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Conformal Coating
Process
AADYAH Aerospace Private Ltd. (AAPL)
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Approval sheet
This Document is the conformal Coating Process Document for MUSIC Engine ECU Unit.
Name
Prepared by
Signature & Date
Angata Maneesha Rani
Lead - Testing
Jithin Sajeev
Reviewed by
Senior Manager – Electronics
Amarnath Reddy
Associate Director – AVP SAT
Arun Kumar
QA
Approved by
Amarnath Reddy
Associate Director – AVP SAT
Mr. Pradeep Kumar
CTO- Chief Technical Officer
Contact Officer for AAPL(Primary)
Contact Officer for AAPL(Secondary)
Himanshu Bhatt
Project Manger
Mob: +91 8800431419
E-mail: himanshu.bhatt@aadyah.com
Jithin Sajeev R
Senior Manager – Power Electronics
Mob: +91 7907622839
E-mail: jithin.sajeev@aadyah.com
Date:
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Conformal Coating
Revision History
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REVISION NUMBER
Document
REV Date
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Contents
Revision History .................................................................................................................................... 3
References: ........................................................................................................................................... 6
Document Purpose: .............................................................................................................................. 7
Conformal Coating Overview .............................................................................................................. 7
What is a Conformal Coating? ........................................................................................................... 7
Types of Conformal Coatings:........................................................................................................... 7
Choice of Conformal Coating: ........................................................................................................... 7
Parylene Coating Process Overview ................................................................................................. 9
Properties of Parylene Coating .......................................................................................................... 9
Parylene Variants .............................................................................................................................. 9
How to Apply Conformal Coating to PCBAs: Steps / Flow .................................................................... 10
PRODUCTION PROCESS FOR THE CONFORMAL COATING ............................................................... 11
1.
Priming ................................................................................................................................ 11
2.
Cleaning: ............................................................................................................................. 11
3.
Baking ................................................................................................................................. 12
4.
Masking ............................................................................................................................... 13
5.
Application .......................................................................................................................... 13
6.
Drying and Curing .............................................................................................................. 15
7.
Demasking and Finishing................................................................................................... 17
8.
Final Inspection .................................................................................................................. 17
9.
Tests: ................................................................................................................................... 18
Music Engine ECU Unit: ................................................................................................................... 18
Coating Removal Methods ................................................................................................................. 19
Rework of Parylenes ........................................................................................................................ 19
Causes of Corrosion in conformal coating (Parylene - C) ................................................................... 19
Summary: ............................................................................................................................................ 20
Corrective Actions / Best Practices: ................................................................................................... 21
Conformal Coating Checklist: ............................................................................................................. 22
Annexure: A ........................................................................................................................................ 23
Why is a conformal coating used?................................................................................................... 23
Conformal Coating Advantages? .................................................................................................... 23
Conformal Coating Types: .............................................................................................................. 23
How to Apply Conformal Coating to PCBAs: Steps ............................................................................ 25
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Problems in PCB Conformal Coating Process..................................................................................... 26
1.
De-wetting ........................................................................................................................... 26
2.
Scaly Skin ............................................................................................................................. 27
3.
Cracking ............................................................................................................................... 28
4.
Corrosion: ............................................................................................................................ 29
5.
Delamination ....................................................................................................................... 29
6.
Orange Peel .......................................................................................................................... 30
7.
Bubbles, Pinholes and Foam ................................................................................................ 31
Parylene Failures:............................................................................................................................... 32
Manufacturing Steps of Via in Pad Process ......................................................................................... 33
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References:
•
•
•
•
•
•
IPC-A-610: Acceptability of Electronic Assemblies.
IPC-CC-830: Qualification and Performance of Electrical Insulating Compounds.
IPC-TM-650: Test Methods Manual.
ECSS-Q-ST-70-08: Manual Soldering of High-Reliability Electrical Connections.
ECSS-Q-ST-70-28: Conformal Coating of Printed Circuit Boards.
Annexure: The annexure section of this document contains general information about the conformal process.
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Document Purpose:
This document outlines the step-by-step process and standards for applying conformal coatings on PCBs
within AAPL. It ensures consistent quality, enhances PCB reliability and longevity, and protects against
environmental factors like moisture, dust, chemicals, and temperature variations. Adhering to these
guidelines maintains high manufacturing standards and meets internal process requirements.
Conformal Coating Overview
What is a Conformal Coating?
Conformal coatings are thin, transparent polymeric layers applied to surface of assembled PCBs to protect them from
contaminants, moisture, and harsh environments that they might face in real world. By understanding and applying
the right types of coatings to ensure reliability in various environments in which the device will operate, the level of
protection needed.
They provide insulation and help reduce issues like crosstalk & electro chemical migration, which are critical as
component sizes and circuit spacing decrease. Users/designers should select coatings based on application needs and
verify their suitability through testing.
Types of Conformal Coatings:
Conformal coatings are categorized into organic and silicone families, with primary types including:
1. Acrylic (AR),
2. Epoxy (ER),
3. Silicone (SR),
4. Urethane (UR), and
5. Poly-para-xylylene (XY)
Choice of Conformal Coating:
Which conformal coating should you choose?
Each type of coating offers unique benefits, so consider the specific needs of your project before making your choice.
• Need flexibility and moisture protection? Go with urethane.
• Looking for easy removal? Acrylic is a good choice.
• Dealing with harsh chemicals and moisture? opt for epoxy.
• Exposure to extreme temperatures and humidity? Silicone is the way to go.
• Need the ultimate protection for high reliability/Space applications? Parylene is your best bet. Outgassing is a
critical consideration in environments like space, Parylene is known for its low outgassing properties, making it
suitable for applications in space.
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•
The following factors are to be considered:
1.
