Time-Lapse Imaging of Ag 3 Sn Thermal Coarsening in Sn-3Ag-0.5Cu Solder Joints
- PDF / 2,383,560 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 69 Downloads / 202 Views
https://doi.org/10.1007/s11664-020-08498-9 Ó 2020 The Author(s)
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
Time-Lapse Imaging of Ag3Sn Thermal Coarsening in Sn-3Ag-0.5Cu Solder Joints J.W. XIAN
,1,2 S.A. BELYAKOV,1 and C.M. GOURLAY1
1.—Department of Materials, [email protected]
Imperial
College,
London
SW7
2AZ,
UK.
2.—e-mail:
The coarsening of Ag3Sn particles occurs during the operation of joints and plays an important role in failure. Here, Ag3Sn coarsening is studied at 125°C in the eutectic regions of Sn-3Ag-0.5Cu/Cu solder joints by SEM-based timelapse imaging. Using multi-step thresholding segmentation and image analysis, it is shown that coalescence of Ag3Sn particles is an important ripening process in addition to LSW-like Ostwald ripening. About 10% of the initial Ag3Sn particles coalesced during ageing, coalescence occurred uniformly across eutectic regions, and the scaled size distribution histograms contained large particles that can be best fit by the Takajo model of coalescence ripening. Similar macroscopic coarsening kinetics were measured between the surface and bulk Ag3Sn particles. Tracking of individual surface particles showed an interplay between the growth/shrinkage and coalescence of Ag3Sn. Key words: Soldering, intermetallics, precipitates, coarsening, time-lapse imaging
INTRODUCTION Ag3Sn particles are a primary strengthening mechanism in Sn-Ag and Sn-Ag-Cu (SAC) solder joints,1,2 and their coarsening in-service plays an important role in solder joint failure.3 During thermal cycling, Ag3Sn particle coarsening has two components: thermal coarsening, and strain-enhanced coarsening. Various studies have shown that accelerated coarsening of Ag3Sn particles occurs in regions of stress concentration which is often in the solder near the package side, and results in a local loss of strength and is a precursor to crack initiation in this region.4–9 Ag3Sn coarsening is relatively rapid in solder joints because they are cycled through a homologous temperature range of up to T/Tm 0.5–0.8, and thermal coarsening of Ag3Sn is a significant challenge in applications where high reliability is required at high maximum operation temperature (e.g. at 125°C).
(Received July 14, 2020; accepted September 17, 2020)
The Ag3Sn particles that give strengthening initially form in a eutectic reaction during solidification and then their size and shape continue to develop in service due to the relatively high homologous temperature. The sizes of Ag3Sn particles after soldering are typically submicron in a range of Sn-Ag-Cu solders,10–18 which can greatly improve the strength and creep resistance of solders through precipitate strengthening. A wide range of eutectic Ag3Sn sizes (from 80 nm to 500 nm) are reported for solders containing similar concentration of Ag12–18 which is related to differences in cooling rate,12–16 nucleation undercooling for bSn,19 and imaging quality of electron microscopy.16,17 These factors are a challenge for studying Ag3Sn coarsening and high
Data Loading...
