density of mullite is higher than that of cordierite. Furthermore, a previous study reported that in batches with > 90 wt% cordierite, the cracks formed by cristobalite transition from β to α weakened the structure of the sample [22].
Figure 5 shows the effect of the cordierite content on the TEC and the flexural strength of the sintered composite pellets. Generally, to improve the thermal shock resistance, the ceramic substrate for probe cards requires a low TEC similar to Si. As shown in Fig. 5(a), the TEC of the samples decreased with increasing cordierite content. The sample containing 70 wt% cordierite was observed to have the lowest TEC (2.88 ppm/℃) among all the samples (except the pure cordierite pellet) and exhibited excellent thermal shock resistance. However, compared to the TEC of Si (3.2—3.9 ppm/℃) [24], it can be assumed that all composite pellets with 30–70 wt% cordierite have good thermal stability. As shown in Fig. 5(b), the result is evidently observed that the flexural strength of the samples containing up to 30 wt% cordierite is higher than that of the samples containing over 50 wt% cordierite. This is because mullite has higher mechanical strength than cordierite. Thus, the flexural strength of samples with a large amount of mullite is higher. In addition, the figure shows the correlation between density and flexural strength. The flexural strength increases with increasing density as shown in Fig. 4 and Fig. 5(b). Therefore, it can be concluded that the composition ratio of 70 wt% mullite and 30 wt% cordierite is optimum for forming mullite-cordierite composites due to its low TEC and high flexural strength.
3.3. Characterization of mullite-cordierite composite pellets sintered at different temperature
Figure 6 shows the TEC and the flexural strength of sintered mullite-cordierite composite pellets of 70:30 wt% for different sintering temperatures ranging from 1300℃ to 1450℃. As shown in Fig. 6(a), the TEC of the samples sintered at 1300℃ and 1350℃ was 3.45 ppm/℃ and 3.42 ppm/℃, respectively, which are similar to the TEC of Si. A significant increase in TEC can be observed for the samples sintered at 1400℃ and 1450℃. This is probably due to the thermal stress induced by the mismatch of the TEC between the glassy phase and the crystalline phases of mullite [25]. This result explains why the sample sintered at 1350℃ is better able to withstand thermal stresses, compared to others samples prepared at different sintering temperatures. The physical and the mechanical properties, such as the density, porosity, flexural strength, and TEC, of the mullite-cordierite composites (70 wt% mullite-30 wt% cordierite) for different sintering temperatures are listed in Table 1. The density at 1350℃ is higher than that at 1300℃. As the sintering temperature increases, some cordierite changes into glassy phase, which fills into the porosity of the mullite-cordierite composites [4, 22]. Over 1400℃, however, all the cordierite melts and the sample consists of only the glassy phase, and the mullite content leads to decrease in density, leading to the large size of pores and the formation of macro cracks [25]. There is a high correlation between the density and the flexural strength, with higher density leading to higher flexural strength. Figure 6(b) can be explained using the density values listed in Table 1. The mullite-cordierite composite pellet sintered at 1350℃ exhibits the most compact structure, which improves its mechanical properties. It can be confirmed that the flexural strength is the highest at 1350℃ (264 MPa). The composite pellets sintered at other temperatures except 1350℃, by contrast, show low flexural strength due to their relatively low density.
Table 1
Bulk density, porosity, flexural strength, and thermal expansion coefficient (TEC) of the mullite-cordierite composites at different sintering temperatures.
