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2005 TRFA Conference
Epoxy, Urethane, Silicone: Choice Of
Encapsulant for High Reliability Magnetic
Components
Robert O. Sanchez
Design Engineer
Sandia National Laboratories
Albuquerque, New Mexico
(505) 844-3130
[email protected]
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,
for the United States Department of Energy under contract DE-AC04-94AL85000.
Outline
2005 TRFA Conference

Background

Magnetic Component Description

Electrical Characteristics

Environmental Requirements

Mechanical Characteristics

Encapsulations of Choice
2005 TRFA Conference
Introduction
 Magnetic components such as transformers, solenoid coils, and
inductors are required for various DOE and DOD programs.
 Component application requirements, materials compatibility,
small package size requirements, resistant to severe
environmental shock, high voltage, and material aging affects are all
considered when designing a magnetic component.
Background
2005 TRFA Conference

Sandia National Laboratories
- Research and Development
- Weapon Programs

Lockheed Martin Corporation

Department of Energy

Sandia Suppliers
Magnetic Component Description
2005 TRFA Conference

Transformers
- Vary in size from 0.25 in3 to 1.25 in3

Inductors
- Vary in size from 0.063 in3 to 2 in3

Coils
- Vary in size from 0.25 in3 to 0.75 in3

Design for Weapon Application
- Severe Environments
2005 TRFA Conference
Encapsulated Magnetic
Component Types
Sandia Has More than 100 Designs of Weapon Magnetic Components that have been
Fielded in Subassemblies.
High Voltage Transformer Design
2005 TRFA Conference

6KV Power Transformer
- Ferrite 2616 Pot Core
- Wire 42 AWG Polyester Insulated
- Wire 34 AWG Polyester Insulated
- Kraft Paper Insulation
- Solder
- Phenolic Microballoon filled Polysulfide
Stress Relief Medium
- Encapsulation
1200 Volt Flyback Transformer
2005 TRFA Conference
6KV Transformer Cross-Section
2005 TRFA Conference
Coil Design
2005 TRFA Conference

Solenoid Coil
- Wire 34 AWG
Polyester Insulated
- Solder
- Tinned Copper/Nickel Pins
- Encapsulation
Electrical Characteristics
2005 TRFA Conference

Inductance (Affected by Mechanical Stress)

Resistance

Turns Ratio

Capacitance (Affected by Mechanical Stress)

Leakage Inductance (Affected by Mechanical Stress)
Typical Environmental Tests
for Magnetics
2005 TRFA Conference
Mechanical Shock
3500 G shock amplitude, 1ms duration
Sinusoidal Vibration
Hz 50-2000-50, 5Hz to 2000Hz
(.001G2/Hz to .4G2/Hz, traverse time
30 min.) acceleration 30G
Steady State
Acceleration
100G, 10 seconds
Temperature Cycles
100 - 200 cycles, -60°C to +93°C
Mechanical Characteristics
2005 TRFA Conference

CTE of Core (Ferrite)

CTE of Wire (Copper)

CTE of Encapsulation

Temperature Range -60°C to +93°C
Typical Material Selection
2005 TRFA Conference

Epoxy for Transformers and Coils

Urethane and Silicones for Stress Sensitive Magnetics

Polyurethane Foam for Low Voltage Magnetics
Encapsulation Mold Designs
2005 TRFA Conference
2005 TRFA Conference
Epoxy, Urethane, Silicone: Choice Of
Encapsulant for High Reliability Magnetic
Components
Howard W. Arris
Materials Process Engineer
Sandia National Laboratories
Albuquerque, New Mexico
(505) 845-9742
hwarris@ sandia.gov
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,
for the United States Department of Energy under contract DE-AC04-94AL85000.
Outline
2005 TRFA Conference
 Introduction
 Epoxy, Silicone, Urethane
 Specific Formulations
 Summary
Introduction
2005 TRFA Conference

Sandia has developed a number of encapsulation formulations

Commercially available formulations sometimes utilized

Use commercial available constituents- minimize variability

Formulations can be generally categorized into epoxies,
urethanes, silicones

Choice of encapsulant determined by: component type,
operating parameters, 40 years manufacturing experience

