**1. Introduction**

Die-attach materials are crucial to power devices. It bonds the chip to the substrate and performs the functions of conducting current and heat. Traditional lead-free and Sn-Pb solders harm the environment and have lower operating temperatures, thus nano-Ag paste as a die-attach material is a feasible option with excellent electrical and thermal conductivity [1–3].

However, silver is prone to electrochemical migration (ECM) and forms dendritic crystals, which result in short-circuit failure [4,5]. Generally, moisture is believed to play a crucial role in the migration of silver, and as a result, silver migration is described as a moisture migration phenomenon. However, recent studies have found that ECM failure of silver occurs even in high temperature and dry environments. The role of O2 in dry environments is similar to that of H2O in the classical moisture migration process [6]. Firstly, silver oxidized at the anode and form the intermediate species Ag2O, which dissociated into silver cations and oxygen anions at a high temperature above 250 ◦C. Driven by the bias voltage, the silver cations move from anode to cathode through the insulating gap, while the oxygen anions move in the opposite direction to maintain charge neutrality. The continuous depletion of silver cations at the anode drives oxidation and dissociation until silver dendrites formed between the two electrodes. Subsequently, silver dendrites gradually appeared and result in short circuits [7,8].

It was concerned that the ECM of the sintered nano-Ag should be a failure risk for packaging wide band-gap (WBG) power semiconductors devices, which can theoretically be operated at extreme temperatures of 500 ◦C, not to mention the great increase of electrical bias [9]. Furthermore, the previous ECM of silver was only focused on thick film conductors

**Citation:** Ding, Z.; Wang, Z.; Zhang, B.; Lu, G.-Q.; Mei, Y.-H. A Reliable Way to Improve Electrochemical Migration (ECM) Resistance of Nanosilver Paste as a Bonding Material. *Appl. Sci.* **2022**, *12*, 4748. https://doi.org/10.3390/ app12094748

Academic Editor: David G. Calatayud

Received: 2 April 2022 Accepted: 6 May 2022 Published: 9 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in conventional printed circuit boards (PCBs) [4,10]. Double-sided cooling (DSC) silicon carbide (SiC) power devices have attracted more and more attention in industries, especially in electric vehicle applications, because of their superior heat dissipation capability and power density [11]. However, the DSC SiC devices have a very small electrode spacing, e.g., 250 μm [12]. When the great sintered nano-Ag was used as die attachment in the DSC SiC devices, it is crucial to avoid the ECM failure of sintered nano-Ag to guarantee the reliability of electric vehicles. Unfortunately, there is a current lack of research on the ECM of silver suppression for the DSC SiC power devices.

Inhibition of the ECM of silver, such as alloying silver with indium or copper to form anti-ECM Ag-In and Ag-Cu alloys, had been studied [13,14]. Recently, we proposed the method of alloying silver with palladium aimed at inhibiting the formation of silver ions through oxidation and decomposition in the anode to delay the ECM of silver [15]. However, these alloying methods need very high processing temperatures in order to improve the resistance to the ECM of silver by alloying more at the higher temperature, not to mention the extremely high cost of indium, palladium, and so on.

In this paper, a nano-Ag composite paste was proposed with the 0.1wt% SiOx nanoparticles (NPs) as fillers. The conventional nano-Ag paste was compared as a reference. The lifetime for reaching the ECM failure was characterized in the first place. Then the thermomechanical reliability of the proposed nano-Ag-SiOx paste was verified in the aspects of die-shearing strength and thermal impedance by thermal shocking tests because it is critical to ensure promising thermo-mechanical reliability besides the improvement of the resistance to the ECM. Microstructures and porosity were analyzed to clarify the effects of SiOx doping on the ECM and the thermo-mechanical reliability. In the end, the proposed nano-Ag-SiOx paste was used as a die attachment for a DSC power device. The improvement of the lifetime of the ECM was validated in the high power density demonstration.
