Fluxless Bonding of Large Silicon Chips to Ceramic Packages Using Electroplated Eutectic Au/Sn/Au Structures

Abstract

A fluxless process of bonding large silicon chips to ceramic packages has been developed using a Au-Sn eutectic solder. The solder was initially electroplated in the form of a Au/Sn/Au multilayer structure on a ceramic package and reflowed at 430°C for 10 min to achieve a uniform eutectic 80Au-20Sn composition. A 9 mm 9 9 mm silicon chip deposited with Cr/Au dual layers was then bonded to the ceramic package at 320°C for 3 min. The reflow and bonding processes were performed in a 50-mTorr vacuum to suppress oxidation. Therefore, no flux was used. Even without any flux, high-quality joints were produced. Microstructure and composition of the joints were studied using scanning electron microscopy with energy-dispersive x-ray spectroscopy. Scanning acoustic microscopy was used to verify the joint quality over the entire bonding area. To employ the x-ray diffraction method, samples were made by reflowing the Au/Sn/Au structure plated on a package. This was followed by a bonding process, without a Si chip, so that x-rays could scan the solder surface. Joints exhibited a typical eutectic structure and consisted of (Au,Ni)Sn and (Au,Ni) 5 Sn phases. This novel fluxless bonding method can be applied to packaging of a variety of devices on ceramic packages. Its fluxless nature is particularly valuable for packaging devices that cannot be exposed to flux such as sensors, optical devices, medical devices, and laser diodes.

Fluxless Tin Bonding Process With Suppressed Intermetallic Growth

“…Our previous experimental results have shown that Sn strongly bonds to Si chips coated with Cr [19]- [21]. Sn atoms bond to Cr atoms without forming IMC at the Si/Cr/Sn interface.…”

“…Thus, we searched for techniques that prevent IMC growth but a little is yet reported. However, the results [19]- [21] reported by our group suggest that Sn solder can strongly bond to the Cr deposited on silicon (Si) chip without forming IMC at the Sn/Cr/Si interface. To evaluate this technique in high-power devices, alumina substrates with direct-bonded Cu (DBC) are selected for bonding study.…”

Performance of Solder Bond on Thermal Mismatch Stresses in Electronic Packaging Assembly

“…Next, the ceramic packaging is being bonded to a silicon chip using Cr/Au material, reflowed at 320 °C for 3 minutes. This new soldering technique has shown that high quality joints can be produced even without using any flux [17].…”

Effect of Zinc Addition on Void Formation in Solid-Liquid Interdiffusion Bonding of Copper

“…In solid-liquid interdiffusion bonding, metals with low melting points such as Sn and In are sandwiched between copper bases and heated above the melting point of the inserted materials to realize die bonding by solid-liquid reaction diffusion. [1][2][3][4][5][6] The liquid phase then reacts with the base copper to form a bond layer of intermetallic compounds (IMCs) that exhibits a higher melting point than the melting point of the inserted material.…”