Three-dimensional morphology of a very rough interface formed in the soldering reaction between eutectic SnPb and Cu

Abstract

We have used scanning electron microscopy to study the interfacial morphology of the Cu-Sn compounds formed between a eutectic SnPb alloy and Cu at 200 °C. A selective etching reveals the three-dimensional morphology of the Cu-Sn compounds. On the solder side, the compounds grow rapidly as big scallops and the interface becomes extremely rough as compared to the Cu side of the interface. In order to understand this rapid and extremely irregular growth of the Cu-Sn compounds, we propose that it is caused by the dissolution of Cu into the liquid solder and the coarsening of the scallop-type compounds by Ostwald ripening. The growth of the Cu-Sn compounds has a serious impact on solder joint rework in electronic packaging.

Effect of thermal aging on the interfacial reactions of tin-based solder alloys and copper substrates and kinetics of formation and growth of intermetallic compounds

“…In the tin/copper and tin-rich-alloys/copper joint systems, the intermetallic Cu 6 Sn 5 (h'-phase) formed immediately upon contact of the liquid solder with the copper substrate [7,8]. Then, the intermetallic compound grew rapidly [9,10]. The interface after thermal aging was found to be very different compared to the initial observations immediately following soldering, as shown in Figure 2, in the micrographs of the Cu/Solder interface at 150 o C for 500 hours.…”

Cross interactions on interfacial compound formation of solder bumps and metallization layers during reflow

“…[6][7][8] Spalling of IMCs were reported when thin film Cu UBM reacted with eutectic SnPb and Pb-free solders. 9,10 A method has been developed to prevent the spalling of IMCs by the opposite interfacial reaction on the substrate side.…”

Modeling of intermediate phase growth

“…[5][6][7] With time scallops grow larger but fewer, indicating that the coarsening process takes place. 6,7 The problem of IMC layer growth has many challenges, the reasons for roughness and coarsening, to name only a few. To describe this process Kim et al 6,8 suggested a two-flux nonconservative Ostwald ripening model based on the assumption of rapid dissolution of Cu into the liquid solder.…”

“…6,7 The problem of IMC layer growth has many challenges, the reasons for roughness and coarsening, to name only a few. To describe this process Kim et al 6,8 suggested a two-flux nonconservative Ostwald ripening model based on the assumption of rapid dissolution of Cu into the liquid solder. Hayashi et al 5 suggested the fast dissolution of Cu 6 Sn 5 along the grain boundary as the reason for scallopedge appearance.…”

“…However, observations of the intermetallic growth in the solid-Cu͑Ni͒/liquid-Sn system ͑above the melting point of the solder͒ instead of a smooth layer always show rough, strongly undulated compound layers with scallops of the intermetallic phase. [5][6][7] With time scallops grow larger but fewer, indicating that the coarsening process takes place. 6,7 The problem of IMC layer growth has many challenges, the reasons for roughness and coarsening, to name only a few.…”