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OEMs with Pb-free solderability problems could benefit from this component and board restoration technique.

ROSA is a surface restoration technique that removes difficult-to-reduce species like metal oxides or sulfides. When it was developed, the focus was on solderability and compliance to environmental regulations. Industry trends and regulatory changes – brought on by the Montreal Protocol – drove much of the concern over environmental compliance. The result: an increase in no-clean and water-soluble fluxes, and the removal of halogenated cleaning chemistries.

Other practices such as just-in-time manufacturing and lifetime purchases of parts also influenced solderability concerns, as cycle times were critical, inventories vulnerable to oxidation and historically no-clean chemistries provided different soldering results than rosin-based activated fluxes.

One attempt at protecting bare copper was organic solderability preservative (OSP). This process of covering bare copper with an organic compound was an unproven technology to protect PCBs. It has found wider favor recently with the development of more reliable coatings and coating process improvements.¹

Re-tinning component leads was also performed, but the process was time-consuming; there was potential for damaging components, and some components were not suited for re-tinning.

A way to restore component and PCB solderability prior to use was needed. In response came the ROSA solderability restoration technique. The work, by an industry consortium involving the U.S. Army Research Laboratories, Rockwell Science Center, Seho USA, and the EMPF Center of Excellence, resulted in a benchtop restoration unit (BART) and ROSA Application Module (RAM) prototype designs.

The conclusions of the project were:

The ROSA process was successful at restoring bare PCBs given a wide range of surface oxide conditions or levels.
The BART cell used for recovering SMT and PTH components was demonstrated in two commercial endeavors.
A benchtop restoration unit (BART) and a ROSA module prototype unit were developed with four BART units and three ROSA units being deployed to four beta-sites. The EMPF was established as a ROSA restoration technology Center of Excellence able to support both commercial industry and DoD depot repair facilities.
The system’s closed-loop design meant no chemicals were consumed. This, plus studies during the project, indicated ROSA was environmentally friendly and safe to use if standard operating procedures were followed.
ROSA chemistry can damage some plastic parts, requiring any ROSA-treated materials to be cleaned.
ROSA removes difficult-to-reduce species like metal oxides through a highly reducing and corrosive aqueous solution. The solution is not active in its uncharged state. Upon application of a potential, V+3 (Vanadium III) is reduced to V+2 (Vanadium II), which oxidizes at the component or board surface, releasing electrons that reduce the metal oxide back to the metal.

The following example with Sn (IV) oxide summarizes the half-cell reactions that occur during ROSA:

The second reaction involves regeneration of the vanadium:

The cell (Figure 1) is separated by a semi-permeable membrane, permitting proton transfer but hindering transfer of the V+2 and V+3 species, where they would be further oxidized to higher valent species. Figure 2 is an example of the improved wetting obtained after ROSA.

The economies of scale of large production runs have defined the disposable part concept, thus eliminating the need to restore components or PCBs. Pb-free created a new potential market for the ROSA technique for a number of reasons. In the case of high-Sn, Pb-free solders like SAC 305, the surface tension is greater than for standard SnPb37 eutectic (548 mN/m vs. 481 mN/m, respectively ^{2, 3}), indicating this solder type would wet less. Another trait of many Pb-free alloys is the tendency to readily tarnish because tin is often the major component, and tin oxides are more stable than those of lead (i.e., lower heat of formation: SnO -69 cal/mol; SnO2 -143 cal/mol; PbO -53 cal/mol; PbO2 -67 cal/mol).⁴ These characteristics of Pb-free solders present a challenge when it comes to solderability.

Some companies have gone completely Pb-free. Others are maintaining lead and Pb-free lines. Companies contractually committed to lead, or in markets where such restrictions do not apply (e.g., military), are finding it difficult to locate lead-plated components and boards. These companies are now relying on aging inventories to maintain raw material streams. The potential need to recover such inventories is high. Re-tinning components can be time-consuming and not cost-effective.

High-rel OEMs, and commercial customers that have transitioned to Pb-free and found solderability problems, could benefit from this component and board restoration technique.

References

M. Carano and J. Hunt, “Improved Organic Solderability Preservatives for Mixed Metal Finishes,” CircuiTree, May 2004.
B. Toleno, Implementation of Pb-free Solder Paste in a Real Manufacturing Environment, company white paper, November 2005.
Z. Moser, W. Gasior and A. Dêbski, “Experimental Results of Studies of the Surface Tension and Density for Data Base of Pb-Free Soldering Materials,” CODATA Conference, November 2004.
Handbook of Chemistry and Physics, 65th edition, CRC Press, 1984-1985.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.