Analysis of Board Defects Using Specular Reflection MicroSpectroscopy

Conduction defects can be caused by stains and contaminants on printed circuit boards. One of the key requirements of preventing similar issues from occurring repeatedly is the identification of these materials.

This article describes a qualitative analysis of contaminants found on a printed circuit board conducted using the AIM-9000 Infrared Microscope. This microscope is specifically designed for defect analysis.

Contaminants/Stains Found on Printed Circuit Board

An analysis of the stains and contaminants found on the terminal of a microSDTM 1) card was carried out. Shown in Figure 1 is an image of the microSDTM card placed on the sample stage of the AIM-9000, captured with a wide field camera. Almost the entire card is within the field of view. Using a wide field camera permits observation of a wide area around the defect with a field of view of around 10 mm × 13 mm. Positional information is shared between the wide field camera and a 15X reflection objective mirror that will be used for analysis. If a defect is detected with the wide field camera, the operator can switch to a 15X reflection objective mirror view of the same location, and only the image focus needs to be adjusted. This characteristic makes it easy to bring a defect within the analytical field of view. Another function that is included as a standard is the auto-centering function. By just double-clicking a point on the wide field image or microscope image, this function spontaneously moves a desired position on the sample stage to the middle of the field of view. This is particularly beneficial in analyzing electrical products with multiple terminals.

Wide Field Camera Image of microSDTM Card.

Figure 1. Wide Field Camera Image of microSDTM Card.

Measurement

The following are the two types of classification of the methods of analyzing contaminants on a terminal surface and other parts of a printed circuit board: attenuated total reflection (ATR) and specular reflection. In specular reflection microspectroscopy, incident light is absorbed by the sample as it passes through the sample, and a detector detects the light reflected by the circuit. This produces identical spectra to those obtained by the transmission spectroscopy, and measurements can be taken without touching the sample. This method is suitable for sample thicknesses of approximately 10 μm. Staining or contamination that is too thick can produce saturated peak tops, and staining or contamination that is too thin can result in indistinct peaks. In ATR microspectroscopy, sample thickness does not affect the peak saturation. However, the sample must be in contact with a prism for taking measurements. This contact can result in issues such as difficulty involved in achieving contact due to sample shape or losing the sample during measurement.

Measurement by Specular Reflection Microspectroscopy

Measurements were carried out by specular reflection microspectroscopy which does not require contact with the sample. In Figure 2, an image of the defect for which a specular reflection spectrum was measured is shown. The blue box shows the location of contamination on the sample. Aperture size was set to 25 μm × 25 μm. Table 1 shows the analytical conditions, and Figure 3 shows the spectrum obtained and a library spectra search result. The contaminant spectrum matched the spectrum for magnesium silicate (talc).

Microscope Image of Contaminant (blue box size: 25 μm × 25 μm)

Figure 2. Microscope Image of Contaminant (blue box size: 25 μm × 25 μm)

Table 1. FTIR Analytical Conditions

Instrument IRTracerTM 2) -100, AIM-9000
Resolution 8 cm-1
Accumulation 20
Apodization Happ-Genzel
Detector MCT

 

Library Spectra Search Result (Top: contaminant, bottom: library spectrum for talc)

Figure 3. Library Spectra Search Result (Top: contaminant, bottom: library spectrum for talc)

Measurement by Specular Reflection and ATR Microspectroscopy

As mentioned previously, good spectra may be obtained by using ATR microspectroscopy but not by using specular reflection microspectroscopy when thick samples are analyzed. Both ATR microspectroscopy and specular reflection microspectroscopy were used to analyze a thin, stain-like contaminant. Figure 4 shows a visual field image of the stain, and Table 2 lists the analytical conditions. For specular reflection measurements, aperture sizes were 25 μm × 25 μm and 50 μm × 50 μm for ATR measurements. In Figure 5, the respective spectra obtained are represented on the same graph.

Table 2. FTIR Analytical Conditions

Instrument IRTracerTM -100, AIM-9000
Resolution 8 cm-1
Accumulation 100
Apodization Happ-Genzel
Detector MCT

 

Image of Stain on Printed Circuit Board (blue box size: 25 μm × 25 μm).

Figure 4. Image of Stain on Printed Circuit Board (blue box size: 25 μm × 25 μm).

Specular Reflection Spectrum and ATR Spectrum of Stain.

Figure 5. Specular Reflection Spectrum and ATR Spectrum of Stain.

In specular reflection microspectroscopy, the peak intensity is lower largely and shows a very slight peak at around 1250 cm-1, though not definite enough for qualitative analysis. The same position measured by ATR microspectroscopy produced a more distinct spectrum as opposed to specular reflection microspectroscopy. A library spectrum search for the ATR spectrum produced the results depicted in Figure 6, with a fluorinated lubricant and a fluorine resin appearing at the top of the list. It was assumed that a thin layer of a fluorine-containing lubricant was adhered to the metal substrate.

Spectrum Search Result. (Top: stain ATR spectrum, middle: fluorinated lubricant library spectrum, bottom: fluorine resin library spectrum)

Figure 6. Spectrum Search Result. (Top: stain ATR spectrum, middle: fluorinated lubricant library spectrum, bottom: fluorine resin library spectrum)

Conclusion

Qualitative analysis was performed on contaminants found on the terminal of a microSDTM card. The wide field camera of the AIM-9000 Infrared Microscope was used to examine a wide area of the sample and determine the contaminant position for analysis.

The choice of using specular reflection microspectroscopy and ATR microspectroscopy of analysis depending on contaminant shape helps the operator to get better spectra.

1) microSDTM is a trademark or a registered trademark of SD-3C, LLC in the United States, other countries or both.

2) IRTracerTM is a trademark or a registered trademark of Shimadzu Corporation in Japan, other countries or both.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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