Project Activities and Issues

1
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 rosanch_at_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.
2
Outline

• Background
• Magnetic Component Description
• Electrical Characteristics
• Environmental Requirements
• Mechanical Characteristics
• Encapsulations of Choice

3
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.

4
Background

• Sandia National Laboratories
• - Research and Development
• - Weapon Programs
• Lockheed Martin Corporation
• Department of Energy
• Sandia Suppliers

5
Magnetic Component Description

• 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

6
Encapsulated Magnetic Component Types
Sandia Has More than 100 Designs of Weapon
Magnetic Components that have been Fielded in
Subassemblies.
7
High Voltage Transformer Design

• 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

8
1200 Volt Flyback Transformer
9
6KV Transformer Cross-Section
10
Coil Design

• Solenoid Coil
• - Wire 34 AWG
• Polyester Insulated
• - Solder
• - Tinned Copper/Nickel Pins
• - Encapsulation

11
Electrical Characteristics

• Inductance (Affected by Mechanical Stress)
• Resistance
• Turns Ratio
• Capacitance (Affected by Mechanical Stress)
• Leakage Inductance (Affected by Mechanical
  Stress)

12
Typical Environmental Tests for Magnetics

• 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 100G, 10 seconds
• Acceleration
• Temperature Cycles 100 - 200 cycles, -60C to
  93C

13
Mechanical Characteristics

• CTE of Core (Ferrite)
• CTE of Wire (Copper)
• CTE of Encapsulation
• Temperature Range -60C to 93C

14
Typical Material Selection

• Epoxy for Transformers and Coils
• Urethane and Silicones for Stress Sensitive
  Magnetics
• Polyurethane Foam for Low Voltage Magnetics

15
Encapsulation Mold Designs
16
Epoxy, Urethane, Silicone Choice Of Encapsulant
for High Reliability Magnetic Components

• Howard W. Arris
• Materials Process Engineer
• Sandia National Laboratories
• Albuquerque, New Mexico
• [phone]
• hwarris_at_ 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.
17
Outline

• Introduction
• Epoxy, Silicone, Urethane
• Specific Formulations
• Summary

18
Introduction

• 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

19
Introduction

• 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

20
Epoxy for Power Transformers

• 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

21
Epoxy for Power Transformers

• 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

22
Urethanes and Silicones for Pulse Transformers

• 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)

23
Urethanes and Silicones for Pulse Transformers

• 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

24
Polyurethane Foam for Low Voltage Magnetics

• 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

25
Polyurethane Foam for Low Voltage Magnetics

• 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

26
Polyurethane Foam for Low Voltage Magnetics

• 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

27
Formulations

28
Epoxy Encapsulation Formulations

• 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

29
Epoxy Encapsulation Formulations

• 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

30
Epoxy Encapsulation Formulations

• 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

31
Epoxy Encapsulation Formulations

• The following processing temperatures have been
  determined
• to be optimum for this formulation and these
  components
• 828 epoxy resin _at_ 71C
• Mica, Al2O3 or SiO2 _at_ 107C
• Curing agent Z _at_ 54C
• Molds with transformers vacuum dried at 71C,
  .2-3 Torr, 2
• hours minimum

32
Epoxy Encapsulation Formulations

• Filler Loading Levels
•   Parts By Weight
• Mica 60
• Al2O3 200
• SiO2 120

33
Epoxy Formulations(New)

• Material Function Parts By Weight
• 828 Epoxy Bis-A Epoxy 50
• MHHPA Catalyst 40
• (Methyl Hexahydrophthalic
• Anhydride)
• Arcol Polyol
• PPG-1025 Flexibilizer 15
• EMI 2,4 Curing agent 2
• (2-Ethyl 4-Methylimidazole)
• KF-105 De gassing aid .05
• (epoxy modified silicone fluid)

34
Epoxy Formulations(New)

• Two Part Formulation
•  
• Part A Formulation
•  
• Ingredient Parts By Weight
•  
• 828 Epoxy 50
• Arcol PPG-1025 15
• KF-105 .05
•  
•  
• Total 65.05

35
Epoxy Formulations(New)

• Part B Formulation
• Ingredient Parts By Weight
•  
• EMI 2,4 2
• MHHPA 40
•  
•  
• Total 42
•  
• Filler loading levels
•  Mica 60
• OR
• Al2O3 200

36
Epoxy Formulations(New)

• The following process parameters have been
  determined to be optimum for this component and
  resin formulations
• Fillers are dried at 107C, 4 hrs., then
  stabilized at 71C
• 828 Epoxy, MHHPA, and PPG-1025 preheated to
  60C
• EMI 2,4 at room temperature
• Molds with transformers vacuum dried at 71C,
  .2-3 Torr, 2
• hours minimum

37
Epoxy Processing

• Typical loading levels may be as high as 40
  volume percent-
• resulting 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

38
Time/Temperature/Viscosity
39
Epoxy Processing

• Determining optimum processing parameters
  requires
• experience and the understanding of the
  effect of
• Time /Temperature/Viscosity
• 5-10C 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

40
Urethanes and Silicones for Pulse Transformers

• 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

41
Urethanes and Silicones for Pulse Transformers

• Silicone
• Material Parts by weight
• Sylgard184 part A 100
• (Dow Corning)
• Sylgard184 part B 10
• GMB, D32/4500 31
• (3M product)
• Processed at room temperature
• Molds are filled and degassed at 1-3 Torr for 2-3
  minutes

42
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.
•  

43
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