On the root cause of Kirkendall voiding in Cu 3 Sn
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Peter Borgesen Department of Systems Science & Industrial Engineering, Binghamton University, Binghamton, New York 13902 (Received 4 June 2010; accepted 4 October 2010)
Soldering to Cu interconnect pads with Sn-containing alloys usually leads to the formation of a layered Cu3Sn/Cu6Sn5 structure on the pad/solder interface. Frequently, microscopic voids within Cu3Sn have been observed to develop during extended thermal aging. This phenomenon, commonly referred to as Kirkendall voiding, has been the subject of a number of studies and speculations but so far the root cause has remained unidentified. In the present work, 103 different Cu samples, consisting of 101 commercially electroplated Cu and two high-purity wrought Cu samples, were surveyed for voiding propensity. A high temperature anneal of the Cu samples before soldering was seen to significantly reduce the voiding level in subsequent thermal aging. For several void-prone Cu foils, the anneal led to significant pore formation inside the Cu. In the mean time, Cu grain growth in the void-prone foils showed impeded grain boundary mobility. Such behaviors suggested that the root cause for voiding is organic impurities incorporated in the Cu during electroplating, rather than the Kirkendall effect.
I. INTRODUCTION
In microelectronics manufacturing, soldering is often made between Sn-containing alloys and Cu interconnection pads. The metallurgical reactions lead to the formation of layered Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) on the pad/solder interface. The Cu3Sn layer is usually thin right after soldering, on the order of nm,1,2 but both IMCs grow by solid state reactive diffusion upon further thermal exposure. Microscopic voids are often observed inside the Cu3Sn IMC and at the Cu/Cu3Sn interface after isothermal aging tests. Severe adverse effects of such voiding on solder joint reliability in impact tests of BGA (ball grid array) assemblies were first presented by Zeng et al.3,4 Since then, several groups have confirmed that voiding in Cu3Sn may indeed lead to significant degradation of solder joint reliability in boardlevel impact tests.5–7 Void-induced brittle failure at the solder/pad interface was also found to occur in mechanical shear and pull tests of solder bumps.4,8 In addition, Cu3Sn voiding was reported in flip-chip joints on electroplated Cu UBM (under bump metallization) or Cu pillars, during extended current stressing at elevated temperatures.9–12 The growth of IMCs and voids were seen to be greatly affected by electromigration induced by current densities on the order of 103–104 A/cm2.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.47 J. Mater. Res., Vol. 26, No. 3, Feb 14, 2011
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The voiding phenomenon is often referred to as Kirkendall voiding or diffusion porosity. It is commonly believed that voids arise from the agglomeration of excess vacancies, as a result of the intrinsic diffusivity difference between the two diffusion species, Cu an
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