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Europe – Asia – Americas www.zestron.com Copyright – V 02
Causes and Cures of Delamination, Cracks and
Blisters in Conformal Coating Layers
iNEMI
Board Assembly Tech Topic Series
Freddy Gilbert, Technology Analyst, ZESTRON Europe
[email protected]
June 30th, 2021
Watch the recorded webinar: https://youtu.be/0WkbAfN-1KA
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ZESTRON Portfolio
Cleaning
Technical support
Process monitoring
R&S – Services
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Zestron Reliability & Surfaces
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ZESTRON R&S Coaching
– When and what for?
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Mechanical
e.g. vibrations, acceleration, crash
Electrical
e.g. moisture, flux, impurities,
overvoltage effects
Corrosive
e.g. humidity, climate
change, salt spray,
harmful gases
Thermal
e.g. temperature
variation,
extremely high/low
temperature
Stress on electronic assemblies
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Coating or potting to protect the
assemblies
Coating protects against
mechanical stress
Coating protects against
moisture-induced failures:
Creeping currents
Electrochemical migration
Corrosion
Signal distortion in high
frequency circuit
… if the surface is sufficiently
clean
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Introduction – about faults
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Coating defects – Delaminations
Easy peeling off of the coating at the edges and components
due to insufficient adhesion
Recognizable through the apparition of light areas at the surface of the coated
assembly
Potential causes: contamination at the surface prior to coating, too thick coating
layer, insufficient curing…
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Contamination influences on coatings,
potting and moulding compounds
Contamination reduces adhesive forces and leads to delamination effects
Contaminants
Improvement of surface cleanliness minimize the risk of delamination
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Coating defects – Cracks
Conformal Coating after Ageing (e.g. temperature cycles)
Potential causes:
Too thick coating layer (heterogeneous mechanical/thermal properties
between the upper coating layer and the bottom layer
Presence of the contamination with a very different coefficient of thermal
expansion
Coating material not suited for the test / application (brittleness)
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Coating defects – Cracks
Conformal Coating after Ageing (e.g. temperature cycles)
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Coating defects – Cracks
Conformal Coating after Ageing (e.g. temperature cycles)
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Coating defects – Blisters (during curing)
Potential causes:
Evaporation of moisture of residual solvent from flux
Drying performed too quickly and/or at too high temperature
Entrapment of air under component (spraying application pattern)
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Other defects
Electrochemical
migration (ECM),
triggered by defects in
coating
Delamination of
silicone lens on
LED components
Delamination / curing
issues of
2K-polyurethane-potting
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Other defects
Insufficient wetting under
components
Immediate dewetting
during coating
Incomplete crosslinking
(e.g. crosslinking blocked by
organotin compounds,
sulphur and ammonium
compounds)
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Preventive measures –
Have a clean surface
47
0
25
72
1 0
20,5
21,5
50
9,5
27,5
87
7
3
23,5
33,5
67,5
32
32,5
132
4,5
8,5
20,5
33,5
0
20
40
60
80
100
120
140
Delamination
uncleaned
Cracks
uncleaned
Bubbles
uncleaned
Total Defect
uncleaned
Delamination
cleaned
Cracks cleaned Bubbles
cleaned
Total defect
cleaned
Total error curve over test cycles - uncleaned vs. cleaned
100 cycles 400 cycles 500 cycles
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Surface evaluation
before Conformal Coating
Optical cleanliness inspection
Ionic contamination: ROSE-Test, Ion Chromatography
Testing for flux activators
Testing for resin residues
Testing for cross-linking disruptors
Measurement of surface energy
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Quality tests for Conformal Coating and
Potting
Inspection of fluorescent coatings
Adhesion tests: X-Cut Tape Test, Cross Hatch Tape Test
Differential Thermal Analysis (DTA)
Environmental simulation tests (e.g. harmful gases with mixtures of
SO2, H
2S, NO
2and Cl
2)
Complete life duration testing (e.g. according to IEC-68-2 or EN
60068-2-60)
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Conventional harmful gas testing:
drawbacks
Expensive and complex experimental set-up necessary (climate
chamber, test chamber, cylinders, taps…)
Harmful gases (H2S, Cl
2,…) High effort and high costs to ensure
operational safety
Long test duration No quick statement possible about the quality of
the Conformal Coating or Potting
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Requirements for extended quality tests
Tests should be carried out quickly, i.e. within a few hours
It should be possible to carry out tests with as little equipment as
possible and at the same time with as high an operational
reliability as possible
Tests should enable a qualitative assessment of the protection
system
Tests are shall be able to be used according to the points 1-3 as
production-accompanying quality monitoring tools
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New, extended quality tests
Quality tests for coatings and potting:
Coating Reliability Test (CoRe-Test) according to GfKORR
Quality test with iodine vapour
ZESTRON®
Coating Layer Test
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Quality tests for protective systems:
Coating Reliability (CoRe) Test
Simplified, accelerated “climate test“ give
examples to the following questions:
Is a protective coating against moisture
necessary?
