A Study on the Effect of Pd Layer Thickness on the Properties of Cu-Ag Intermetallic Compounds at the Bonding Interface

TL;DR

This study examines how Pd layer thickness affects the formation of Cu-Ag intermetallic compounds during high-temperature storage.

Contribution

The paper reveals how Pd thickness influences the Kirkendall effect and intermetallic compound growth at bonding interfaces.

Findings

Thinner Pd layers (Pd100) lead to larger voids, cracks, and uneven intermetallic compound growth.

Thicker Pd layers (Pd120) reduce microcracks and promote uniform, thinner intermetallic compounds.

Pd suppresses the Kirkendall effect and slows the growth of Cu-Ag intermetallic compounds.

Abstract

This paper conducted a high-temperature storage test (HTST) on bonded samples made of Pd100 (Pd-coated Cu wire with a Pd layer thickness of 100 nm) and Pd120, and studied the growth law of Cu-Ag intermetallic compounds and the inhibitory mechanism of Pd thickness on Cu-Ag intermetallic compounds. The results show that the Kirkendall effect at the bonding interface of the Pd100-bonded sample is more obvious after the HTST, the sizes of voids and cracks are larger, and the thickness of intermetallic compounds is uneven. But, the bonding interface of the Pd120-bonded sample has almost no microcracks, the Kirkendall voids are small, and the intermetallic compound size is uniform and relatively thin. The formation sequence of intermetallic compounds is as follows: Cu atoms diffuse into the Ag layer to form Ag-rich compounds such as CuAg4 or CuAg2, and then the CuAg forms with the increase in diffused Cu elements. Pd can significantly reduce the Kirkendall effect and slow down the growth of Cu-Ag intermetallic compounds. The growth rate of intermetallic compounds is too fast when the Cu bonding wire has a thin Pd layer, which results in holes and microcracks in the bonding interface and lead to the peeling of the bonding interface. Voids and cracks will hinder the continuous diffusion of Cu and Ag atoms, resulting in the growth of intermetallic compounds being inhibited.

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