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Are you having problems encapsulating Magnetic Components?

 

Basic Considerations for Encapsulating Magnetic Components using Epoxy or Polyurethane Systems

 

Things to keep in mind:

  • Polyurethanes tend to have shorter pot life and, as a rule, exhibit more shrinkage during cure
  • Unfilled product tend to have higher CTE’s which may result in broken wires, cracked cores and / or a change in filled products exhibit less shrinkage, lower CTE and better thermal conductivity.
  • In most cases, the pot life (working time) can be adjusted by a slight alteration in the chemistry, with Epoxy systems having a wider range in this regard.

 

 

The following are some of the issues that can impact part performance:

Formulation

  • Consider pot life, mixed viscosity, shrinkage during cure, coefficient of thermal expansion (CTE), adhesive properties
  • Ideally what you need:

 

Property

Impact

Ideal

Pot Life (useable time)

Working time

As long as possible

Filler Content

Shrinkage, Thermal conductivity

As high as possible while yielding suitable viscosity

Filler Type

Dielectric strength, Heat dissipation.

Good dielectric strength and thermal conductivity

Hardener vapour pressure

De-airing

No hardener fumes in trapped bubbles

Exotherm

Shrinkage, Embedded components

Low exotherm = less shrinkage, no damage to embedded components (i.e. powder cores inductors etc.)

CTE

Embedded components

As low as possible. No damage to embedded components, reduced effect on bonding to the case etc.

 

Component Design

  • Appropriate insulation for the operating voltage
  • Eliminate or minimize sharp edges and pointed corners
  • Wherever possible, direct contact with heat sinks
  • If it is a high voltage device, stay away from carbon black pigments, iron oxide pigmented systems are much better


Mould Design

  • No Leakage Allowed. If the mould leaks, air will be sucked in when changing from vacuum to atmosphere resulting in bubbles in the casting. Use “O” rings if necessary.
  • Plenty of vent openings especially in areas (i.e. corners) where air tends to trap.
  • If it is a closed mould, provide a good size reservoir. The reservoir should be located and sized so that it will stay liquid for the longest time. This will allow the encapsulant to replenish itself as it shrinks during gellation thereby eliminating shrink marks.
  • If it is a closed mould, locate the pour hole over a sparsely populated area with to minimize the amount of disturbance to the stream of material entering the cavity.
  • The mould cavity, including the vents, should be highly polished or Teflon coated for good part finish and easy de-airing.


Key Steps in Casting / Potting

  • Make sure the device to be cast or potted is thoroughly dry, free of moisture.


Epoxy:

  • Thoroughly dry the components to be potted or encapsulated in an oven at 80 - 100oC
  • Pre-heat the mixed material to a temperature that will result in the lowest viscosity while leaving ample pot life
  • Pre-heat the mould to a suitable temperature (slightly higher than the resin temperature) in order to maintain the material mix temperature during pouring.
  • Vacuum de-gas the mix for 2 - 3 minutes at 1-3 Torr (1.3 - 4 Millibars) after the initial foaming while under vacuum breaks.
  • Fill the mould, being careful to pour into one spot, or slowly through the pour hole, allowing the material to rise slowly, pushing the air ahead of it as it is rising.
  • Once filled, place the tool into a vacuum chamber and degas for 2-3 minutes at 1 - 3 Torr then return to atmosphere.


Polyurethane:

  • Polyurethanes are moisture sensitive. Since these products will pick up moisture from the surrounding air, open containers only for as long as necessary. Make sure the containers a purged with Nitrogen before storage.
  • Thoroughly dry the components to be potted or encapsulated in an oven at 80 - 100oC
  • By their nature, Polyurethanes exhibit shorter pot life and working time than Epoxies, therefore it is not feasible to significantly preheat either the mould or the mixed material. However, experience has shown that slightly warming both the mix and the mould is very helpful.
  • Prepare the mix and de-air under 1 - 3 Torr for only 2 - 3 minutes after the foaming collapses, not to use up too much pot life.
  • Fill the mould, using the technique recommended for epoxy (above), and de-air for 2 - 3 minutes after which return to atmosphere. Be sure not to vacuum too long at this stage in order to avoid blemishes.


Other Considerations:

  • Fast curing products will generate more exotherm than slower curing systems.
  • Unfilled systems will shrink considerably more then filled systems.
  • Room temperature curing systems rely on exotherm (heat generated during the reaction) for cure. The higher the volume of material in a given location, the higher the exotherm and the quicker the cure will be in that location. This must be considered especially if the casting involves areas of vastly different encapsulant thicknesses.
  • Too high an exotherm can damage internal components in the casting.
  • Heat cured system rely on external energy (heat) + exotherm for curing. In some cases, starting low and increasing the temperature at slow intervals can help in producing a better, stress free casting.
  • Some products require “post cure” to fully develop full physical properties.
  • Be careful not to inadvertently thermal cycle parts which are not fully cured by subjecting them to significant variations in temperature prior to full cure.


It is always best to discuss the details of each project with your supplier and obtain the best product for your device. This is far better than purchasing an off - the - shelf product and making it fit the process. The number of trade-offs will negate any initial cost savings.

 

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