Nobuo Iwase

High thermal conductivity aluminum nitride ceramic substrates and packages

Breakthrough technologies for achieving a whole line of AlN products for wider use in the electronics market are described. These products are: (1) plain substrates; (2) metallized substrates; (3) direct bond copper (DBC) substrates; (4) substrates for thin-film circuits; (5) substrates for thick-film circuits; and (6) cofired multilayer packages. The basic properties of AlN and problems with AlN from the manufacturing process point of view are summarized. The discussion of the breakthrough technologies needed for AlN applications focuses on four problem areas: the realization of the highest thermal conductivity by sintering, pin/lead brazing, the metallization problem for the cofired case and the postfired case, and polishing for a thin-film circuit. AlN applications and market trends are discussed. >

Thick Film and Direct Bond Copper Forming Technologies for Aluminum Nitride Substrate

Thick film and direct bond copper (DBC) were successfully formed on aluminum nitride (AIN) ceramics. Commercially available gold (Au), silver-palladium (Ag-Pd), and copper (Cu) thick film pastes for alumina (AI 2 O 3 ) were formed on AIN under exactly the same firing conditions as on AI 2 O 3 . From X-ray micro analysis (XMA) and X-ray diffraction (XRD) analysis, copper (Cu), lead (Pb), silicon (Si), and oxygen (O) were observed at the adhesive boundary of the thick films and ceramics. On the contrary, poor bondings were observed when bismuth (Bi) segregated at the adhesive boundaries. Pre-oxidation of AIN ceramics at high temperature around 1250°C was required for the bonding of DBC to AIN. Cu 2 O was formed at the boundary between the Cu metal and oxidized AIN, thus copper was directly bonded to AIN firmly. Bonding strength, peel strength, and wire bondability were enough for practical use in applications to high power modules and large-scale integrated (LSI) packages.

Solder bumper formation using electroless plating and ultrasonic soldering

Aluminum electrodes in the Si wafer were surface-treated in the sequence palladium activation, Ni-P electroless plating, ultrasonic soldering with Pb-1Sn solder, and dipping in Sn-37Pb to form second-stage solder bumps. The average bump height and the shear force were 20.3 mu m and 26.8 g/pad, respectively. The shear force did not decrease after the heating test (150 degrees C*1000 h) or thermal cycle test (-65 degrees C (--) room temperature (--) 150 degrees C, 300 cycles). Aluminum diffused to amorphous Ni-P through the Pd layer, Ni-Sn intermetallic compounds were formed at the Pb-1 Sn/Ni-P interface. The low-cost polyester tape-automated bonding was utilized to melt low temperature second solder on inner lead bonding. No bridge occurred when applying this technique to an LSI device with 376 electrodes, 100 mu m wide with 150 mu m pitch.<<ETX>>

Influence of ceramic surface treatment on peel-off strength between aluminum nitride and epoxy-modified polyaminobismaleimide adhesive

Peel-off strength between aluminum nitride (AlN) ceramics and a polyaminobismaleimide (PABM) adhesive is investigated after surface treatments. The surface treatments of the AlN substrates were oxygen plasma exposure, K/sub 2/O/spl middot/n(B/sub 2/O/sub 3/) aqueous solution immersion and a combination of the two. Each of the three methods increases the peel-off strength compared to that in the case of no surface treatment. In the case of the combination of oxygen plasma exposure for 60 s and K/sub 2/O/spl middot/n(B/sub 2/O/sub 3/) aqueous solution immersion for 10 min, average peel-off strength was over 1.6 N/mm, whereas that in the case of no surface treatment was 0.14 N/mm. Oxygen plasma exposure and K/sub 2/O/spl middot/n(B/sub 2/O/sub 3/) aqueous solution immersion decreased the relative amounts of carbon and hydrocarbon on the surface of as-sintered AlN substrates. On the other hand, the relative amount of hydrophilic groups, such as COO, C=O, and C-O relatively increased. This chemical change is effective for increasing peel-off strength. The results of measurement of surface free energy of the AlN substrate surface indicate that these surface treatments increase surface energy of AlN substrates from about 47 to 60 mJ/m/sup 2/. When AlN substrate was immersed in K/sub 2/O/spl middot/n(B/sub 2/O/sub 3/) aqueous solution, tiny protrusions were formed on the AlN grain surfaces. The approximate height and pitch of the protrusions were about 30 nm and 60 nm, respectively, in the case of immersion for 10 min. Most AlN grains were etched, although not all. This change in shape of grains brings about resistance to peeling and contributes to enlargement of the surface area. Due to these effects, average peel-off strength of AlN substrates with both oxygen plasma exposure and K/sub 2/O/spl middot/n(B/sub 2/O/sub 3/) aqueous solution immersion was 1.1 N/mm even after 800 cycles of thermal cycling and the value is still larger than that required for the practical package application.

