Effect of Sn concentration on massive spalling in high-Pb soldering reaction with Cu substrate
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The massive spalling of Cu3Sn in the soldering reaction between high-Pb solders and Cu substrates was studied to identify the mechanism behind this rather interesting and frequently observed phenomenon. Four different alloys (99.5 Pb 0.5 Sn, 99 Pb 1S n, 97 Pb 3 Sn, and 95 Pb 5 Sn, in wt%) were soldered at 350 C for durations ranging from 10 s to 600 min. At low Sn concentration (0.5 or 1 Sn), massive spalling occurred as early as 20 min. However, at high Sn concentration (3 or 5 Sn), massive spalling was not completed even after 600 min. To the best of our knowledge, these results are the most detailed observations ever reported on the sequence of events that occur during massive spalling. The Pb–Sn–Cu phase diagram is used to rationalize the phenomenon. I. INTRODUCTION
Solder alloys play an important role in electronic packaging; in particular, PbSn alloys are very frequently used for flip-chip applications.1 During the soldering reaction, one or more intermetallic compounds are formed at the substrate–solder interface, and the integrity of these intermetallics has a profound effect on the reliability of the produced electronic devices. Recently, considerable attention has been paid to the massive spalling of intermetallics, that is, large-scale detachment of intermetallics from the interface during soldering or aging. Spalling has been observed in several substrate– solder systems, for example, PbSn soldered on Ni,2 SnCu on Ni,3 SnAgCu on Ni,4,5 SnZn on Cu,6 and PbSn aged on Cu.7 A unified thermodynamic rationalization was subsequently proposed to explain the driving force behind the massive spalling in these systems.8 According to this argument, if the most reactive constituent in the solder is present in a limited amount and if its concentration decreases below a certain level during the reaction, the original reaction product formed at the interface is no longer in local thermodynamic equilibrium with the solder. The original reaction product as a result detaches itself from the interface and spalling occurs. Because the amount of the most reactive constituent has a marked influence on massive spalling, it is desirable to quantitatively analyze the effect of reactive elea)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/JMR.2009.0398 J. Mater. Res., Vol. 24, No. 11, Nov 2009
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ment concentration on massive spalling. In high-Pb solders, Sn is the reactive constituent, and the objective of this study is to understand the effect of Sn concentration on the massive spalling during the high-Pb/Cu soldering reaction. II. EXPERIMENTAL
Rectangular Cu substrate plates of dimension 25 10 4 mm were mirror-polished on both faces, etched with 50% nitric acid to remove the surface oxide layer, and fluxed with a commercial
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