A customer recently called the EMPF helpline requesting that printed circuit boards be analyzed to determine the cause of corrosion.

Over the past few years, the EMPF has helped OEMs and contract manufacturers solve many manufacturing and assembly issues. The EMPF also has an extensive battery of analytical equipment to perform detailed investigations on unique and common real world problems, such as the corrosion of PCBs.

Test Methods
In this instance, the EMPF analytical services staff recommended elemental analysis through Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEMS/EDS) to determine what kind of corrosion was encountered on the customer supplied PCBs. The breakdown of materials to their core constituents through this analysis provided great insight in the investigation of the corroded areas of the PCBs.

Modes of Corrosion
Before reviewing the results of the analysis, the EMPF examined the two most common modes of corrosion. While other types of corrosion occur, only the following were addressed:

• Oxidation – deterioration of intrinsic properties due to reactions within an environment.
• Galvanic – degradation caused by an element (water/humidity) promoting electrical contact of dissimilar metals. This type of corrosion is accelerated if there is significant difference in voltage between the metals. The electromotive potential on various metals were characterized. [1]

Results
A SEM/EDS analysis revealed, in most instances, the observed residue was composed of lead (Pb), tin (Sn) or copper (Cu), and oxygen (O) in some combination. Further, these results were confirmed with a flaky, non-metallic material. The color of the residue was blue/green for copper and white for both tin and lead. Figure 3-1 and Figure 3-2 illustrate the effects of the corrosion. Notice the high levels of the aforementioned metals.

Conclusions
Based on the results of the analysis, oxidation was the primary cause of corrosion on the PCBs. While not shown in the figures, it is worth noting that, in most instances, the PCBs were corroded near the connector interface side of the assemblies. Incidentally, humidity or some other form of moisture had a relative easy entry point at this interface. In the other case, galvanic corrosion was ruled out, since any combination of lead, copper, and tin does not form a galvanic pair. The elimination of galvanic corrosion was important since that kind of corrosion would have required a significant design change to the PCB. However, only corrosion through the oxidation process had to be addressed.

Recommendations
By far, the greatest improvement for the customer’s PCB – which was not initially being utilized – would have been to apply a protective conformal coating to the PCBs. The application of this conformal coating (acrylic, silicone, polyurethane) would serve as a barrier against adverse effects such as moisture/humidity, dust, dirt, and salt air. The EMPF suggested that the customer add a conformal coating step in the manufacturing process to improve the overall reliability. Simple, low-cost, and effective solutions, such as conformal coatings, can successfully protect sensitive electronics in harsh environments.

Many customers have taken advantage of the SEMS/EDS analytical services at the EMPF. Additional analytical services from the EMPF are also available as a resource to the electronics industry.

REFERENCE
[1] Holloman, James K. Jr., Surface-Mount Technology for PC Board Design, pp. 206-207

For more information, please contact the EMPF helpline at (610) 362-1200 or via email at helpline@empf.org.