XRF Calibration Standards: options for coating thickness measurements

Utilizing quality reference standards is critical to maximizing the performance of many x-ray fluorescence analyzers. While offered for many applications (RoHS testing, geological interests, petrochemical analysis, etc) a majority of reference standards that the in-house XRF standards laboratory of Eastern Applied Research develops are for coating thickness measurement applications.

One of the questions we will ask buyers of new reference standards is if they want plated or foil pieces. Some may not realize what those options are...

When it comes to purchasing XRF calibration standards for coating thickness analysis you should know what type of format you are seeking as there are two styles to choose from: plated or foil. Both are commonly used throughout industry and provide reliable results.

When determining the best fit for you consideration must be made to the model of XRF analyzer the standards will be used on, constituents of interest and target thicknesses, whether the calibration curve will be created with or without use of fundamental parameters (FP) and how the reference materials will be used. ​

Example of Plated XRF Standards (Cu/Fe)

Foil Thickness of ENi being placed on base material

About Foil Standards: this style is created with the foil of desired material and thickness attached to a stainless steel foil holder. The foil is then temporarily secured on top of a plastic chip containing the desired base material. When a range of thicknesses are required, the user will have several foils of difference thicknesses to span the desired range. 

Pros of Foil Thickness Standards:

• Versatility: foil standards can be placed over any desired substrate, thereby increasing the range of applications
• Multi-Layer Flexibility: they are generally stacked on top of one another for a reliable method of multi-layer analysis. A common question regarding stacking foils is whether or not the air gaps influence results - the answer is no; any air gaps are so small that they are negligible in the thickness reading.
• Usable Curves: when using stacked foils for multi-layer analysis there are essentially three useful calibration curves, two single layer curves and one multi-layer curve. By nature, one of the single layers is the first plated layer and can be used to monitor the plating in-process. 

Cons to Foil Thickness Standards:

• Fragile: while more versatile, the foils themselves are fragile and must be handled with care to avoid punctures. Generally speaking, foil standards are not ideal for everyday use or in environments where they might come into contact with a lot of dirt as they cannot be cleaned. 

About Plated XRF Standards: these are created by electroplating the desired material of specific thickness onto a permanent base material. When a range of standards is required, the user will have standard pieces of several different thicknesses to span the desired calibration range (layers and base material in one piece for each thickness). 

Plated Thickness Standard Pros:

• Durability: compared with foil pieces, plated standards tend to be more durable and tolerate heavy handling. However, they are still prone to damage when the measured surface comes into contact with an object that scratches it. Even the user touching the surface can cause damage due to the dirt from the environment or oils on a user's hands.
• Extended Life: fortunately, if scratches or damages do occur on a plated standard, all is not lost. In most cases, the standard can be polished with a fine grit polish and remarked to reflect the new, lower, value. 

Cons to Plated Thickness Standards:

• No Versatility: with the coating permanently fixed onto a base material, a user will need to have a different plated standard for each thickness and base material combination they require.
• Potential Variation: it is well documented that certain plated metals such as lead, gold, silver and tin (to name a few) will migrate over time; thus potentially changing the thickness of the reference material. This change takes a long time but can easily be monitored by having reference standards certified yearly by an accredited laboratory (like ours). 

These are just a few thoughts to help you begin the discussion with Eastern Applied Research's XRF standards laboratory when it is time to purchase new reference standards. Request a phone call or email by a lab technician to discuss further.

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