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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.
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How Conformal Coating Can Kill
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What Do Conformal Coatings Do?
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What Are the Baddies?
Meets the Spec
Physical Objects (insects, leaves, hair etc.)
Liquids (Short-Term: Water, etc.)
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How Do We Choose Conformal Coatings?
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How Do We Choose Conformal Coatings?
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How Do We Choose Conformal Coatings?
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How Do We Choose Conformal Coatings?
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o Conformal Coating Market will be $2.3 billion by 2013
o CAGR of 8.9%, with Medical / Biomedical seeing largest growth
o Acrylic Dominates (70% of the Market)
o $1 to $2 per medium size board (no masking)
o Cheap, Dry Quickly, Easy Rework
o Silicone is Dominant Second Choice (20% of the Market)
o $6 to $10 per medium size board (no masking)
o More Robust, Higher Temperature
Conformal Coating Legacy
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o Break Components
o Concentrate Contaminants
How To Catch the Killer
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o Primary concern is stress due to CTE mismatch
o Very sensitive to thickness
o Similar specs in IPC2221, J-STD-001, and IPC-HDBK-830
Breaking Components: Stress vs. Thickness
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o Cracked Components
o Especially glass MELF Diodes
o Cracked Solder Joints
o Primarily cylindrical components
Failure Modes
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o Problem 1: Does Not Consider Low Standoff Components
o QFN standoff can be less than five mil (125 microns)
o Problem 2: Does Not Consider Glass Transition
Temperature (Tg)
o Dirty little secret: Companies do a horrible job of measuring coating thickness
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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
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o Dip coated assembly with BGA technology
o Passed ALT (-40C / 100C)
o Failing quickly in the field
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o All amorphous materials have a glass transition
temperature (Tg)
Hard/Brittle Soft/Rubbery
Breaking Components
-65C 125C
Silicone
Urethane
Acrylic
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Tg Behavior
o Near the glass transition temperature (Tg), CTE changes more rapidly than modulus
o Changes in the CTE in polymers tend to be driven by changes in the free volume
o Changes in modulus tend to be driven by increases in translational / rotational movement of the polymer chains
o Increases 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
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Expansion and Modulus at Tg
0.01
0.10
1.00
10.00
35 45 55 65 75 85 95 105
Temperature (oC)
Sto
rag
e M
od
ulu
s (
MP
a)
0
20
40
60
80
100
120
140
CT
E (p
pm
/ oC)
Storage Modulus
CTE
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Rise in Stress
High stresses generated
due to CTE increase
before modulus decrease
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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
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Elastic Modulus - DMA
Datasheet - Specification (8.7 MPa, 1260 psi)
Tg = 15°C
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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
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o Between -30C / 40C
o Actual field conditions
o Thick/nominal coating
o Bulbous/starved solder
Finite Element Analysis – Results
o Approx. 2000
cycles to move
solder over 3 mil
(75 microns)
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o Traditional Conformal Coatings are NOT hermetic
o Moisture will diffuse through
o Requires good adhesion to the circuit board
o Bubbles/Voids/Delam can drive micro-condensation
o Can make it electrochemical migration MORE likely
Concentrate Contaminants
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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
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Encapsulated Hybrid (cont.)
o Silicone coating was removed using Dynasol
o Visual inspection revealed black corrosion product
throughout the hybrid
o Most severe in areas with no solder or solder mask covering silver
thick film traces
o Attack through the solder mask in some locations
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Sulfur Corrosion Sites
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Elemental Analysis
o Sulfur and silver peaks detected
o Note: No migration products were observed
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o Several field issues reported in thick film resistors
o Use silver as the base conductor (cost, stability, oxide resistance, compatibility with ruthenium oxide)
o Failures reported in environments with high levels of sulfur-based gases
o E.g., hydrogen sulfide (H2S), sulfur dioxide (SO2), and carbonyl sulfide (COS),
o Failure mode is increasing resistance (electrical open)
Elevated Resistance of SMT Resistors
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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.)
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SMT Resistors (Observations)
o Observation that a majority of failures occurred in assemblies that were encapsulated in silicone
o Silicone structure could act as a ‘sponge’ for sulfur-based gases.
o Behavior is not uncommon for gases and polymeric compounds; observed with water molecules and epoxy resins
o In epoxies, water can exist in two forms (bound and unbound)
o Bound molecules are attracted to the polymer chains through hydrogen bonding and become immobilized.
o If ‘bounding’ exists with H2S or SO2 and silicone, it may provide the gases time to react with the silver conductor
o Alternate theory: Presence of moisture and H2S / SO2 in silicone create aggressive chemistry
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o Outdoor application
o Elevated failure rates
after 12 months
o Exposed to mercaptan
o Specific design uses
silicone conformal coating
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o Sulfur 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
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o Traditional, wet-applied conformal coatings can induce
failures (‘kill’) as much as they protect
o Decision to use them must be made very carefully
o Electronics community does not understand the influence of
Tg on time to failure
o Many coating manufacturers do not even know Tg of their
materials!
Summary
