Sometimes, you may receive some power inductors from Digi-Key with visual cracking in their bodies. Is this an issue? How did these inductors end up damaged this way? The answer is that in most cases they’re not damaged at all.
Some power inductor cores, such as the WE-LHMI series from Würth Elektronik, is not a single, solid chunk of ferrite. They are molded powder inductors. That means a mixture of iron powder and a resin binder, which are pressed around the inductor coils.
A clean visual appearance is expected, but due to the manufacturing processes involved in molding these parts, there are limits to the core material’s ability to maintain a perfect visual appearance. Molded powder inductors have passed all industry standard tests regardless of whether they have visual defects, including testing for thermal shock, mechanical shock, terminal strength, vibration, humidity, humidity resistance, service life, etc. These defects also do not affect the reliability or performance of the products.
Here are some examples of common visual imperfections
Hairline Crack
Cracks around the core are nearly impossible to eliminate as the molding process involves curing the resin binder and iron powder together. During this process small cracks are caused by wall friction when ejecting the component. Unlike in ferrite powder inductors, minor to moderate and major
cracks may occur and they do not affect the performance products.
The length of the hairline crack should be less than one third of the ‘H’ on the left and right sides of the terminal, otherwise it is considered a defective. ‘H’ is defined by the above figure., showing the distance between the solder terminal and the side of the part.
Visual Crack on terminals
This figure shows an acceptable minor crack on the left side of the terminal.
This figure shows an acceptable major crack on right side of terminal.
The above 2 figures shows visual cracking but it is within specification and still usable as the crack is less than 1/3 of height ‘H’. All other cracks must be less than the height H to be accepted.
Crack on the surface with Marking
Any cracks appearing on the top surface of the inductor are considered unacceptable.
Crack on Pad Surface
Cracks may also occur on the bottom surface near the surface of the pad. If the crack length is less than 2mm, the part is considered acceptable.
To learn more, please check out Würth Elektronik’s application note:
ANP020: Cracks on Molded Power Inductors