Pros and Cons of Analytical Techniques Used for Residue Analysis

The tools used to analyze residue-related electronics assembly failures are varied and each is capable of providing useful data. While it is important to understand the capabilities of each method, it is just as important to know the limitations. Following is a high-level overview of the pros and cons for five common test methods. 

FTIR – Fourier Transform Infrared Spectroscopy

FTIR provides specific information about chemical bonding and molecular structures.

FTIR is particularly useful for:

• Identifying the molecular structure of organic compounds for contamination analysis;

• Identification of organic particles, powders, films, and liquids (material identification).

Potential drawbacks of FTIR:

• A large sample library of materials must be available to match material signatures for identification of unknown residues or contaminants;

• Because it is a qualitative analytical technique, FTIR is not able to provide any information regarding elemental content, nor is it able to quantify ionic content of a sample.

SEM / EDX – Scanning Electron Microscope

Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing X-rays emitted by the matter in response to being bombarded with charged particles.

SEM / EDX is well-suited for:

• Analyzing a metal surface for tin whiskers or looking at the grain boundaries and intermetallic formation of a solder joint;

• Elemental analysis or chemical characterization of a sample with high levels of ionic elements (carbon, oxygen, copper, silver, aluminum, etc.) – including relative elemental composition information.

However, SEM / EDX has some drawbacks:

• It is a potentially destructive test because items may need to be cut to size, sputter coated, or otherwise modified before being placed in the chamber;

• Thin film materials, such as chloride and sulfate, can be volatilized under sublimation and carried away in the vacuum - not detected.

XRF – X-Ray Fluorescence

X-ray fluorescence is a quick, non-destructive analytical technique that uses X-rays to excite electrons in the test sample.

XRF is particularly useful for:

• Non-destructive elemental analysis of metal or plastic materials, e.g. for RoHS compliance;

• Determining the elemental makeup of a material;

• Measuring plating thicknesses.

However, XRF has some drawbacks:

• It is not well suited to thin film metals;

• It is not well suited to organic and ionic materials.

HPLC - Ion Chromatography

High-performance (AKA high-pressure) liquid chromatography is used frequently in biochemistry and analytical chemistry to separate, identify, and quantify ionic and organic compounds. Ion-exchange chromatography (AKA ion chromatography) is a process that allows the separation of ions and polar molecules based on the charge properties of the molecules. These methods can be used for almost any kind of charged molecule including large proteins, small nucleotides and amino acids.

These methods are particularly useful for:

• Determining which ions / organic residues are present on a corrosion site ( even a location that shows no dendrite but is shorting due to stray voltage) and whether the residues are corrosive;

• Use in protein purification, water analysis, and quality control.

Drawbacks for HPLC and ion chromatography:

• This is not necessarily a drawback, but it is important: the critical step is to get the ionic residue into a solution for analysis. Traditional bag extractions are limited to extracting large areas of the board and components when failures and residue areas are typically localized in nature. This is why Foresite developed the C3 (Critical Cleanliness Control) localized extraction system.

ROSE Testing per IPC TM-650 2.3.25

Foresite uses an Aqueous Technologies Zero-Ion G3 Ionic Contamination Tester for ROSE testing. We also use the 1978 Omega Meter 600R system for general process ionic analysis.

ROSE testing is useful for:

• Obtaining a gross measure of general surface ionics;

• Determining pass/fail in order to meet old contractual requirements.

Limitations of ROSE testing include:

• Dilution when relatively small test samples are submerged in the large volume of solution. This can lead to minute conductivity changes in the test solution and erroneous good test results;

• High sensitivity to environmental CO2, which is absorbed into the system, reacts to become carbonate and is very detectable with these systems;

• “Deadband” issues for the Omega Meter. This occurs when the solution is cleaned to a resistivity level that exceeds the measurable range of the resistivity probe in the system. In this case, much of the contamination present will not be measured until the resistivity drops to a measurable range.