Energy Dispersive Spectroscopy (EDS) is an elemental identification technique that uses X-rays emitted from specimens to identify elemental species. Typically, EDS is used in correlation with scanning electron microscopy (SEM). EDS has the capability to produce qualitative and quantitative data, allowing users to determine what elements (e.g. tin and lead) are present. It can also to determine weight and atomic percentages of those elements. In addition, EDS systems have the capability to image the source of the X-rays allowing for elemental mapping (also known as Xray mapping). When coupled with scanning electron microscopy, EDS users are capable of identifying defects that reduce the performance and reliability of electronic packages and assemblies.

How EDS Works
To create EDS spectra, characteristic X-rays must be excited from the surface of a sample. These X-rays are produced when an incident electron beam strikes a sample. The electron gun of a scanning electron microscope is the source of an electron beam. Initially, the incident electrons, stemming from the beam, strike the specimen creating a collision with the atom's many inner shell electrons. This collision causes the ejection of the samples inner shell electrons. Outer shell electrons, which carry much more energy than inner shell electrons, seize the opportunity to reduce their energy by replacing the ejected electron from the inner shell. As the outer shell electron moves to the lower energy inner electron shell, a characteristic X-ray is emitted. The X-ray's energy is a function of the atomic number of the element investigated.

A silicon-lithium detector receives the emitted X-rays and transfers its energy into an X-ray analyzer. The analyzer produces amplified electric signals that are then processed into spectra. These X-ray spectra can have multiple peaks correlating to multiple electron shell interactions. For example, electron interactions with the element gold may produce spectra from M electron and L electron shell X-ray emissions.

Quantitative analysis of elements present is accomplished by determining the number of characteristic X-rays emitted from a sample. With computer correction of absorbency and fluorescence, quantification results are quite accurate, producing small relative error.

Uses for EDS in Electronics Failure Analysis
EDS can be a powerful tool for scientists and failure analysis engineers. New innovations in EDS software and hardware have expanded the capabilities of EDS systems and allows users flexibility with the collection and reporting of data. Some of these capabilities include electron imaging and dimensioning, X-ray imaging and mapping, electron depth simulation, automated peak deconvolution, and individual automated particle analysis.
When examining electronics, EDS can be utilized for tasks such as:

• Elemental identification of microstructure
• Contaminate identification
• Diffusion mapping
• Determination of solder composition
• Plating thickness dimensioning

Inconsistent plating of gold or silver can allow underlying metallization to be exposed to oxidation and reduce solderability of the pad or component. X-ray imaging provides visual proof of this defect. Gold contamination of tin-lead solders enbrittles the solder joint and reduces reliability. EDS can identify the quantity of gold in the solder and map its position. Additionally, EDS identification of resin contamination at trace-barrel separation in multilayer PWB's conveys problems with the hole drilling processes.

Limitations of X-ray Spectra

Scientists and engineers at the EMPF use the SEM/EDS on a daily basis to perform failure analysis and other investigations. If you have any questions regarding microscopy, failure analysis or EDS, call the EMPF Helpline at610-362-1320.