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Improving Solderability and Corrosion Resistance for Final Finishes, Part 1

Introduction

The process of forming a reliable solder joint between the component and the printed circuit board is paramount with respect to the manufacturing of robust circuit board assemblies. For example, an electronic connection between circuits using a through-hole is typically carried out by coating the through-hole walls and other conductive surfaces of a printed wiring board with hot, molten solder to make electrical connections by wetting and filling the spaces between the conductive through-hole surfaces and the leads of electrical components which have been inserted through the through-holes.

Soldering inconsistencies (e.g., inconsistent or weak adherence to the conductive surfaces) are often the result of difficulties in keeping the conductive surfaces of the printed circuit board clean and free of tarnishing and corrosion prior to and during the soldering process. There are several techniques to protect the solderability of the printed circuit board and prevent soldering inconsistencies which have been developed.

The most common involves the deposition of a coating of metal or a combination of metals on the conductive surfaces of the printed circuit board. The deposited metal coatings are often referred to as “final finishes.” Common final finishes include, for example, electroless nickel (EN), electroless palladium (EP), electroless nickel/immersion gold (ENIG), elec troless nickel/electroless palladium/immersion gold (ENEPIG), immersion silver, and electroless nickel/electroless palladium (ENEP). However, there has been new research with respect to protecting solderable finishes from corrosion and in the preservation of solderability. Several published research reports detail the issue of creep corrosion. One new development that may show promise both in preserving solderability and minimizing creep corrosion is the use of self-assembled monolayers.

Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto clean metal surfaces have been the focus of research chemists and engineers for several years. These molecules have shown promise as a way to control oxidation of active metals such as copper and silver. In addition, there is speculation that these SAMs may also be effective in bonding with copper under certain conditions and essentially acting as an organic solderability preservative (OSP).

The molecules typically possess a functional group that has an affinity for the substrate, also known as a head group, and a tail group. In forming a self-assembled monolayer, the head groups of molecules chemisorb to the substrate, arraying the tail groups to form a dense assembly that extends from the surface of the substrate. Known head groups include thiols, silanes, and phosphonates. In many applications, the tail group of the molecule is functionalized to provide the resulting monolayer with desired properties relating to, for example, wetting adhesion, chemical resistance, biocompatibility, and the like. Due to the strong affinity of the thiol head group to metal substrates, alkanethiols have often been used in the formation of self-assembled monolayers. Alkanethiol selfassembled monolayers have found applications in electronics, for example, for modifying the surface properties of metal electrodes[1].

Why is this important?

Many circuit components and printed boards used in electronic equipment are exposed to harsh environments. This description includes PCB assemblies under the hood of an automobile or in instrumentation and controls subjected to corrosive environments. The latter would apply to electrical units deployed in paper mills, clay modeling studios and other areas where sulphur in the atmosphere may come in contact with any susceptible exposed metals. A good example of corrosion is shown in Figure 1.

Electroless nickel/immersion gold coatings are not the only finishes susceptible to creep corrosion. In Figure 2, immersion silver (with soldermask) exhibits corrosion.

Specifically, the corrosion product seen here is copper sulphide. Creep corrosion is the migration of copper sulphide across the circuit. For creep corrosion to occur, there must be exposed copper (on the PCB) and sulphur in the atmosphere. Silver sulphide will also form when exposed to sulphur in the environment. There is ample documentation highlighting the vulnerability of certain solderable finishes to creep corrosion.

Robert Veale of Rockwell Automation determined that immersion silver and ENIG exhibited corrosion when subjected to Battelle Class III harsh environment and ASTM B845[1]. The IPC B-25 comb pattern was the test vehicle used in the study. The center comb pattern was biased with 5V DC and all the other areas were unbiased. There were two sets of test boards with one set having no soldermask applied and a second set with a soldermask overlaying the comb pattern conductors as described below. (The results with the soldermask applied will be presented in a future column.) The test conditions are shown in Table 1.

At the end of the 20-day test period, the test vehicles were examined for the extent of corrosion. In a future column, more information on creep corrosion prevention will be presented as well as the performance of other solderable finishes under corrosive environmental conditions[2]. What has not been determined in this study is the effect, if any, on corrosion resistance with the use of self-assembled monolayer chemistry. That will be the subject of a future column.

Summary

Corrosive environments have an adverse effect on some solderable finishes. In this edition of “Trouble in Your Tank,” the corrosion resistance of ENIG and immersion silver was discussed. Creep corrosion is a definite issue for these finishes when subjected to a corrosion environment under conditions described in ASTM B845 and Battelle Class III. Future studies will be undertaken to determine the effect if any of self-assembled molecules on solderability improvements.

References

1. C. Zhou, M. R. Deshpande, and M. A. Reed L. Jones II and J. M. Tour, “Nanoscale metal/selfassembled monolayer/metal heterostructures. ”

2. R.K. Veale, “Reliability of PCB Alternate Surface Finishes in a Harsh Industrial Environments,” SMTA, 2005.

Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here.

Editor's Note: This article originally appeared in the March 2017 issue of The PCB Magazine.