An Introduction to Contamination Analysis
A common question asked of laboratories in the third-party testing world is – What is this unknown material on my product? This question is then typically quickly followed by others…
- How can I figure out “exactly” what the substance is?
- Where is this substance coming from?
- Should I be worried about this substance in respect to long-term reliability of my product?
- Can I / Should I clean off the substance?
For answers to any of these questions, you will likely need a Contamination Analysis which begins with a set of basic evaluations that on most occasions will answer the “What is this?” question. The protocol contains four parts:
- Visual Examination
- Organic Analytical Testing by Fourier Transform Infrared (FTIR) Spectroscopy
- Elemental Analytical Testing by Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDS)
- Ionic Analytical Testing by Ion Chromatography (IC)
Contamination Analysis Methodologies
The start to any test investigation should always be a comprehensive inspection with the naked eye and a stereomicroscope (or similar).
A great deal of information can be gathered by simply looking at the contaminant (“unknown”) material. When performing your inspection, here are some general questions you should ask yourself:
- What “form” is the substance – solid, liquid, “gel”, etc.? What texture?
- What color is the substance?
- Is the substance in a specific region of the test specimen or is it randomly distributed?
- Is this an isolated occurrence for this specimen type or is it “wide spread”?
Although apparently non-important at the start of the analysis, the answers to these questions can help hone in on a final answer once the analytical testing has been completed.
A real world example: An analysis of an unknown solid, “bluish” material finds that the substance is copper based and inorganic. Analysis of the substance via SEM/EDS detects copper and oxygen, but cannot distinguish between copper (II) oxide – CuO – and copper (II) hydroxide – Cu(OH)2, as hydrogen cannot be detected by EDS.
If some reference book reading is performed, one would find that the hydroxide is blue / blue-green in color, while the oxide is black / brown in color. From here, the “simple” visual characteristic of color ends up answering the “What is this?” question.
Fourier Transform Infrared (FTIR) Spectroscopy
Organic Analytical Testing by FTIR is a standard analytical technique that is used to identify, qualitatively, the presence of organic-based materials. The results of an FTIR analysis obtained are in spectral form, as shown in Figure 1 (below), with the peaks representing different chemical bonds within the sample. By obtaining this spectrum, various new questions can then be answered in an attempt to identify the contaminant material.
- Is the substance organic or inorganic?
- If organic, what characteristic chemical bonding peaks are present?
- Were any spectral “matches” found when comparing this spectrum to one’s spectral libraries?
- Do the matched spectra “make sense” for the issue at hand?
This first of two analytical techniques can provide a significant amount of information about the substance, especially if it is organic based, and at times, it may even answer the “What is this?” question directly.
A real world example: The spectrum in Figure 1 was obtained for a material that was causing an open circuit in a switch contact. When comparing this spectrum to global spectral libraries, a match for phenoxy resin was found. When investigating further, it was found by the originator that one of the assembled parts contained a phenoxy-based release agent that was being transferred to the contacts during assembly.
Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDS)
Elemental Analytical Testing by SEM/ESD is actually a combination of two techniques into one. The SEM portion is a visual technique that allows one to view a sample in a slightly different manner than with the naked eye or with an optical microscope. The image created by the SEM is grayscale and is based off of the electrons that are present at the location being inspected – with heavier elements showing up brighter and lighter elements showing up darker.
At times, contaminant material that is not visible to the naked eye is visible via SEM. For instance, materials that are optically transparent are not transparent in SEM. Also, thin residues or residues that can only be seen at high magnification are more visible in SEM than with optical microscopy. Figure 2 (below) shows an example of just such a thin residue observed surrounding a printed circuit board pad.
Switching sides on SEM/EDS, the EDS analysis is a standard analytical technique that is used to identify elemental species that are present in a given area of interest. The results obtained are in spectral form, as shown in Figure 3 (below), similar to the FTIR results shown in Figure 1 (above), with each peak representing an individual element.
From this spectrum, quantitative results can be obtained for any elemental species present (above carbon on the periodic table) in the area of interest. Further, this additional information can then be used to help answer more questions in an attempt to identify the contaminant material.
- Is the substance organic or inorganic?
- Is the material metallic?
- Are certain elements represented in greater concentrations?
- Do the elements detected suggest a specific chemical compound or compounds?
As with FTIR, this analytical technique can also provide a significant amount of information about the substance, and at times, it can also answer the “What is this?” question directly.
A real world example: A spectrum was obtained from an unknown substance found in a condensate pan. The scan is dominated by the presence of aluminum, sulfur, and oxygen. With aluminum acting as the cation in a salt-like compound, sulfur and oxygen would likely be present as a sulfate anion, suggesting that the unknown substance is aluminum sulfate. When investigating further, the condensate pan was found to be formed from aluminum, which would suggest that something in the condensate liquid is likely corroding the pan to form the corrosion product.
Ionic Analytical Testing by Ion Chromatography (IC)
IC is an analytical method that is used to identify and quantify ionic compounds. An ionic compound is a chemical compound consisting of a cation, a positively charged ion, and an anion, a negatively charged ion, which are bonded by electrostatic forces. Salts, such as NaCl, are ionic compounds. When dissolved or in the presence of moisture or humidity, ionic compounds can become electrically conductive. For this reason, ionic contamination on electronics is particularly problematic as increased ionic content can lead to low resistance and short circuits. Thus, by quantitatively identifying ionic contamination, IC can be used to gather more information and identify a contaminant.
- Is the substance ionic?
- What are the ionic species present? And how much of each are present?
- Is the quantity detected likely to cause a problem?
- Do the ionic species detected suggest a possible source?
In most cases, the results obtained from each of the Contamination Analysis segments – Visual Examination, FTIR, SEM/ESD, and IC – are used in combination with the other to come to a singular result. It is not entirely uncommon for one technique to provide more information than the others; however, the use of all in tandem better paints the whole picture.
If more information is needed in respect to the “What is this?” question, the Contamination Analysis can be expanded to include other analytical techniques.