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How Conformal Coating Can Kill
© 2004 - 2007 
2010 
What Do Conformal Coatings Do?
© 2004 - 2007 
2010 
What Are the Baddies? 
Meets the Spec 
Physical Objects 
(insects, leaves, hair etc.) 
Liquids (Short...
© 2004 - 2007 
2010 
How Do We Choose Conformal Coatings?
© 2004 - 2007 
2010 
How Do We Choose Conformal Coatings?
© 2004 - 2007 
2010 
How Do We Choose Conformal Coatings?
© 2004 - 2007 
2010 
How Do We Choose Conformal Coatings?
© 2004 - 2007 
2010 
oConformal Coating Market will be $2.3 billion by 2013 
oCAGR of 8.9%, with Medical / Biomedical seei...
© 2004 - 200107
© 2004 - 2007 
2010 
oBreak Components 
oConcentrate Contaminants 
How To Catch the Killer
© 2004 - 2007 
2010 
oPrimary concern is stress due to CTE mismatch 
oVery sensitive to thickness 
oSimilar specs in IPC22...
© 2004 - 2007 
2010 
oCracked Components 
oEspecially glass MELF Diodes 
oCracked Solder Joints 
oPrimarily cylindrical co...
© 2004 - 2007 
2010 
oProblem 1: Does Not Consider Low Standoff Components 
oQFN standoff can be less than five mil (125 m...
1©4 2004 - 200107 
Conformal Coating and QFN 
o Care must be taken when using conformal coating over QFN 
o Coating can in...
© 2004 - 2007 
2010 
oDip coated assembly with BGA technology 
oPassed ALT (-40C / 100C) 
oFailing quickly in the field
© 2004 - 2007 
2010
© 2004 - 2007 
2010 
oAll amorphous materials have a glass transition temperature (Tg) 
Hard/Brittle  Soft/Rubbery 
Break...
© 2004 - 2007 
2010 
Tg Behavior 
oNear the glass transition temperature (Tg), CTE changes more rapidly than modulus 
oCha...
© 2004 - 2007 
2010 
Expansion and Modulus at Tg 
0.010.101.0010.0035455565758595105Temperature (oC) Storage Modulus (MPa)...
© 2004 - 2007 
2010 
Rise in Stress 
High stresses generated due to CTE increase before modulus decrease
© 2004 - 2007 
2010 
During use, excessive solder deformation causes adjacent leads to short together. Failures only occur...
© 2004 - 2007 
2010 
Elastic Modulus - DMA 
Datasheet - Specification (8.7 MPa, 1260 psi) 
Tg = 15°C
© 2004 - 2007 
2010 
Glass Transition Temperature Tg ≈ 5 to 15°C 
Coefficient of Thermal Expansion - TMA 
Below Tg CTE – 1...
© 2004 - 2007 
2010 
oBetween -30C / 40C 
oActual field conditions 
oThick/nominal coating 
oBulbous/starved solder 
Finit...
© 2004 - 2007 
2010 
oTraditional Conformal Coatings are NOT hermetic 
oMoisture will diffuse through 
oRequires good adhe...
© 2004 - 200107 
Sulfur Attack of Coated Hybrid 
o Silicone coating, 
ceramic hybrid 
o Used in industrial controls 
o Cus...
© 2004 - 2007 
2010 
Encapsulated Hybrid (cont.) 
oSilicone coating was removed using Dynasol 
oVisual inspection revealed...
© 2004 - 2007 
2010 
Sulfur Corrosion Sites
© 2004 - 2007 
2010 
Elemental Analysis 
oSulfur and silver peaks detected 
oNote: No migration products were observed
© 2004 - 2007 
2010 
oSeveral field issues reported in thick film resistors 
oUse silver as the base conductor (cost, stab...
© 2004 - 200107 
o Sulfur attack of silver occurs at the 
abutment of the glass passivation layer 
and the resistor termin...
© 2004 - 2007 
2010 
SMT Resistors (Observations) 
oObservation that a majority of failures occurred in assemblies that we...
© 2004 - 2007 
2010 
oOutdoor application 
oElevated failure rates after 12 months 
oExposed to mercaptan 
oSpecific desig...
© 2004 - 2007 
2010 
oSulfur vapor has a substantial heat of solution in silicone and can be sorbed by them to concentrati...
© 2004 - 2007 
2010 
oTraditional, wet-applied conformal coatings can induce failures (‘kill’) as much as they protect 
oD...
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How Conformal Coating Can Kill

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Conformal Coating is applied to circuit cards to provide a dielectric layer on an electronic board. This layer functions as a membrane between the board and the environment. With this coating in place, the circuit card can withstand more moisture by increasing the surface resistance or surface insulation resistance (SIR). With a higher SIR board, the risk of problems such as cross talk, electrical leakage, intermittent signal losses, and shorting is reduced.

