2.1 Particle synthesis and paste preparation

Filler particles mixed with Cu and Cu formate were synthesized using a simple wet reaction [²⁷]. First, 25–75 g of Cu₂O powder (95%, Daejung Chemical) was added into 252 mL of formic acid (HCOOH, 85%, Samchun Pure Chemical). Second, the slurry was stirred at 250 rpm for 15-25 min. The resultant slurry was washed three times with ethanol. Finally, the sludge was decanted and dried in a lowvacuum oven for 5 h. Subsequently, a paste containing the obtained particles was prepared by mixing the dried particles with α-terpineol (98.5%, Samchun Pure Chemical) at the weight ratio of 7:3 using a spatula.

2.2 Sinter-bonding

Dummy Cu substrates and chips were polished with a 2000-mesh sandpaper and then etched in a 10% H₂SO₄ solution for 1 min. The prepared paste was screen-printed on a dummy Cu substrate over an area of 3×3 mm, followed by drying at 150 °C for 3 min to decrease the amount of solvent by evaporation. Subsequently, a 3×3×1 mm dummy Cu chip was placed on the printed pattern to form a sandwich structure. For sinter-bonding, the sandwich-structured sample was heated at 250 °C at a continuously applied external pressure of 5 or 10 MPa using a thermo-compression bonder, and held for 1-10 min at that temperature.

2.3 Characterization

The morphology of the initial Cu₂O and synthesized Cu complex particles, the cross-sectional microstructures of the bond-lines, and the fracture surfaces were characterized using high-resolution scanning electron microscopy (HR-SEM, SU8010, Hitachi High-Technologies Co.). Thermal analysis of the particles was carried out while heating to 400 °C in air, at a heating rate of 10 °C/min using thermogravimetry-differential thermal analysis (TG-DTA, DTG-60, Shimadzu) to estimate the thermal behavior of the Cu complex particles during heating. Phase transformation in the particles after wet synthesis or heating was determined by X-ray diffraction (XRD, DE/D8 Advance, Bruker). The solidity of the formed bond-line was measured during shearing at 200 μm/s at a shear height of 200 μm using a bond tester (DAGE 4000, Nordson Corp.), and the bonding strength was defined as the measured maximum stress.

3.1 Synthesis of Cu complex particles

Fig 1 shows the morphologies of the initial Cu₂O particles and back-scattered electron (BSE) images of particles mixed with Cu formate, and Cu particles obtained using different amounts of Cu₂O, following wet synthesis for 15 min in formic acid. The synthesized particles indicated two completely different morphologies. The white and tiny pseudo-spherical shapes and grey flake-type particles in the BSE images represent pure Cu and Cu formate phases, respectively [²⁷], which were in situ mixed during the synthesis. However, the degree of aggregation of the synthesized particles differed significantly based on the initial amount of Cu₂O. The best dispersion among the samples was accomplished when 50 g Cu₂O was used.

Fig 2 shows the BSE images of the Cu complex particles obtained at various synthesis times when 50 g Cu₂O particles was added. In principle, the aggregation between particles intensified and the number of pure Cu particles decreased with increasing synthesis time. The XRD patterns of the particles measured as a function of synthesis time are shown in Fig 3.

In every sample of 15-25 min synthesis times, pure Cu, Cu formate, and slight Cu oxide phases were indexed. However, the peaks of Cu formate were slightly higher with an increase in synthesis time, indicating that the amount of Cu formate increased with time. The Cu oxide phase seemed to be the result of the oxidation of the synthesized pure Cu in air.

The formation of Cu formate from Cu₂O can be accomplished through following two reactions [²⁷]:

Thus, with insufficient synthesis time, a pure Cu phase will be obtained. The 15-min synthesis using 50 g Cu₂O particles was chosen as an optimal synthesis condition to prepare a filler for a paste with both less aggregation and a higher total amount of pure Cu particles in the filler material.

3.2 Thermal properties of the synthesized particles

The TG-DTA results of the synthesized particles mixed with Cu formate and Cu are presented in Fig 4.

