Inductance Maximization by Mitigation of Encapsulation Stresses of PCB Embedded Ferrite Broadband Transformers

Abstract

The maximization of bandwidth in broadband magnetic devices, done reliably in mass production, has led to a strong industry interest in substrate embedded ferrite core materials with windings fabricated by photolithography. Ferrites, when embedded within a substrate, can lose significant magnetic permeability due to stress from the cured bonding adhesive. In this paper, an experimental survey is conducted, encapsulating sample ferrites in various types of bonding adhesive and measuring the change in the ferrite core's inductance. The adhesives include epoxies, cyanoacrylates, polyurethanes, and silicone sealants, and each adhesive is evaluated on curing procedural limitations, bonding strength, and resulting loss in inductance when used as a ferrite encapsulant. Though trade-offs exist, the best choice based on bonding strength and minimal loss of inductance was found to be a low shrinkage, rigid epoxy.

Inductance optimization of miniature Broadband transformers with racetrack shaped ferrite cores for Ethernet applications

“…The inductance of the transformers was found in reasonable agreement with what was predicted, though the ferrite core effective permeability can vary widely due to mechanical processing. 10,11 …”

“…However, material permeability is both dispersive and sensitive to stress and temperature, making it highly variable. 10,11 Superimposed over FIG. 3a and FIG.…”

“…However, the permeability of the racetrack shaped ferrite material is known to vary widely due to the extensive mechanical processing required to create the shape. 10,11 The predicted inductance assumes the permeability at 100kHz published by the ferrite manufacturer. The effective permeability of the specific core used in each device is not precisely known, but has been observed to vary up to 50% from the published manufacturer's value.…”