The reaction of indium (In) and silver (Ag) during the electroplating process of indium over a thick silver layer was investigated. It was found that the plated In atoms react with Ag to form AgIn 2 intermetallic compounds at room temperature. Indium is commonly used in the electronics industry to bond delicate devices due to its low yield strength and low melting temperature. In this study, copper (Cu) substrates were electroplated with a 60-lm-thick Ag layer, followed by electroplating an In layer with a thickness of 5 lm or 10 lm, at room temperature. To investigate the chemical reaction between In and Ag, the microstructure and composition on the surface and the cross section of samples were observed by scanning electron microscopy (SEM) with energydispersive x-ray spectroscopy (EDX). The x-ray diffraction method (XRD) was also employed for phase identification. It was clear that indium atoms reacted with underlying Ag to form AgIn 2 during the plating process. After the sample was stored at room temperature in air for 1 day, AgIn 2 grew to 5 lm in thickness. With longer storage time, AgIn 2 continued to grow until all indium atoms were consumed. The indium layer, thus, disappeared and could barely be detected by XRD.

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Silver (Ag) foils are bonded to alumina substrates by a low temperature solid state bonding process. The alumina substrate is premetalized with 40 nm titanium tungsten (TiW) and 2.54 μm gold (Au). The bonding temperature is just 260 °C, compatible with the peak reflow temperature of lead-free (Pb-free) solders used in electronic industries. The Ag foil is quite soft and ductile. It can deform to mate with the Au surface on alumina. Thus, only 1000 psi of static pressure is needed to bring Ag atoms and Au atoms within atomic distance on the interface. Ag has superior physical properties. It has the highest electrical and thermal conductivities among the metals. Scanning electron microscope (SEM) images show that the Ag foil is well bonded to the Au layer on alumina. A standard shear test is performed to determine the shear strength of the bonding. The shear strength of five samples tested far exceeds the strength requirement of MIL-STD-883 G standard.

A novel process of bonding silver (Ag) foils to copper (Cu) substrates has been developed. This direct bonding method does not use any intermediate layer in between. An important application of this process is electronic packaging where semiconductor device chips are bonded to Cu substrates or Cu electrodes fabricated on substrates. Cu is chosen as the major material for substrates and electrodes due to its high electrical and thermal conductivities, high strength, adequate rigidity, easiness in forging and machining, and low cost. On the other hand, Cu has a large mismatch in the coefficient of thermal expansion with most semiconductors, particularly with silicon. This makes it very difficult to bond large device chips to Cu substrates with a metallic joint. We thus design the Ag-cladded Cu structure to overcome this difficulty. Ag is quite soft and ductile. It can function as a strain buffer between the semiconductor chip and the Cu substrate. Ag also has superior physical properties. It has the highest electrical and thermal conductivities among all metals. In the beginning, we used an electroplating process to produce Ag-cladded Cu substrates. However, it is time consuming and costly to electroplate thick Ag layers. To obtain thick Ag layers (more than 200μm), this new laminating process is developed. The Ag foil is laminated to the Cu substrate directly with a static load of 1000psi at 250°C in a 50mtorr vacuum to suppress oxidation. No bonding medium is used. Scanning electron microscopy images on cross sections of bonded samples exhibit a perfect Ag–Cu bond.

Silver flip-chip joints between silicon (Si) chips and copper (Cu) substrates were fabricated using a solid-state bonding process without any solder and without flux. The bonding process was performed at 250°C, compatible with typical reflow temperature for lead-free solders. During the bonding process, there was no molten phase involved. The Ag joints fabricated consisted of only pure Ag without any intermetallic compound (IMC). Thus, reliability issues associated with IMCs and IMC growth do not exist anymore. Silver has the highest electrical conductivity and highest thermal conductivity among all metals. It is also quite ductile and able to deform to release stresses caused by thermal expansion mismatch. Flip-chip joints of high aspect ratio can be accomplished because the joints stay in a solid state during the bonding process. It looks like that silver is the ultimate joining material for flip-chip as well as through-Si-via interconnect technologies. In this study, the solid-state bonding process was first developed using a pure Ag foil to bond a Si chip to a Cu substrate in one step. The bonding strength on two interfaces, Si/Ag and Ag/Cu, passes the MIL-STD-883G Method 2019.7. To demonstrate Ag flip-chip interconnects, Si chips were electroplated with Ag bumps, followed by the solid-state bonding process on Cu substrates. The flip-chip bumps are well bonded to the Cu substrate. It would take some time for this new technology to be probably accepted and utilized in production. On the other hand, the preliminary results in this study show that Ag flip-chip joints can indeed be fabricated at 250°C.