Copper dissolution in rework, selective and wave soldering can be a serious issue, what can be done to reduce the risk of copper degradation and reliability reduction?

Tin-silver-copper (SAC) has received much publicity in recent years as the lead-free solder of choice. The IPC Solder Value Product Council in the United States has endorsed the SAC305 alloy 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 alloys. 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 recently are gaining more global acceptance in wave and selective soldering operations.

Lead-free alloys in the SAC family of solders have an affinity for copper and dissolve copper from copper tracks and through-hole barrels more rapidly than conventional tin-lead. The dissolution can lead to complete removal of the copper especially in the elbow area of the through-hole; once this occurs bonding is jeopardized. Since the solder is now in contact with the laminate material, a suitable inter-metallic bond has failed to form causing reduced reliability.

This issue has been documented in several papers recently and one of the most revealing is by Craig Hamilton, Matthew Kelly and Polina Snugovsky in the June 2007 Circuits Assembly issue.  The paper is titled ”Effects of SAC Alloy Copper Dissolution Rates on PTH Processes”.

From a solder material perspective several alloys based on tin-copper have shown promise in reducing copper dissolution. Tin-copper solders with certain additives in small quantities have shown a reduction in copper dissolution rates; however tin-copper without additives has not.

Below is Figure 1, showing the reduction seen with tin-copper based solders when compared to SAC. Kester’s K100LD alloy is referenced as Sn-Cu-Ni-Bi below. In respect to copper dissolution, this alloy shows a dramatic reduction and if this were an issue it would be a preferred choice. Tin-copper based solders do have a higher melting point but in wave, selective and rework processes this is translates to only a few degrees higher than SAC solders. Wetting speeds are slightly slower with tin-copper based solders also but slight process changes such as longer contact time at the solder usually alleviates this.

Figure1. Comparing SnCu based solders to SAC305

The other advantages of lower dissolution rates for copper are reduced solder pot maintenance, less solder analysis frequency. The solder alloy remains more elementally uniform and a more consistent solder results in better process control. Less contamination of the solder is always a plus; copper dissolution also tends to increase the melting point of the lead-free solder. The higher melting point will result in a more sluggish flow of solder resulting in lesser hole-fill.

To summarize several other differences between both alloys Figure 2 shows the typical solder pot temperatures used to achieve the same results as with SAC solder. The longer contact time enables SnCu based solders to wet completely the through-hole part. The immersion depth of the board was also critical here and is best to use at least ½ of the board thickness. In thicker assemblies such as 0.093 inch or more ¾ is best. The flux used in soldering will also play an important role in hole-fill, the higher the activity and stability of the activators used in the flux formulation will enable better hole-fill. Since contact time is longer with SnCu based solders a flux designed for this application will perform better. 

Figure2. Comparing process variables of lead-free solders to leaded

The obvious advantage is also cost.  Figure 3 shows the relative cost of tin-cooper based solders versus tin-lead and SAC. The lower cost of using alloys without silver will impact initial pot fills but also operating costs long term. It must be noted that the dross rate may be higher with some alloys but alloys such as the K100LD contain dross reducer, which therefore help reduce oxides to levels close to tin-lead.

Figure3. Solder Comparison

Further information can be obtained at Kester on this alloy and several technical papers discussing its use.

About the author:
Peter Biocca is Senior Market Development Engineer with Kester in Des Plaines, IL. He is a chemist with 24 years experience in soldering technologies. He has presented around the world in matters relating to process optimization and assembly. He has been working with lead-free for more than eight years, involved in numerous national and global consortia within this time; he has assisted many companies implement lead-free successfully. He is an active member of IPC, SMTA, IMAPS and ASM. He is the author of over a 100 technical papers delivered globally. He is also a Certified SMT Process Engineer.
   

For further information please contact Peter Biocca at Kester, 972.390.1197; E-mail: click here .