Coating Guide

Core Finish

All standard toroidal cores, composite toroid cores, rectangular toroidal cores and balun cores are provided with a protective coating. In addition a number of other geometries will come standard with coating on non-mating surfaces. The geometries with protective coating on nonmating surfaces include bus bar cores, HC/IC and HD/ID cores. Please refer to the individual part datasheet for information if core finish is included as well as coating type.

Toroid sizes with outside diameter of 0.20in./5.1mm (“T20” part prefix) and smaller are typically coated under vacuum with Parylene C. The larger cores are coated with a two color epoxy finish with colors being used to identify material. It is important to note Parylene C is not compliant with European Union Directive 2002/95/EC on Restriction of Hazardous Substances (ROHS) for Halogens. It may be possible to substitute to RoHS compliant parylene coating, please contact the factory or local representative for further details.

The epoxy coated parts are RoHS compliant and UL approved for Flame Class UL94V-0 per file #E140098 (S). A copy of the Yellow card can be downloaded or located on the UL website.

All finishes have a minimum dielectric strength of 500 Vrms at 60 Hz. Toroidal cores can be double or triple coated for greater dielectric strength. Plastic core caps are available for “T400” and “T520” part series for very heavy gauge wire or when greater dielectric strength is required. Parts can also be supplied without coating. Contact local sales assistant for further details.

All finishes resist most cleaning solvents, however, extended exposure to certain solvents may have detrimental effects. All coatings will tolerate elevated temperatures for a limited time for Reflow solder, IR or Vapor Phase soldering operations. The typical solder temperatures encountered are 200°C to 240°C for up to 25 seconds of exposure time. Parylene coated parts are most susceptible to damage if exposed too long to elevated temperature. The coating can soften and possibly “blister” under worst case exposure. The epoxy coated parts will tolerate solder temperatures for up to 2 minutes and not suffer any long-term damage.

Uncoated cores may be subject to surface oxidation. Micrometals recommends that all uncoated cores should be sheltered from high humidity or moisture. It is suggested that bare cores are handled with gloves in order to avoid formation of surface blemishes. Surface oxidation or discolorations are cosmetic and will not affect core performance.

Micrometals iron powder cores have an organic content and undergo thermal aging.  When cores are exposed to or generate elevated temperatures, a permanent decrease in both inductance and quality factor (Q) will gradually occur.  The extent of these changes is highly dependent on time, temperature, core size, frequency, and flux density. It is essential that these properties are considered in any design operating at or above 75ºC. Iron powder cores tolerate temperatures down to -65ºC with no permanent effects.

In high power applications where core loss is contributing to the total temperature, a decrease in quality factor will translate into an increase in eddy current losses which will further heat the core and can lead to thermal runaway.  Designs where core loss exceeds copper loss should be avoided. Hysteresis losses are unaffected by the thermal aging process.

A thermal aging predictor has been incorporated into our inductor design software.  Please do not hesitate to contact our application engineers for specific application questions and free design consultation at


Trademark Notice

Color combinations used on toroidal shapes are recognized by the United States Patent and Trademark Office as registered trademarks of Micrometals Incorporated.  Use of the color combinations identifying material type listed on this website and published catalogs are exclusive to Micrometals Inc., further details can be located at the U.S. Trademark Office.