The Cu formate in the particles began to decompose at approximately 200 °C via the sequential reactions of Equations (3) and (4) [²⁸,²⁹], resulting in abrupt weight loss and heat generation:

Accordingly, exothermic peaks of the two steps were observed in the DTA curve during the weight-loss period; the appearance of the peaks was slightly delayed compared to that observed in a previous report due to the fast heating rate [²⁸]. The generated heat (26±2 kJ/mol [³⁰]) can substantially contribute to sinter-bonding behavior. The weight loss approached approximately 30% at 251 °C, which implies that the amount of Cu formate in the mixed particles was 62 % in weight.

The XRD result indexed after heating the Cu complex particles at 300 °C is shown in Fig 5. The peaks of Cu formate disappeared and those of pure Cu were strongly indexed as a product of the pyrolysis. The Cu generated in situ during the pyrolysis would be inherently active, thus we expected that it would exhibit strong sinterability under physical contact. In the results, the peaks of copper oxides were also indexed owing to the heating in air. However, the degree of oxidation can be significantly reduced in a real bond-line if the bond-line can be rapidly compacted by an external pressure, unlike the heating conducted without pressure for this measurement. This result clearly demonstrates that the Cu formate used in the Cu complex particles was transformed to Cu by pyrolysis through heating in air.

3.3 Sinter-bonding properties of the paste containing synthesized particles

Fig 6 shows the average shear strength values of dummy Cu chips sinter-bonded at 250 °C in air as a function of bonding time. Overall, the strength increased, with an increase in bonding time under 10 MPa pressure. The strength finally approached the excellent value of 22.2 MPa after bonding for 10 min, surpassing the strength of Pb-5Sn bond-lines [³¹]. When the bonding pressure was 5 MPa, however, the strength decreased to less than half for an identical bonding time. These results indicate that rapid sinter-bonding within 10 min at 250 °C in air using Cu-based particles is feasible under a pressure of 10 MPa, which is a significantly faster method compared to those performed at higher temperatures in inert atmospheres using Cu-based particles [³²].

The BSE images of bond-lines sinter-bonded at 250 °C under 10 MPa are shown in Fig 7. In cases where the density of the formed bond-line is high, there may be instances where micro voids are filled due to the influence of debris during the polishing process. However, in this study, it was distinctly observed that there were hardly any instances of voids being filled, as the density of the bond-line was not high or only macro voids primarily exist.

For the samples bonded up to 3 min, compact microstructures were not formed in the bond-lines although three-dimensional connections between the remnant particles were formed. However, compact bond-line structures were observed in the 5-min bonded samples, and bonding areas at the chip/bond-line particles and bond-line particles/substrate interfaces were significantly enhanced in the samples. Detailed observation of the bond-lines revealed the formation of oxide shells on the surfaces of the remnant Cu particles. The formation of such oxide shells is anticipated to have a somewhat detrimental effect on the thermal conductivity property within the bond-line.

3.4 Fractography

Fig 8 shows images of the fracture surfaces on the dummy chips sinter-bonded at 250 °C with respect to bonding time. Bond-line/substrate interfacial failures were observed in the 1- and 3-min bonded samples, indicating that the interface was weakest owing to insufficient sintering. However, the failure mode changed to coherent failure within a bond-line from the bonding of 5 min, implying that sinter-bonding at the bond-line/substrate interface was reinforced. Furthermore, shear bands indicating the ductile fracture were clearly observed on the fracture surfaces, and the number of shear bands increased in the 10-min samples. Although oxide layers existed on both substrate and chip, the sinter-bonding at the interface seemed to be responsible for the in-situ reduction of the oxide layers during the bonding. The hydrogen gas emitted by Equations (3) and (4) during the pyrolysis facilitated robust interfacial sintering on the Cu finish by reducing the surface oxide at that temperature with the assistance of external pressure. Immediately after the reduction, active Cu already reduced in the bond-line began to sinter with the Cu finishes of both substrate and chip, and solidity at the interfaces was attained with an increase in the interfacial sintering area by the increase in bonding time.