Mechanical and microstructural properties of SnAgCu solder joints

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Abstract

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85 °C for 1000 h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag3Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.

Introduction

Environmental and health concerns of lead have increased the pressure towards the lead-free soldering in electronics industry. Most of the commercial electronic products, which are put into market in EU, will be urged to qualify the lead-free regulations defined by RoHS directive, which will come into effect as soon as 1 July 2006. Therefore, a large scale transition from SnPb to lead-free solder alloys is on hand globally. SnAgCu has quite commonly been considered as the most viable replacement for SnPb. However, detailed studies of the effect of PCB surface finish and SnAgCu composition on the microstructure and mechanical properties of SnAgCu solder joints have been quite limited.

Organic solderability preservative (OSP) surface finish is used on Cu pads mainly to prevent oxidation of Cu. Experimental studies have shown that OSP does not affect the metallurgy of the solder joints. The use of NiAu surface finish is generally argued by the fact that Ni barrier prevents the diffusion of Cu atoms into the solder and thus excessive growth of intermetallic compounds [1]. The role of a thin Au layer used on Ni is to improve wetting properties, and to protect the pads from corrosion. The strength properties of SnAgCu solder joints on OSP and NiAu have been studied by a few research groups [2], [3], [4]. Their results are, however, contradictory with each other.

Several SnAgCu compositions for replacement of traditional SnPb solder alloys have been proposed by different consortia; in Japan 96.5Sn3.0Ag0.5Cu, in US 95.5Sn3.9Ag0.6Cu, and in EU 95.5Sn3.8Ag0.7Cu. The mechanical and microstructural properties of solder alloys in this composition scale have been studied in some extent. Kim et al. [5] found that both the cooling rate and the composition will affect the size of Ag3Sn dispersions in SnAgCu solder alloys, and this in turn was suggested to affect the mechanical properties of the alloys. Shiau et al. [6] proposed that Cu content in SnAgCu solder significantly affects the formation of intermetallic layers. Reliability of SnAgCu solder joints with different compositions in BGA packages have also been studied [7], [8].

Detailed studies on the effect of PCB surface finish on the shear strength of solder joints and their microstructural features are scarce. In the present work these features are studied.

Section snippets

Experimental

Mechanical properties of SnAgCu solder joints were measured by using special single-overlap shear specimens. The PCB specimens were made of glass epoxy FR-4 with a glass transition temperature Tg of 180 °C. The PCB material consisted of four layers. Copper pads in the specimen, thickness 50 μm, were plated either with organic solderability preservative (OSP) or immersion gold over electroless nickel (NiAu). The design of the specimens used is shown in Fig. 1. The lower part of the specimen had a

Shear test results

The composition of SnAgCu solder affected the shear strength of solder joints, as can be seen in Fig. 3. In the joints on OSP, the joints with eutectic composition had the highest strength value after the reflow. The hypoeutectic SnAgCu joints had the second highest strength value, and the hypereutectic joints the lowest strength value of the SnAgCu joints. In the SnPb joints the strength values were lower than those in all SnAgCu compositions. In the joints on NiAu, the differences between

Effect of PCB surface finish on the microstructure

The PCB surface finish affects, as described in context the results, the composition of intermetallic layer, the formation of AuSn4 intermetallic phase, and the amount of Cu6Sn5 phase in the bulk SnAgCu solder. In the joints on OSP, the intermetallic layer formed is Cu6Sn5, and it is (Cu,Ni)6Sn5 in the joints on NiAu. The Ni-barrier at the interface between solder and Cu pads prevents significant diffusion of Cu atoms from the pads into the solder, and the (Cu,Ni)6Sn5 layer is based mainly on

Conclusions

The shear strength and microstructure of SnAgCu solder joints with three different compositions on OSP and NiAu surface finishes were studied in both the as-reflowed state and after thermal aging. The results are summarised as follows:

  • (1)

    If the total area of the copper pads is substantial in the solder joints, the PCB surface finish on the pads may affect the cooling rate of the solder joints. It was noticed that in the solder joints on NiAu the cooling occurs slower than on OSP because the

Acknowledgements

The authors would like to thank the Technical Staff of Aspocomp Oy, Elcoteq Network, Nokia Mobile Phones and Tellabs Oy for their support and technical advice. Also the help of Ms. Elina Välipakka and Mr. Tuomo Saarinen in sample preparation is gratefully acknowledged. This study was funded by the National Technology Agency of Finland (Tekes).

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