Need to determine whether a solder alloy has the proper concentration of silver? Do you ever want to know whether assemblies are RoHS compliant? It can all be done with one single method: X-ray Fluorescence.

What is X-ray Fluorescence?

When determining the elemental composition of materials, X-ray Fluorescence (XRF) is among the best techniques. It is non-destructive and relatively inexpensive. XRF determines the chemistry of a sample by exciting a primary X-ray source, then measuring the fluorescent or secondary X-ray emitted from the sample.

As with other analytical techniques, each of the elements present in a sample produces a set of characteristic fluorescent X-rays that is unique to that element. XRF can both perform quantitative elemental analysis and measure thickness in an integrated fashion. It can also examine each component on an assembly individually through a small spot analysis method. XRF can provide a meaningful and accurate measure of the RoHS compliance of a product, including identifying any areas of non-compliance.

XRF Applications

• Quantitative analysis of solid, powder, liquid, or paste-like materials

• Thickness and element composition measurements of individual coatings in coating systems with up to twenty-four different elements

• Substrate materials analysis through coating systems

XRF Advantages

• Non-destructive analytical method

• Analyzes small solder joints

• Focal distance of 0.2mm spot analysis

• Able to differentiate layer thicknesses

• With known stack-up structure (e.g. ENIG, ENIPIG), able to program the system to measure each layer

• Automated system to look at one joint of every component to ensure RoHS compliance

• Designed to exclude interval leaded constituents

• Does not include the interfacial aspects of the components, only the external solder surfaces: joint and board layers

Our use of XRF technologies

Here at Foresite, we have utilized our XRF system to aid in failure analysis in a variety of scenarios, including solder de-wet/non-wet, board fabrication/plating issues, determining alloy concentrations, and evaluating and confirming electrochemical migration (dendrites). We recently used XRF to measure the plating thickness and concentration (by percentage) of the tin and tin/lead plating over copper and Kovar wire. We designed this experiment to look for 100% tin, 99% tin, and 95% tin with the balance for each group of lead. Using XRF, we were able to show the plating thickness and concentration.

We also use XRF regularly to measure the plating thickness on PCBs and component leads to determine whether the plated surfaces meet the requirements defined in the print/drawings for the part number. XRF can be used to determine whether the wave solder or HASL pot has increased above the acceptable levels of copper when soldering OSP or at the bare copper board level.