Epoxy and silicone formulations utilize fillers to alter material
properties
Introduction
2005 TRFA Conference
 Component design, fabrication techniques, core materials,
component functionality- dictate encapsulant,
epoxy, urethane, foam, silicone
 Development of formulations consists of:
- Identifying component types for each formulation
- Completing component evaluations
Epoxy for Power Transformers
2005 TRFA Conference
 Complete impregnation is required
 Voids in encapsulant can cause HVB during testing and
operation
 Filled formulations, process at elevated temperatures to
reduce viscosity
 Sufficient pot life to facilitate impregnation of secondary
winding
Note: It is important to balance TIME/TEMPERATURE/VISCOSITY
Epoxy for Power Transformers
2005 TRFA Conference
 Failure modes after encapsulation may include cracking of
encapsulant or ferrite cores, and breakage of windings
 Encapsulation stresses due to cure shrinkage, CTE
differences can lead to component failure
The only encapsulants that have been used
successfully for this type of component are filled epoxy
formulations
Urethanes and Silicones for Pulse
Transformers
2005 TRFA Conference
 Obtaining complete impregnation of pulse transformers is not
as critical as with power transformers
 Sandia pulse transformers vary in size from 1in3 to .25in3
 Typical design might consist of: 5 turn primary winding of
28AWG and a secondary winding of 75 turns of 38AWG on a
torroidal core
 Core materials: molypermalloy powder or ferrite (ferrite cores
are stress sensitive)
Urethanes and Silicones for Pulse
Transformers
2005 TRFA Conference
 Urethane encapsulants historically used, more recently filled
silicone resin
 Silicone formulations filled with glass micro balloons (GMB)
- GMB helps reduce high CTE
 Urethane formulation has outstanding electrical properties;
however, a short pot life
 Silicone formulation has long pot life; however, we must
account for high CTE during cure and “poisoning” associated
with silicone
Polyurethane Foam for Low Voltage
Magnetics
2005 TRFA Conference
 Low voltage magnetics include: pulse transformers, current
viewing resistor transformers, inductors, and coils
 Utilize various core types, materials, winding configurations,
package configurations
 Obtaining complete impregnation of low voltage transformers
is not required
 Cure stress of encapsulant must be minimized
Polyurethane Foam for Low Voltage
Magnetics
2005 TRFA Conference
 Polyurethane foams induce least amount of stress during
encapsulation and cure of all of our resin systems
 Foams are used to facilitate packaging requirements and
mitigate shock during testing and use
 10-14 lb/ft3 most commonly used, Toluene Diisocyanate foams
used for 30 years
 Mold design enabling complete flow are critical to robust
package
Polyurethane Foam for
Low Voltage Magnetics
2005 TRFA Conference
 Environmentally conscious foams, ploymeric diisocyanate
developed, component evaluations started
 Foam components are manufactured at one of our production
facilities, formulations and processing will not be presented here
2005 TRFA Conference
Formulations
Epoxy Encapsulation Formulations
2005 TRFA Conference
 Epoxy formulations used for high voltage power transformers
historically filled with mica, more recently aluminum oxide and
fused silica investigated
 4X Mica, (Mineralite Corp.), T-64 Al2O3, ALCOA (Aluminum
Corporation of America), Teco-Sil- 44CSS, SiO2, (C-E Minerals)
 Use of filler reduces CTE (coefficient of thermal expansion)
-reduces stress on encapsulated units
 Striking a balance between filler loading levels, pot life, viscosity
are critical to this application
Epoxy Encapsulation Formulations
2005 TRFA Conference
 Aluminum Oxide and Silica loading levels were determined
experimentally
 Units are encapsulated, cured, and sectioned to analyze
impregnation into the secondary winding
