Metallography Capability and Methodology at Electro-Spec Inc.

ESI has multiple analytical tools available to satisfy specific customer requirements along with military and industrial standards.  Among these are Cross-Sectioning capabilities, Metallography, Scanning Electron Microscope, (SEM), and Energy Dispersive Spectroscopy, (EDS).  These tools are also used in conjunction with plating wisdom to troubleshoot problematic plating deposits on varying materials.

Cross-sectioning, having been in use since the 19th century, is one of the oldest tools in a competent plater’s toolbox.  This involves mounting a sample in a material that can then be ground and polished, (i.e. 2-part epoxy mounting material), for further investigation.  Standard cross-sectioning techniques provide basic information such as multiple plating layer thicknesses.  Beyond standard cross-sectioning techniques, micro cross-sectioning can provide determination of defects in the micro-structure of deposit and substrate for root cause/failure analysis.  Parts can also be mounted in conductive epoxy in order to be analyzed with SEM/EDS for a more thorough and detailed plating thickness and elemental analysis.

A Metallograph is used for understanding metallurgy at the microstructure level of a given material.  It provides an intense bright field in a thin, highly reflective focal plane that is necessary to determine grain structures and grain boundaries of base and plated metals at 50x-1000x.  An operator can use different optical filters, prisms, and effects including Differential Interference Contrast, (DIC), which can be used for further defect analysis.  For a skilled operator, a Metallograph is a quick and easy tool to use for troubleshooting activities.

The SEM is a non-optical microscope that uses energized electrons to reflect off of the surface of the sample to create an image.  A metallic sample will reflect back good imagery without any prior preparation.  Non-metallic items will morph and charge up without any prior preparation.  This preparation would include a metal/carbon sputter coater.  SEM detection can be made to be extremely sensitive to surface contamination and topography due to the 3D imaging and large magnification factor available.  It is exceptionally useful when used in conjunction with X-ray spectroscopy.  Cross-sections can be utilized in the SEM/EDS but must use conductive media with the mounting material.

As previously mentioned, SEM/EDS is an X-ray spectroscopy used with an SEM.  This provides elemental identification but cannot identify molecular structure except in very rare instances.  SEM/EDS is extremely useful for FOD, (Foreign Object Debris), identification and analysis by interrogating individual particles at high magnification, (up to 10,000x).  SEM/EDS can also be used for elemental surface mapping.  ESI utilizes this ability for FOD contamination/dispersion.

With ESI’s metallurgical tool set, up to and including SEM/EDS, we are able to provide our customer and supplier base with advanced troubleshooting and failure analysis on a variety of parts.  ESI has a track record of assistance with defect investigations, root cause analysis, and process optimization.

Using a series of steps, ESI has the capability to process defect samples thoroughly beginning with standard stereo-microscope visual inspection, which transitions into cross-sectioning in conjunction with the Metallograph, and if necessary, further cross-sectioning utilizing SEM/EDS.  This failure analysis road map provides ESI’s customer and supplier base with various data points and images for enhanced conclusions.

ESI is able to fully exploit this enhanced capability by having a staff of skilled and knowledgeable individuals who perform the analysis.  ESI’s staff has the ability to make determinations based on experience that places us at a top level for surface finishing analysis and investigation.


 

X-Ray Fluorescence (XRF)

X-Ray Fluorescence (XRF) is a fast, convenient, and non-destructive method used for material analysis, elemental composition, and plating thickness testing for all types of materials. This measurement method is recognized in several industry standards including the B568-98 ASTM standard. XRF requires no use of chemicals and sample preparation is not necessary. XRF provides an accurate thickness measurement in as little as 15 seconds. This happens by measuring the fluorescent (secondary) X-ray emitted from a sample after it has been excited by a primary X-ray source. Since all of the elements present on the sample have special characteristics that are unique, this allows the XRF to be able to identify each element and makes it an excellent source for qualitative and quantitative analysis of material composition and thickness.