Design and characteristics of a newly developed cavity-up plastic and ceramic laminated thin BGA package

The key requirements for a package are high electric and thermal performance, thinness, light weight, small size or high assembly density, and low cost. Plastic packages are superior in terms of electrical performance and cost whereas highly thermally conductive ceramic packages are superior in terms of thermal performance, weight, and size. However, these conventional plastic or ceramic packages cannot simultaneously satisfy all the requirements A new cavity-up plastic and ceramic laminated package (PCLP) has been developed that not only has superior electrical and thermal characteristics simultaneously without a heat sink, but also a thin profile and small size and is cost-effective. For example, the frequency range applicable to the PCLP exceeds 500 MHz, the maximum power dissipation is 4 W under natural convection, and the thickness is less than 2 mm. The PCLP is composed of two substrates: an electrically conductive plastic substrate and thermally conductive ceramic substrate. The plastic substrate, made of liquid crystal polymer (LCP) and copper, forms a flexible printed circuit (FPC). LCP is a suitable material since it has low water absorption, low dielectric constant, and low dielectric loss. The ceramic substrate is cofired tungsten-metallized aluminum nitride (AlN). It has high thermal conductivity and its coefficient of thermal expansion (CTE) is close to that of silicon. The AlN substrate also supports mechanically both the FPC and the semiconductor chip. The package is made using simple processes: both FPC and AIN substrate are single insulation layers; interconnection technologies are simple, for example, screened bump interconnection and lamination; and a conventional pattern formation is used, for example, screen printing. The measured electrical resistance is 450 m/spl Omega/ (line length 14.7 mm, width=50 /spl mu/m), which was about 1/10 of that for a simple ceramic cofired package of the same dimensions with a tungsten conductor. The measured thermal resistance is 10.8/spl deg/C/W under natural convection without a heat sink. In this paper the PCLPs design concept, configuration and performance characteristics are reported.

Titanium nitride-molybdenum metallizing method for aluminum nitride

A paste containing molybdenum (Mo) and titanium nitride (TiN) powders was printed on aluminum nitride (AlN) substrates and postfired. The adhesion strength of metallized substrates with Ni/Au plate was about 25 kgf/2.5 mm and was unchanged after the thermal cycle test. TiN-Mo does not adhere to the grain boundary phase in AlN substrate, or to the surface oxide layer, but to the AlN grain itself. This method, therefore, seems to be applicable to any kind of AlN substrate, which may have different grain boundary oxide phases and thermal conductivities. This TiN-Mo metallized AlN substrate was tried as a replacement for a beryllium oxide (BeO) heat sink, which has been used for RF power transistors. There was no trouble in assembling the AlN heat sinks into transistors. Thermal resistance and electrical properties for transistors with AlN heat sinks were almost equal to those for transistors with BeO heat sinks. The TiN-Mo metallized AlN substrates were found to be suitable for replacing BeO substrates as the heat sinks for semiconductor devices. >

