2005 trfa conference epoxy, urethane, silicone: choice of encapsulant for high reliability magnetic...

2005 TRFA Conference

Epoxy, Urethane, Silicone: Choice Of Encapsulant for High Reliability Magnetic

Components

Robert O. SanchezDesign Engineer

Sandia National LaboratoriesAlbuquerque, New Mexico

(505) [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

Background

Magnetic Component Description

Electrical Characteristics

Environmental Requirements

Mechanical Characteristics

Encapsulations of Choice

2005 TRFA ConferenceIntroduction

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

Sandia National Laboratories

- Research and Development

- Weapon Programs

Lockheed Martin Corporation

Department of Energy

Sandia Suppliers


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


Encapsulated Magnetic Component Types

Sandia Has More than 100 Designs of Weapon Magnetic Components that have been Fielded in Subassemblies.


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


1200 Volt Flyback Transformer


6KV Transformer Cross-Section


Solenoid Coil- Wire 34 AWG

Polyester Insulated

- Solder

- Tinned Copper/Nickel Pins

- Encapsulation

Coil Design


Inductance (Affected by Mechanical Stress)

Resistance

Turns Ratio

Capacitance (Affected by Mechanical Stress)

Leakage Inductance (Affected by Mechanical Stress)

Electrical Characteristics


Typical Environmental Tests for Magnetics

Mechanical Shock3500 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, -60°C to +93°C


CTE of Core (Ferrite)

CTE of Wire (Copper)

CTE of Encapsulation

Temperature Range -60°C to +93°C

Mechanical Characteristics


Typical Material Selection

Epoxy for Transformers and Coils

Urethane and Silicones for Stress Sensitive Magnetics

Polyurethane Foam for Low Voltage Magnetics

2005 TRFA ConferenceEncapsulation Mold Designs


Epoxy, Urethane, Silicone: Choice Of Encapsulant for High Reliability Magnetic

Components

Howard W. ArrisMaterials Process EngineerSandia 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

Introduction

Epoxy, Silicone, Urethane

Specific Formulations

Summary


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


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


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


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


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)



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



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 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



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 ConferenceFormulations


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



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



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



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



Filler Loading Levels

Parts By Weight

Mica 60

Al2O3 200

SiO2 120


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)



Two Part Formulation

Part “A” Formulation

Ingredient Parts By Weight

828 Epoxy 50

Arcol PPG-1025 15

KF-105 .05

Total 65.05



Part “B” FormulationIngredient Parts By Weight EMI 2,4 2MHHPA 40 Total 42 Filler loading levels Mica 60OR

Al2O3 200



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

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


0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60

Z/Alox(70)

Z/Alox(90)

CTBN/DEA/GMB(70)

CTBN/DEA/GMB(90)

vis

cosi

ty (

P)

time (min)

Time/Temperature/Viscosity


Epoxy Processing

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



Conap EN-7™- Urethane

Material Parts by weight

EN-4 part “A” 100

EN-7 part “B” 18.8

Processed at room temperature




SiliconeMaterial Parts by weight

Sylgard™184 part “A” 100(Dow Corning)

Sylgard™184 part “B” 10

GMB, D32/4500 31(3M product)

Processed at room temperature


2005 TRFA ConferenceSummary

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 ConferenceAcknowledgements

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

2005 trfa conference epoxy, urethane, silicone: choice of encapsulant for high reliability magnetic...

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