3.4. The reliability and durability of the large-area mullite-cordierite composite substrate
Based on the above results, a large-area mullite-cordierite composite substrate was fabricated by sintering at 1350℃ using granules having a composition ratio of 70 wt% mullite-30 wt% cordierite. The large-area ceramic composite substrate is 320 mm in diameter. The thickness of the substrate is approximately 5.7 µm, and the relative standard deviation of the thickness is 0.07%, which means that it has a highly uniform thickness as shown in Fig. 7(a). To form the fine pitch of the probe card, 30000 micro-holes were processed in the large-area substrate through ultrasonic drilling. The size and position deviation of the holes were measured to investigate the shrink rate of the large-area substrate that occurred during the sintering process, as shown in Fig. 7(b) and 7(c). The SEM images in Fig. 7(b) show the cross sections of the ceramic substrate with the processed micro-holes. The measured average sizes of the top, middle and bottom section of the processed holes were 416.3 µm, 400.1 µm and 387.8 µm, respectively, having a deviation of < 5%. Figure 7(c) shows enlarged SEM images of the four sections marked #1 to #4 in Fig. 7(a). The distance between the holes was measured in four places in each of the SEM images, and the standard deviation of the distance was found to be < 3%. These results reveal that the processed holes of the large-area ceramic substrate are formed evenly and that the shrinking of the substrate hardly occurred during the sintering process.
In order to evaluate the reliability and the durability of the large-area ceramic composite substrate, the substrate was cut into specimens with dimensions of 80 mm \(\times\) 80 mm and subjected to environmental tests, such as the HTS test, DH test, and TS test. Figure 8 shows the changes in the sheet resistance of the specimens during the environmental tests. In the HTS test, the specimens were placed in an oven at 200℃ for more than 100 h. There is no significant difference between the changes in the sheet resistance of the specimens before and after the HTS test. It is clear that the ceramic substrate is stable even when exposed to high temperatures for a long time. In the DH test, the specimens were exposed to a relative humidity of 85% at 85℃. The rate of change in resistance was nearly constant. It can be seen that the absorption of moisture hardly occurred due to the high temperature and the high humidity. Compared to the other environmental tests, a larger change was observed in the sheet resistance of the specimen when it was subjected to TS test of over 100 cycles in a temperature range from − 40℃ to 85℃. The change in sheet resistance was caused by the impact of the thermal shock. However, the rate of change in the resistance was not large, which means that ceramic substrate was not significantly affected by sudden temperature changes. In Kiattisaksophon et al. [4], a mullite-cordierite composite of 30:70 wt% was subjected to the TS test by heating the sample to 1100℃ and cooling to room temperature. No damage was observed on the surface of the composite body until the 8th cycle, but the spot of micro-cracks was observed in the 9th cycle. Compared to previous studies [4, 9, 14, 24, 26], the TS test was performed in a harsher environment at low temperature, although the applied temperature range is slightly different. In all the environmental test results, the fabricated specimens exhibited a low resistivity change < 10%, and no micro-cracks on the surface of the sample were observed even after 100 cycles. These results prove that the ceramic substrate has excellent reliability and durability.
The influence of the environmental tests on the flexural strength of the ceramic substrate is shown in Table 2. Specimens with dimensions of 3 mm \(\times\) 4 mm \(\times\) 36 mm were prepared for the measurement of flexural strength, and the result was computed as the average of ten measurements of the specimens. On comparing the flexural strength of the specimens before and after each environmental test, the flexural strength of all the specimens was observed to decrease slightly after the environmental tests. It can be assumed that the thermal stress can generate an impurity, which affects the physical properties [26]. In the research of Cheng et al. [27], it was reported that the flexural strength of the specimen with the best thermal property was about 90 MPa and the loss rate of flexural strength was 13.12%, after 30 thermal shock cycles. Compared to the composites reported in previous studies [27, 28], the fabricated large-area mullite-cordierite composite substrate has excellent mechanical strength. This is because it has a high flexural strength of 200 MPa or more and the rate of change in the flexural strength is less than 10%, after 100 cycles of all environmental tests.
Table 2
Variation of the flexural strength with the degradation time under high temperature storage test (200℃), damp heat test (85℃/85%), and thermal shock test (between -40 and 85℃).
Consequently, the large-area substrates fabricated using spherical 70 wt% mullite-30 wt% cordierite granules with homogeneous particle size distribution were not considerably affected after environmental tests to accelerate fatigue failure. These results prove the high reliability and durability of the fabricated composite.