This reduction in moisture will also help to reduce metallic growth called dendrites and corrosion or oxidation. Conformal coating will also serve to shield a circuit card from dust, dirt and pollutants that can carry moisture and may be acidic or alkaline.

There are several types of conformal coating materials and the selection of one for your application must consider several variables. Silicones, polyurethanes, acrylics, epoxies and some of the newer hydrophilic materials offer many options to the user. However, the incorrect selection can result in huge problems with your CCA. For example, if you use silicone as your conformal coating material in a high sulfur environment, the silicone absorbs the sulfur and enhances the probability of silver migration on chip resistors and other forms of corrosion.

Published in: Technology

How Conformal Coating Can Kill

  1. 1. How Conformal Coating Can Kill
  2. 2. © 2004 - 2007 2010 What Do Conformal Coatings Do?
  3. 3. © 2004 - 2007 2010 What Are the Baddies? Meets the Spec Physical Objects (insects, leaves, hair etc.) Liquids (Short-Term: Water, etc.)
  4. 4. © 2004 - 2007 2010 How Do We Choose Conformal Coatings?
  5. 5. © 2004 - 2007 2010 How Do We Choose Conformal Coatings?
  6. 6. © 2004 - 2007 2010 How Do We Choose Conformal Coatings?
  7. 7. © 2004 - 2007 2010 How Do We Choose Conformal Coatings?
  8. 8. © 2004 - 2007 2010 oConformal Coating Market will be $2.3 billion by 2013 oCAGR of 8.9%, with Medical / Biomedical seeing largest growth oAcrylic Dominates (70% of the Market) o$1 to $2 per medium size board (no masking) oCheap, Dry Quickly, Easy Rework oSilicone is Dominant Second Choice (20% of the Market) o$6 to $10 per medium size board (no masking) oMore Robust, Higher Temperature Conformal Coating Legacy
  9. 9. © 2004 - 200107
  10. 10. © 2004 - 2007 2010 oBreak Components oConcentrate Contaminants How To Catch the Killer
  11. 11. © 2004 - 2007 2010 oPrimary concern is stress due to CTE mismatch oVery sensitive to thickness oSimilar specs in IPC2221, J-STD-001, and IPC-HDBK-830 Breaking Components: Stress vs. Thickness
  12. 12. © 2004 - 2007 2010 oCracked Components oEspecially glass MELF Diodes oCracked Solder Joints oPrimarily cylindrical components Failure Modes
  13. 13. © 2004 - 2007 2010 oProblem 1: Does Not Consider Low Standoff Components oQFN standoff can be less than five mil (125 microns) oProblem 2: Does Not Consider Glass Transition Temperature (Tg) oDirty little secret: Companies do a horrible job of measuring coating thickness
  14. 14. 1©4 2004 - 200107 Conformal Coating and QFN o Care must be taken when using conformal coating over QFN o Coating can infiltrate under the QFN o Small standoff height allows coating to cause lift o Hamilton Sundstrand found a significant reduction in time to failure (-55 / 125C) o Uncoated: 2000 to 2500 cycles o Coated: 300 to 700 cycles o Also driven by solder joint sensitivity to tensile stresses o Damage evolution is far higher than for shear stresses Wrightson, SMTA Pan Pac 2007
  15. 15. © 2004 - 2007 2010 oDip coated assembly with BGA technology oPassed ALT (-40C / 100C) oFailing quickly in the field
  16. 16. © 2004 - 2007 2010
  17. 17. © 2004 - 2007 2010 oAll amorphous materials have a glass transition temperature (Tg) Hard/Brittle  Soft/Rubbery Breaking Components -65C 125C Silicone Urethane Acrylic
  18. 18. © 2004 - 2007 2010 Tg Behavior oNear the glass transition temperature (Tg), CTE changes more rapidly than modulus oChanges in the CTE in polymers tend to be driven by changes in the free volume oChanges in modulus tend to be driven by increases in translational / rotational movement of the polymer chains oIncreases in CTE tend to initiate before decreases in modulus because lower levels of energy (temperature) are required to increase free volume compared to increases in movement along the polymer chains Polymer Science and Technology, Chapter 4: Thermal Transitions in Polymers, Robert Oboigbaotor Ebewele, CRC Press, 2000
  19. 19. © 2004 - 2007 2010 Expansion and Modulus at Tg 0.010.101.0010.0035455565758595105Temperature (oC) Storage Modulus (MPa) 020406080100120140 CTE (ppm/oC) Storage ModulusCTE
  20. 20. © 2004 - 2007 2010 Rise in Stress High stresses generated due to CTE increase before modulus decrease