Solder bump formation using electroless plating and ultrasonic soldering

Aluminum electrodes in the Si wafer were surface-treated in the sequence palladium activation, Ni-P electroless plating, ultrasonic soldering with Pb-1Sn solder, and dipping in Sn-37Pb to form second-stage solder bumps. The average bump height and the shear force were 20.3 mu m and 26.8 g/pad, respectively. The shear force did not decrease after the heating test (150 degrees C*1000 h) or thermal cycle test (-65 degrees C (--) room temperature (--) 150 degrees C, 300 cycles). Aluminum diffused to amorphous Ni-P through the Pd layer, Ni-Sn intermetallic compounds were formed at the Pb-1 Sn/Ni-P interface. The low-cost polyester tape-automated bonding was utilized to melt low temperature second solder on inner lead bonding. No bridge occurred when applying this technique to an LSI device with 376 electrodes, 100 mu m wide with 150 mu m pitch. >

A thin and low thermal resistance aluminum nitride BGA package for high speed DSP devices

Low thermal resistance and high TCT (temperature cycle test) reliability have been attained by thin AlN BGA package structure. High speed signal transmission for multimedia DSP (digital signal processor) has been analyzed by computer simulation. The package has 0.6 mm body thickness, which is 1/3 of ordinary ceramic packages. A low thermal resistance of 4.8 /spl deg/C/W has been attained without a heat sink and with no air cooling while mounted on a PWB having 4 conductive layers. Scattering parameters were measured up to 9 GHz and applied to 250 MHz clock signal simulation. No deterioration of signal waveforms was observed during the simulation. The effects of package thickness and package size have been discussed from the thermal resistance and TCT reliability point of view. Thicker and larger package size provided lower thermal resistance. The developed package thickness and size (35/spl times/35 mm) have been determined by taking into account the application field of mobile PCs (personal computers) and through simulation. The package thickness versus TCT reliability has been discussed, and then the TCT has been carried out under an assembled condition on a PWB, and MTTF of 1 k cycles has been derived.

An organic and ceramic laminated BGA package with high thermal and electrical performance characteristics

A new structural face-up LSI package has been developed. The package shows low thermal resistance without a heatsink and low inductance, capacitance, and resistance values. The package is thin enough for portable multimedia equipment applications, with a thickness of 0.5 mn (not including ball height). The measured thermal resistance was 11/spl deg/C/W under natural convection without a heatsink. The simulated inductance and capacitance were 6.7 nH and 1.1 pF respectively, and measured resistance was 520 m/spl Omega/ (line length=14.7 mm, width=60 /spl mu/m). The package consists of a resin film and a ceramic substrate. The film is a liquid crystal polymer (LCP) and the substrate is aluminum nitride (AlN). LCP is a suitable material for buried-bump interconnection technology (B2it/sup TM/). AlN has high thermal conductivity and its coefficient of thermal expansion (CTE) is close to that of silicon. Both materials were laminated by an adhesive agent. This material combination provides a thin structure, low thermal resistance, and low LCR which are suitable for portable multimedia electrical equipment. This paper reports the configuration and performance characteristics of this newly developed package.

Densification of Tungsten Conductors in Cofired Aluminum Nitride Multilayer Substrates by the Addition of Manganese Oxide

The sintering behavior of tungsten (W) conductors embedded in aluminum nitride (A1N) multilayer substrates was improved by adding manganese dioxide (MnO2) to A1N green sheets. To obtain A1N suspensions, MnO2 powder (purity: 99.99%) was mixed with A1N powder, yttrium oxide (Y2O3) powder (as a sintering additive), casting organic additives, and organic solvents by ball milling. Each suspension was cast into A1N green sheets and tungsten conductors were then printed on them. To obtain A1N multilayer substrates, the green sheets were laminated and then sintered at 1780, 1820, and 1850°C for 4 hours in a nitrogen atmosphere. The microstructure of the fracture surface of the A1N multilayer substrate with added MnO2 indicated that the densified W conductor layers were without pores. Segregation of manganese (Mn) was observed among W grains by EPMA in the A1N multilayer substrate heated up to 1500°C. Moreover, rapid W grain growth was observed in the specimen, and Mn wetted these W grains well. These results indicated that molten Mn penetrated into W grain boundaries from the A1N matrix and accelerated W grain growth by liquid phase sintering during A1N multilayer substrate cofiring. Therefore, the A1N multilayer substrates, including the densified W conductors, were prepared by the addition of MnO2 into A1N green sheets.