Trend and issues with low-Ag flux-cored wire solder

As the use of Sn3.0Ag0.5Cu solder to replace the conventional SnPb becomes more common due in part to a recommendation by JEITA, most users designate this alloy type of flux-cored wire solder.

However, changes in the recent economic environment and widespread of Sn3.0Ag0.5Cu solder usage have exposed two major concerns. One is the shift in demand to low-Ag containing materials caused by increased material cost, such as tin and silver. Another is characteristics of the lead-free solder that erode base materials, which leads to disappearance of finer copper lines and accelerated consumption of soldering bit (Fig. 1). Measures against the issues need to be taken.

In this article, the trend with low-Ag solder and the response in the industry are discussed.

Fig.1 Cross section: soldering bit after 50,000 shots

● Features of Co added low-Ag flux-cored wire solder

1. Wetting

S03X7Ca-56M¡¦is a flux-cored wire solder that has an alloy composition of Sn0.3Ag0.7Cu with 0.03% Co (cobalt) and minute amount of a substance to complement the function of Co, and is described as Sn0.3Ag0.7Cu0.03Co+α. Surface tension, one of the indexes to evaluate wettability, is said to drop by adding Co, which was confirmed in our lab test. It also offers the same level of solder spread as Sn3.0Ag0.5Cu on the copper plate (Table 1). Furthermore, solderability of low-Ag solder is comparable to Sn3.0Ag0.5Cu as there is no notable difference in wetting time within the range of process temperatures.

Table.1 Comparison: surface tension and solder spread

2. Erosion of base metals

To evaluate resistivity of Co-added solder to iron (Fe) erosion, an Fe plate was put into the molten solder of 450℃ and mixed in for one hour. The eluted amount of iron was about the same as that of tin-lead solder (Sn40Pb).　Next, 20,000 shots of soldering were performed at the soldering bit temperature of 390 ℃ using soldering machine. Cross section images after the test are shown in Fig 2. Co-added and SnPb solders showed the same level of erosion into Fe plating. As seen in EPMA mappings (Fig 3), Co-added solder has superior resistivity against Fe erosion because, presumably, a barrier layer of SnFe-SnFeCo-SnCo was formed in the interface and prevented erosion.

Fig.2 Cross section: soldering bit after 20,000 shots

3. Reliability

Co added solder forms a type of compound called (Cu, Co)6Sn5 by substituting Cu from the metallic compound of Cu6Sn5 formed in the interface between the solder and copper layers. This metallic layer is relatively thick and even, and prevents growth of compound. EPMA mapping indicates that Co is dispersed in the form of fine SnCo. Reliability has been achieved by improved joint strength of the metallic layer in Co interface and by dispersion of Co into the solder.

Fig.3 Fe ¡¦EPMA mapping of solder interface