 Examined under 20x magnification
 Impregnation on these units was excellent
Epoxy Encapsulation Formulations
2005 TRFA Conference
828/Mica/Z (historically used)
Material
Function
Parts by Weight
Shell Epon 828
Bis-A epoxy
60
Mica
Filler
40
Ancamine “Z”
Curing agent
12
Epoxy Encapsulation Formulations
2005 TRFA Conference
 The following processing temperatures have been determined
to be optimum for this formulation and these components
 828 epoxy resin @ 71°C
 Mica, Al2O3 or SiO2 @ 107°C
 Curing agent “Z” @ 54°C
 Molds with transformers vacuum dried at 71°C, .2-3 Torr, 2
hours minimum
Epoxy Encapsulation Formulations
2005 TRFA Conference
Filler Loading Levels
Mica
Parts By Weight
60
Al2O3
200
SiO2
120
Epoxy Formulations
(New)
2005 TRFA Conference
Material
Function
828 Epoxy
Bis-A Epoxy
MHHPA
Catalyst
(Methyl Hexahydrophthalic
Anhydride)
Arcol
PPG-1025
Polyol
Flexibilizer
EMI 2,4
Curing agent
(2-Ethyl 4-Methylimidazole)
KF-105
De gassing aid
(epoxy modified silicone fluid)
Parts By Weight
50
40
15
2
.05
Epoxy Formulations
(New)
2005 TRFA Conference
Two Part Formulation
Part “A” Formulation
Ingredient
828 Epoxy
Arcol PPG-1025
KF-105
Total
Parts By Weight
50
15
.05
65.05
Epoxy Formulations
(New)
2005 TRFA Conference
Ingredient
Part “B” Formulation
Parts By Weight
EMI 2,4
MHHPA
2
40
Total
42
Filler loading levels
Mica
OR
Al2O3
60
200
Epoxy Formulations
(New)
2005 TRFA Conference
The following process parameters have been determined to be
optimum for this component and resin formulations
 Fillers are dried at 107°C, 4 hrs., then stabilized at 71°C
 828 Epoxy, MHHPA, and PPG-1025 preheated to 60°C
 EMI 2,4 at room temperature
 Molds with transformers vacuum dried at 71°C, .2-3 Torr, 2
hours minimum
Epoxy Processing
2005 TRFA Conference
 Typical loading levels may be as high as 40 volume percentresulting in high viscosity formulations
 Processing temperature is essential to obtaining complete
impregnation
Time/Temperature/Viscosity
 Low processing temperature produces a high viscosity
formulation resulting in voids or incomplete impregnation
 High processing temperatures results in shortened pot life that
may lead to incomplete impregnation
Time/Temperature/Viscosity
2005 TRFA Conference
400
350
viscosity (P)
300
250
200
150
Z/Alox(70)
100
Z/Alox(90)
CTBN/DEA/GMB(70)
50
CTBN/DEA/GMB(90)
0
0
10
20
30
time (min)
40
50
60
Epoxy Processing
2005 TRFA Conference
 Determining optimum processing parameters requires
experience and the understanding of the effect of
Time /Temperature/Viscosity
 5-10°C can drastically affect formulation viscosity
 Heat loss must be minimized to maintain optimum viscosity
 Molds are filled and degassed at 1-3 Torr for 2-3 minutes
 Molds are returned to atmosphere and the cure is initiated
Urethanes and Silicones for Pulse
Transformers
2005 TRFA Conference
Conap EN-7™- Urethane
Material
Parts by weight
EN-4 part “A”
100
EN-7 part “B”
18.8
Processed at room temperature
Molds are filled and degassed at 1-3 Torr for 2-3 minutes
Urethanes and Silicones for Pulse
Transformers
2005 TRFA Conference
Silicone
Material
Parts by weight
Sylgard™184 part “A”
(Dow Corning)
100
Sylgard™184 part “B”
10
GMB, D32/4500
(3M product)
31
Processed at room temperature
Molds are filled and degassed at 1-3 Torr for 2-3 minutes
2005 TRFA Conference
Summary
Encapsulation of magnetic components is essential if they are to
survive the environmental requirements. Selection of the
encapsulant, either epoxy, urethane, or silicone is dependent on
the type of transformer. Choice of the correct formulation is
critical in providing high reliability components.
2005 TRFA Conference
Acknowledgements
Sandia National Laboratories
Manny O. Trujillo - Formulation, Process Development
Patrick Klein - Materials Characterization
Scott Campin - Materials Characterization
Mil-Spec Magnetics
Shelly Gunewardena- CEO
Tony Gunewardena - President