Electro-Spec utilizes XRF technology in its inspection processes to determine plating deposit thicknesses, as well as the composition of the plating. Using XRF, up to three layers of a deposit can be routinely measured. Limitations of this method are dependent on the substrate. For example, measuring Au over Ni over Cu alloy substrates is done relatively easily.  Conversely, measuring “like” or similar metals to the substrate like Cu over brass is difficult because of the similar elemental composition of the deposit over the base material.

Plating thicknesses can be measured in a wide range from 5-500µ” depending on the number of layers and their density. Units of measure typically are in micro-inches and microns. Thicknesses are reported with mean, range, standard deviation, and Cp/Cpk values if applicable. An accuracy of 95% within a computed confidence interval is possible. Electro-Spec has the ability to perform GR&R studies as well as in-house cross-sectioning correlation studies with other thickness testing equipment and XRF machines. Electro-Spec XRF capabilities also include alloy composition analysis on plating and un-plated substrates.

Technology/Process/Equipment

Electro-Spec utilizes Fischerscope XRF technology for inspection of deposit thicknesses and material identification. During this process, a heated cathode emits electrons in the X-ray tube. These electrons are accelerated by a high voltage supply. The energy of the electrons is then converted to electromagnetic radiation.  A large percentage of this is X-ray radiation. This constitutes the primary X-ray beam. The radiation then passes through the optical imaging device known as the collimator. The collimator is what allows the electromagnetic radiation to focus on the measurement area of the substrate. The atoms in the measurement area of the substrate then emit their own x-rays. A radiation detector uses a proportional counter to convert the x-ray count into data that will be used to compute a spectrum of intensity. The amount of fluorescent x-rays generated is proportional to the thickness of the deposit.

Samples are staged inside the unit and the specific program file is selected by the operator.  Once the proper program file is selected and the samples are in the proper position relative to collimator size, shape, and part geometry – measurement begins.

Upon completion of the measurement, the unit produces a thickness and composition report that is used for analysis and disposition of samples. Statistical data is also captured for process control and continuous improvement.

How Does Electro-Spec Ensure XRF Accuracy?

Issue – XRF accuracy with similar metal compositions between the base substrate and subsequent plating layers.

  • Standards calibration and base material corrections are part of our process
  • Electro-Spec has the ability to check multiple layers of plating over multi-layer substrates, composite substrates, & alloy substrates. A typical example would be Au over Ni over a brass substrate.  Au over Ni over Cu strike over Cu substrate, however, potentially confounds the Cu strike with any Cu alloy substrate. Other limitations of this method include the total number of plating layers and the thickness of each layer.
    • Alternative methods for thickness verification (dissimilar substrate, coupons, or even cross-section) are utilized by Electro-Spec if necessary

Issue – The right size collimator on the XRF Machine.

  • Understanding the importance/significance of collimator size as it relates to part geometry
  • The collimator of the XRF directs the primary x-ray beam to the material under test. Collimators vary in size and shape and are dependent on the geometry and size of the test sample.
    • Standard protocol by Electro-Spec inspection personnel is to ensure that the right size collimator is used for XRF verification to improve the accuracy of measurement
    • Micro-miniature parts should utilize a small collimator to ensure accuracy and prevent interference. Conversely, a larger part should use a larger collimator to provide more appropriate measurements for the larger surface area.

Issue – Interference of the X-ray beam that distorts the calculated value of the measurement

  • The orientation of the parts in the x-ray chamber and beam interference is critical to ensure the most accurate results
  • Parts must be oriented in such a manner to allow the beam to reach the detector unobstructed. This makes the orientation and position of the part inside the XRF critical to achieving accurate results.
  • Best practices are such that the surface being tested should be flat if circumstances allow. Test sample surfaces that are concave, convex, knurled, or threaded, present a unique challenge that should be avoided when possible.