Is the existing coating sufficient to provide
protection against moisture?
Is the coating quality/adhesion negatively
influenced by contaminants?
Are there defects or weak points in the
coating?
Could the quality of the coating be
improved after optimizing the coating
process?
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Quality tests for protective systems:
Coating Reliability Test (GfKORR)
Gas development
at imperfections
(02
and H2
as a
result of electrolysis
of water)
+
–
- 5 V
- 12 V
- 24 V
Dendrite growth
= ELECTROCHEMICAL MIGRATION (ECM) at defects
Standby current
over time
R protection
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Coating Reliability (CoRe) Test –
Current characteristics (examples)
Not fully crosslinked coating layer
- broken through
Coating layer –
- Resistant to short term condensation
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Quality test with iodine vapour
Objective for a new and fast quality test for potting and
molding compounds:
Localization of cracks and penetration paths in molds and potting
compounds
Localization of adhesion weaknesses between assembly surface
and molding/potting
Focus on power electronics encapsulation (especially DCB
substrates)
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Quality test with iodine vapour
Test specifications/ performance iodine vapor test:
Further development of the “Pressure-Cooker-Tests” (IEC 60749-
33:2004), which is used as an accelerated humidity test for
encapsulated (molten) components without power supply
Iodine is reactive but also easy to handle (as solid or saturated aqueous
solution)
Test equipment: Beaker/autoclave; fume cupboard with suitable filter
system; optical microscope with polarizing filter (interference contrast)
Iodine evaporates from the saturated solution at elevated temperature -
a saturated iodine vapor atmosphere is established (resublimation of
the iodine on cooling)
Combined humidity and humidity pressure test possible (adjustable via
test temperature)
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Quality test with iodine vapour
Test conditions: ~60 °C; 1-3 h
Optical evaluation via microscope transparent systems are preferred
Saturated
iodine solution
Iodine and/or
water vapour
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Quality test with iodine vapour
Test result: case study
- Cu-DCB power electronics module (chips soldered on; Al Bonds)
- Silicone potting: 1-1.5 cm layer thickness
Before the test:
Clear, transparent silicone
After the test:
Turbidity of the silicone
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Quality test with iodine vapour
Test results: Case study
Inorganic substances (salts, ...) can be made visible via the interference
contrast in the light microscope
In areas with insufficient potting adhesion - especially in the immediate
vicinity of the chip solder joints - copper iodide has formed
Copper
iodide
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Application area
Localised detection of protective coating
layer defects on electronic assemblies
black coloring
No fluorescent tracer necessary
Reacts with metal surfaces:
Tin
Copper
Nickel or nickel containing
compounds
Other less precious metals such as
ferrite (Fe, Fe/NiZn, Fe/Mn)
Other less precious alloys
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Easy and fast application with a test
result within minutes
1. Apply test indicator 2. Wait for max. 3 minutes
3. Optical analysis 4. Rinse off or dab off
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Conformal Coating
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Conformal Coating after Ageing
(e.g. temperature cycles)
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Coating Layer Test
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Summary
Quality Management/
Safety
Drying
Coating
Application
Machine
Geometry of
PCB
Surface
PCB Production
Picture courtesy of GfKORR
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Where do I find support?
Technical articles
GfKorr Handbook
IPC – Guide-
line – 830
NASA Handbook
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Thank you!
Do you have questions?
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Topic Date
PCBA Cleaning – Stefan Strixner July 8th, 2021
iNEMI
Board Assembly Tech Topic Series
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Contact
Zestron Reliability & Surfaces
ZESTRON Europe
Freddy Gilbert
Technology Analyst
Phone : +49 841 635 115
Email : [email protected]