Our thanks to Kester for allowing us to reprint the following article.
By Peter Biocca, Senior Market Development Engineer, Kester
Tin-silver-copper has received much publicity in recent years as the lead-free solder of choice. SAC305 was endorsed by the IPC Solder Value Product Council in the United States as the preferred option for SMT assembly; most assemblers have transitioned to this alloy for their solder paste requirements. The SAC305 alloy due to its 3.0% content of silver is expensive when compared to traditional 63/37 for this reason many wave assemblers are opting for less costly options such as tin-copper based solders for their wave, selective and dip tinning operations.
In recent years tin-copper based solders with a variety of elemental additives have emerged which improve the overall properties and performance of tin-copper solders. Tin-copper solder without the incremental additions of certain elements is rarely used but the addition of nickel or nickel and bismuth as found for example in K100 and K100LD respectively do offer improvements in wetting, joint cosmetics and in some cases solder joint reliability.
These alternative SnCu based solders are not normally used in reflow soldering but are gaining use globally in wave and selective soldering operations. This paper will describe how to achieve solder joint reliability using SnCu based solder with nickel and other additives such as bismuth.
Comparing SAC and Tin-Copper Based Solders
At this time there is no doubt more technical information exists for SAC solder. The number of users for SnCu based solders has steadily increased in recent years and about 30% of assemblers are now using various alternatives of SnCu solder in wave or selective systems. SAC solders still account for about 60% and 10% use other lead-free solders such as tin-silver and tin-bismuth. However the rate of conversion to tin-copper has increased in 2006 and into 2007. What are the main differences between SAC and SnCu based solders? Is it as reliable as SAC in wave soldering applications? These are common questions often asked by engineers.
One of the main attraction and difference is cost. Below is Figure 1 indicating the relative cost of SAC to SnCu. The cost of material will impact the initial cost of loading but also the operating costs of wave and selective systems. The approximate elemental concentrations of K100LD and K100 are detailed; it must be noted that very few use tin-copper solder without some additives...
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