  21. 21. © 2004 - 2007 2010 During use, excessive solder deformation causes adjacent leads to short together. Failures only occur when excessive conformal coating is present Case Study – Solder Spreading Cross-section cut Conformal Coating Solder Lead Field failures could not be duplicated using –55 to 125°C thermal cycling
  22. 22. © 2004 - 2007 2010 Elastic Modulus - DMA Datasheet - Specification (8.7 MPa, 1260 psi) Tg = 15°C
  23. 23. © 2004 - 2007 2010 Glass Transition Temperature Tg ≈ 5 to 15°C Coefficient of Thermal Expansion - TMA Below Tg CTE – 170 ppm/°C Above Tg : CTE – 340 ppm/°C
  24. 24. © 2004 - 2007 2010 oBetween -30C / 40C oActual field conditions oThick/nominal coating oBulbous/starved solder Finite Element Analysis – Results oApprox. 2000 cycles to move solder over 3 mil (75 microns)
  25. 25. © 2004 - 2007 2010 oTraditional Conformal Coatings are NOT hermetic oMoisture will diffuse through oRequires good adhesion to the circuit board oBubbles/Voids/Delam can drive micro-condensation oCan make it electrochemical migration MORE likely Concentrate Contaminants
  26. 26. © 2004 - 200107 Sulfur Attack of Coated Hybrid o Silicone coating, ceramic hybrid o Used in industrial controls o Customer reported failures after 12 to 36 months in the field o X-ray identified several separations ‘Good’ hybrid ‘Bad’ hybrid
  27. 27. © 2004 - 2007 2010 Encapsulated Hybrid (cont.) oSilicone coating was removed using Dynasol oVisual inspection revealed black corrosion product throughout the hybrid oMost severe in areas with no solder or solder mask covering silver thick film traces oAttack through the solder mask in some locations
  28. 28. © 2004 - 2007 2010 Sulfur Corrosion Sites
  29. 29. © 2004 - 2007 2010 Elemental Analysis oSulfur and silver peaks detected oNote: No migration products were observed
  30. 30. © 2004 - 2007 2010 oSeveral field issues reported in thick film resistors oUse silver as the base conductor (cost, stability, oxide resistance, compatibility with ruthenium oxide) oFailures reported in environments with high levels of sulfur-based gases oE.g., hydrogen sulfide (H2S), sulfur dioxide (SO2), and carbonyl sulfide (COS), oFailure mode is increasing resistance (electrical open) Elevated Resistance of SMT Resistors
  31. 31. © 2004 - 200107 o Sulfur attack of silver occurs at the abutment of the glass passivation layer and the resistor termination o Cracks or openings can allow the ingress of corrosive gases, o Reaction with the silver to form silver sulfide (Ag2S) o Large change in resistance o rAg = 10-8 ohm-m; rAg2S = 10 ohm-m o Up 20K ohms (0.01 x 0.01 x 0.5mm) o Manufacturers’ solutions o Sulfur tolerant – silver alloys o Sulfur resistant – silver replacement SMT Resistors (cont.)
  32. 32. © 2004 - 2007 2010 SMT Resistors (Observations) oObservation that a majority of failures occurred in assemblies that were encapsulated in silicone oSilicone structure could act as a ‘sponge’ for sulfur-based gases. oBehavior is not uncommon for gases and polymeric compounds; observed with water molecules and epoxy resins oIn epoxies, water can exist in two forms (bound and unbound) oBound molecules are attracted to the polymer chains through hydrogen bonding and become immobilized. oIf ‘bounding’ exists with H2S or SO2 and silicone, it may provide the gases time to react with the silver conductor oAlternate theory: Presence of moisture and H2S / SO2 in silicone create aggressive chemistry
  33. 33. © 2004 - 2007 2010 oOutdoor application oElevated failure rates after 12 months oExposed to mercaptan oSpecific design uses silicone conformal coating
  34. 34. © 2004 - 2007 2010 oSulfur vapor has a substantial heat of solution in silicone and can be sorbed by them to concentration levels 100,000X larger than in the surrounding atmosphere. IBM Journal of Research and Development Date of Publication: March 1977 Author(s): Berry, B. S. / Susko, J. R. Volume: 21 , Issue: 2 Page(s): 176 - 189
  35. 35. © 2004 - 2007 2010 oTraditional, wet-applied conformal coatings can induce failures (‘kill’) as much as they protect oDecision to use them must be made very carefully oElectronics community does not understand the influence of Tg on time to failure oMany coating manufacturers do not even know Tg of their materials! Summary

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