An analysis of four common methods starts with FTIR.

Beginning this month, we will discuss the applications of four typical analysis techniques and their pros and cons in regard to understanding electronics residues. The techniques include FTIR (Fourier transform infrared spectroscopy), SEM/EDX (scanning electron microscopy/electron dispersive b-ray), XRF and ion chromatography.

Eric W. Weisstein explains

In its simplest form, a Fourier transform spectrometer consists of two mirrors located at a right angle to each other and oriented perpendicularly, with a beamsplitter placed at the vertex of the right angle and oriented at a 45° angle relative to the two mirrors (Figure 1). Radiation incident on the beamsplitter from one of the two “ports” is then divided into two parts, each of which propagates down one of the two arms and is reflected off one of the mirrors.

The two beams are then recombined and transmitted out the other port. When the position of one mirror is continuously varied along the axis of the corresponding arm, an interference pattern is swept out as the two phase-shifted beams interfere with each other.

FTIR provides specific information about chemical bonding and molecular structures, making it useful for analyzing organic materials and certain inorganic materials. Chemical bonds vibrate at characteristic frequencies, and when exposed to IR radiation, they absorb the radiation at frequencies that match their vibration modes. Measuring radiation absorption as a function of frequency produces a spectrum that can be used to identify functional groups and compounds.


Applications and limitations include identifying the molecular structure of organic compounds for contamination analysis; identification of organic particles, powders, films, and liquids (material identification); and the large (20,000 item) library of materials to match material signatures, which permits observations on the pure material and aged materials (for changes).

The assessment of unknown visible residues on electronics involves looking at samples from the top of soldermask, laminate and metallization. When samples are analyzed, they tend to include these other materials. The analysis shows the presence of combined materials, and no separation is done using this technique; and while it can identify a family, it cannot quantify the residue amount. Residue assessment will reveal that a white flaky or gooey material around a solder joint is a flux, but that’s where it stops. With 70% of the industry using no-clean fluxes, this provides slightly more information than in known pretesting.

Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. This column appears monthly.

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