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

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@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. Solenoid Coil - Wire 34 AWG Polyester Insulated - Solder - Tinned Copper/Nickel Pins - Encapsulation Coil Design

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, -60°C to +93°C

13. Mechanical Characteristics • CTE of Core (Ferrite) • CTE of Wire (Copper) • CTE of Encapsulation • Temperature Range -60°C to +93°C

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 (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.

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 Diisocyanatefoams 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) MaterialFunctionParts 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 @ 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

32. Epoxy Encapsulation Formulations Filler Loading Levels Parts By Weight Mica 60 Al2O3 200 SiO2 120

33. Epoxy Formulations(New) MaterialFunctionParts 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 IngredientParts By Weight 828 Epoxy 50 Arcol PPG-1025 15 KF-105 .05 Total 65.05

35. Epoxy Formulations(New) Part “B” Formulation IngredientParts 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 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

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 • 5-10°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

40. Urethanes and Silicones for Pulse Transformers Conap EN-7™- Urethane MaterialParts 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 MaterialParts by weight Sylgard™184 part “A” 100 (Dow Corning) Sylgard™184 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