Yasutoshi Kurihara

Ag-Pd thick film conductor for AlN ceramics

An Ag-Pd thick-film conductor for aluminum nitride (AlN) ceramics was developed. The conductor showed adhesion strength and reliability almost comparable to those of commercial conductors for alumina ceramics. The influence of physical properties of the lead oxide glass frit on conductor performance was studied, and the most suitable glass was selected for the frit material. The softening point of frit glass has the largest effect on conductor performance; this temperature should be approximately 500 degrees C. If the softening point was either too high or too low, compared to the sintering temperature of the conducting metals, poor adhesion strength resulted. Use of glass with too high a softening point caused poor wettability but resistance to erosion by Pb-Sn solder, while it resulted in good wettability but poor erosion resistance when the softening point was too low. Additionally, Cr-, Ti, and Zr-oxides were identified as wettability-promoting agents. The molten glass did not penetrate into the AlN ceramics; rather, it remained on the surface. This behavior differed from that in the case of alumina ceramics, for which the molten glass penetrated along the grain boundaries. >

The influence of moisture on surface properties and insulation characteristics of AlN substrates

The influence of moisture on the surface properties and insulation characteristics of sintered aluminum nitride (AlN) substrates was investigated. Two AlN substrates, one nontreated and the other with an oxidizing heat treatment, were exposed to moisture under several sets of conditions. For the nontreated substrate, changes in weight and surface properties were not observed, and neither was degradation of insulation characteristics under high temperature and humidity test conditions (80 degrees C, 90% RH). Under more severe conditions of a pressure cooker test (121 degrees C, 2 atm, water vapor), the surface morphology changed and the insulation characteristics were degraded. These changes are attributed to formation of a new surface layer on the substrate. Verification of this hypothesis was undertaken by carrying out an immersion test in warm water (80 degrees C). Clear changes in surface morphology were seen. Meanwhile, the oxidized substrates were found to experience no changes in weight or surface morphology under the pressure cooker test. It is concluded that the thermal oxidation process is an effective way to protect the AlN substrates against moisture and the subsequent degradation of the insulation characteristics. >

Pb - Sn Solder for Die Bonding of Silicon Chips

The thermal fatigue life of silicon devices die bonded with lead-tin (Pb-Sn) alloys of 14 different compositions, from Pb-3%Sn to Pb-95% Su was investigated. A silver-plated silicon chip and a nickelplated copper substrate were soldered together using a piece of thin foil (100µmt) solder. Soldered silicon devices were exposed to thermal cycling tests of - 55 to 150°C (1 cycle/h). The thermal fatigue life of a device was defined as the number of thermal cycles when the heat resistance of the device reached 1.5 times its original value. The maximum thermal fatigue life was observed for Pb-50%Sn and was about 9 times that for Pb5%Sn. Solder grain growth during the thermal cycling was also observed, and scanning electron microscope (SEM) measurements showed that cracks in the solder propagated selectively in the at (Pb-rich) region.

Bonding mechanism between aluminum nitride substrate and Ag-Cu-Ti solder

The bonding mechanism between aluminum nitride (AlN) substrate and silver-copper-titanium solder was studied. The AlN surface was bonded to the solder with the TiN intermediate layer containing free Ti and Al, produced at the interface between the AlN substrate and the solder. There were garnet phases consisting of aluminum oxide and yttrium oxide partially existed on the AlN substrate surface and these were also bonded to the solder. It was observed that the interface between the garnet phases and the intermediate layer had the highest bonding strength of all the regions. The reasons for variations in bonding strength of a copper plate to AlN substrate were also investigated, focusing on the differences in the preparation procedures of the AlN surface. The adhesion strengths of the copper plate were higher for samples using as-fired AlN substrates than those of lapped ones. This was attributed to the existence of a large amount of the garnet phases and no mechanical damage on the as-fired AlN surface, while the lapped AlN surface had a small amount of the garnet phases and was damaged mechanically. >

Thick film resistors for AlN ceramics

RuO/sub 2/ thick-film resistors for aluminum nitride (AlN) ceramics with controlled temperature coefficient of the resistor (TCR) and reliabilities have been developed. The resistors, in the range of approximately 10 Omega / Square Operator to 10 k Omega / Square Operator were obtained using a crystallized glass powder and a RuO/sub 2/ powder, binder materials, and a conducting material. The glass had a low PbO content of 6 wt %, a low thermal expansion coefficient of 3.6*10/sup -6// degrees C, and a softening point of 610 degrees C. The resistors, to which MnO/sub 2/ was added to the above combination, had excellent TCRs of +or-250 p.p.m./ degrees C at 30 Omega / Square Operator to 30 k Omega / Square Operator . These resistors showed small resistance changes, which were less than +1.0% after a thermal cycle test (-55-150 degrees C, 1000 times) and high-temperature storage test, ( 150 degrees C, 1000 h). They had excellent resistance stability. Further, the effect of particle size of glass and RuO/sub 2/ powders on electrical properties of the resistors containing the crystallized glass was similar to that in resistors containing noncrystallized glass for alumina ceramics. >

Laser trimming of thick film metal resistors on aluminum nitride substrates

Problems and their countermeasures in laser trimming of thick-film metal resistors on aluminum nitride (AIN) substrates are studied. Trimming was made possible by selection and combination of suitable Q-switching frequency, laser power, and number of irradiations. For example, under the conditions of 1 kHz, 2.0 W, and double irradiation, resistance values could be controlled to within +or-0.3% of target values. Resistance changes of trimmed metal resistors were less than +5.5% and comparable to those on alumina after thermal cycle tests (-55-150 degrees C, 1000 times), high-temperature storage tests (150 degrees C, 1000 h), and high-temperature and humidity storage tests (80 degrees C, 90% RH, 1000 h). Since the insulation characteristics of an irradiated portion strongly depended on the irradiation conditions, suitable conditions were selected for practical applications. The degree of insulation degradation after the laser irradiation was different for the two types of AlN substrates studied. This is attributed to the different amounts of free Al formed on the AlN surfaces. Further, AlN has a higher absorption factor and smaller reflectivity than alumina for a YAG laser beam. Thermal and electrical damage of AlN are affected by these properties. >