When a printed circuit board assembly (PCBA) is returned with corrosion, the analyst needs to find the source of the contamination causing the corrosion in order to eliminate it. Contamination can come from a variety of sources such as manufacturing operations, packaging, installation, and the environment.
A variety of methods can be used to determine the composition of the contaminant. Once the composition is known, possible sources can be determined. By talking through an example, this paper explains the process analysts use to identify a source of contamination on a corroded PCBA.
Several corroded PCBAs were returned from a customer. The PCBA was heavily corroded, but only in spots near one edge. Images of the corrosion from two PCBAs are shown in Figures 1 and 2.
In both figures, there is an open circuit in the lower conductor. The soldermask had been removed from the conductor, and the exposed copper was heavily oxidized. On both PCBAs, there was evidence of conduction between the conductors as indicated by burned epoxy between them. This can indicate that the corrosion is ionic in nature, and that the environment allowed a thin layer of water to adhere to the PCBA.
Elemental Identification
A scanning electron microscope (SEM) was used to image the corroded area. The SEM is usually equipped with an energy dispersive x-ray detector (EDX). The EDX system determines the elemental composition of the sample. The EDX system cannot determine whether two elements are chemically combined, but knowing which elements are present can lead to an understanding of chemical composition.
Figure 3 shows the SEM image of the contaminated area. This is the area where the EDX data for the corrosion, shown in Figure 4, was obtained.
Figure 4 shows a typical EDX spectrum of an area of soldermask near the corroded area and a spectrum of the corrosion. The blue spectrum is from an area of uncorroded solder mask, the red spectrum is the corroded area. The two spectra show marked differences.
The solder mask (blue spectrum) contains silicon (Si), which is used to control the viscosity of the liquid solder mask prior to curing. It also contains barium (Ba) and sulfur (S), which are combined as barium sulfate as a fire retardant. All of these elements are expected in the solder mask, and are shown by the blue spectra.
The contaminated areas have copper (Cu) and chlorine (Cl), which were not found in the solder mask. The copper, which originates from the copper conductors, has been etched and chemically removed from the metallic conductors and is now deposited on the solder mask. Chlorine is a contaminant that is not supposed to be present on the PCBA.
Chlorine can be found in a variety of places in the environment. Salt contains chlorine. Chlorine is present in many chemicals used to manufacture the PCBA. However, these chemicals should not be a source of chlorine because they should be rinsed off during washes that the PCBA goes through during its manufacturing process. Chlorine was not found on the soldermask sample, so the PCBA manufacturing process can be discounted.
Chlorine has been used historically during the soldering operation. Chlorinated compounds were added to solder fluxes to facilitate good solder connections. These compounds quickly removed any oxide layer from the copper, allowing for a good solder connection. During the switchover from eutectic solder to RoHS compliant solders, a change was made to the type of flux used. Instead of chlorinated rosin fluxes, low solids fluxes containing organic acids were used in the flux to clean the copper surfaces for soldering. Organic acids are normally decomposed by heat during the soldering process into innocuous products. These PCBAs were manufactured after the introduction of RoHS compliant solders and were built using low solids organic-based fluxes.
The corrosion was isolated to only one area of the PCBA. If the chlorine was present in the air from salt water mist or chlorine vapors, it would be present over the entire PCBA. Chlorine is used in a variety of industrial processes (and in swimming pools) leading to chlorine and chlorine compounds in the air in these areas.
A visual examination of the PCBA revealed that several components near the corroded areas had been reworked. The visual examination revealed that the solder flux residues were not from a low solids, organic acid based flux.
These reworked sites were examined in the SEM using the EDX system. The solder flux residues were found to have chlorine in them. The presence of the chlorine in the flux indicated that an old type of activated flux had been used during the reworking of the components. The source of the flux at the assembly house was found and eliminated as a corrective action. A single rework station had a bottle of the old liquid flux that had not been purged from the factory.
If the chlorine had not been traced to the flux used during the rework process, the necessary corrective action could not have taken place. Corrective actions would have been taken in several possible areas to eliminate the contamination. In this particular case, the activated rosin flux was a holdover from the previous process and had not been purged from this rework station. Other actions to eliminate the contamination would have been ineffective. Corrective actions must be based on evidence gathered during the investigation. A shotgun approach to eliminating the chlorinated flux may not have caught the bottle at the single rework station and would not have eliminated the issue.
How does EDX work?
The EDX system captures x-rays from the sample that are generated when electrons change their energies to fill up a vacant spot within an atom. It tallies the number of x-rays captured at each energy level. Each electron surrounding the nucleus of an atom has a specific energy. These energies differ from one element to another. Because of these differing energies, the atomic composition of the sample can be determined.
The electrons that the SEM shoots at the sample will interact with the electrons of each atom. In some cases, this interaction will cause an atom’s electron to escape the atom, leaving a vacancy. If a second electron from the atom with a higher energy fills the vacancy, a particular amount of energy must be released. This released energy takes the form of an x-ray of a specific energy, or wavelength. By capturing and measuring these x-rays, we can assemble a list of the elements present. The software of the EDX system displays the data on a chart. The energy of the x-ray is shown on the horizontal axis and the number of the counted x-rays is shown on the vertical axis.