The followings points are to be considered:
Coating Thickness: Follow industry typical thickness ranges: As per IPC-CC-830,
Type of Coating
Thickness
AR
25-75 μm [0.98-2.95 mil]
UR
25-75 μm [0.98-2.95 mil]
ER
25-75 μm [0.98-2.95 mil]
SR
50-200 μm [1.97-7.87 mil]
XY
12.5-50 μm [0.49-1.97 mil]
Date:
05/02/2025
AR- acrylic (AR), epoxy (ER), silicone (SR), urethane (UR), and poly-para-xylylene (XY)
AS per IPC-CC-830, Thickness shall be measured in accordance with ASTM-D-1005 or by micrometre or
indicator accurate to 12.5±2.5 µm. for type XY the thickness may measure optically.
1. Key details should be given to vendors prior to coating:
➢ Designers/Engineers need to specify the areas to be coated and outlines specific requirements.
➢ Designers/Engineers need to Ensure that coatings do not interfere with electrical performance of the
Components / connectors etc.
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➢ Designers/Engineers need to Confirm that applied coatings are compatible with PCA substrates, Coating
Material, Coating Thickness.
➢ After Coating Engineers/QC/Intender Need to validate the suitability of coatings for specific applications.
➢ Life Cycle Reliability: Ensures long-term protection in the end-use environment.
Parylene Coating Process Overview
Parylene coatings are applied at ambient temperatures with specialized vacuum deposition equipment. Parylene
polymer deposition takes place at the molecular level, where films essentially ‘grow’ a molecule at a time:
• A solid, granular raw material, called dimer, is heated under vacuum, and vaporized into a dimeric gas.
• The gas is then pyrolyzed to cleave the dimer to its monomeric form.
• In the room temperature deposition chamber, the monomer gas deposits on all surfaces as a thin, transparent
polymer film.
• Because Parylene is applied as a gas, the coating effortlessly penetrates crevices and tight areas on multilayer components, providing complete and uniform encapsulation. Optimal thickness of the polymer coatings
is determined based on the application and the coating properties desired.
Properties of Parylene Coating
•
•
•
•
•
•
•
Ultra-thin, uniform films
Chemical and moisture barriers
Excellent electrical properties
Superior thermal stability
Biocompatibility
Dry film lubricity
Halogen-free variants
Parylene Variants
•
•
•
•
•
•
Parylene N is a primary dielectric, exhibiting a very low dissipation factor, high dielectric strength, and a low
dielectric constant invariant with frequency.
Parylene C has a useful combination of electrical and physical properties, plus a very low permeability to
moisture and corrosive gases. Parylene C is colourless and transparent. It meets requirements of MIL
46058C/IPC-CC-830.
Parylene C is created by adding a chlorine atom to the aromatic ring in the Parylene N monomer.
Parylene D is similar in properties to Parylene C with the added ability to withstand slightly higher use
temperatures.
Parylene HT® is useful in high temperature applications (short term up to 450°C) and those in which longterm UV stability is required.
ParyFree® is a halogen-free Parylene variant that offers the advanced barrier properties of Parylene C and
adds improved mechanical and electrical properties compared to other commercially available Parylenes.
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How to Apply Conformal Coating to PCBAs: Steps / Flow
The following Steps are to be Followed for the Conformal Coating Process of Any PCBs:
Step 1: Coating Material Selection & Planning
Step 2: PCB Preparation (E.g.: Inspection, Priming, Cleaning, Baking, Masking) (During Masking – Antistatic gloves to
be used to avoid fingerprints, Oil traces)
Step 3: Application of Conformal Coating (E.g.: Brushing, Spraying, Dipping, Vacuum Deposition)
Step 4: Drying/Curing (E.g.: Oven Cure, UV Cure, Moisture Cure Etc.)
Step 5: De-Masking
Step 6: Final Inspection and Testing
Step 7: Packaging and Storage
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PRODUCTION PROCESS FOR THE CONFORMAL COATING
1. Priming
Before coating, substrates are cleaned to ensure reliability and adhesion. Priming or adhesion promotion may
be used after cleaning to enhance adhesion.
• Priming/Adhesion Promotion for Poly-para-xylylene (Parylene)
The parylene chemical vapor deposition process for coating PCBs often requires adhesion promotion, which can be
done via gas phase deposition or liquid immersion using an organosilane ester (A-174).
1. Vapor Phase Process: Clean parts are placed in a deposition chamber, and an organosilane coupling
agent is deposited just before coating.
2. Immersion Process: Parts are dipped in an IPA solution with the coupling agent for 15-30 minutes, dried,
rinsed in fresh IPA, and baked before masking, fixturing, or deposition.
2. Cleaning:
The cleaning removes different residues from the operator handling, soldering, machine, and environmental
contamination.
Cleaning agents: Electronic Grade isopropyl alcohol, deionized water
Verify cleanliness through:
• Ionic contamination testing: IPC-TM-650 TEST METHOD 2.3.25
Guidelines on cleaning solutions could be found in the following IPC documents:
a. IPC-SC-60 POST SOLDER SOLVENT CLEANING HANDBOOK.
b. IPC-SA-61 POST SOLDER SEMIAQEOUS CLEANING HANDBOOK.
c. IPC-SA-61 POST SOLDER AQEOUS CLEANING HANDBOOK.
d. IPC-CH-65 GUDELINES FOR CLEANING OF PRINTED BOARDS AND ASSEMBLIES.
Factors that influence corrosion and, or electrochemical migration, usually revolve around ionic contamination the
cleanliness of a PCB. These contaminants come from electronic processes such as: Board Fabrication, Components,
Assembly Equipment, Soldering, Equipment Handling, Incorrect cleaning.
Poor/ improper Cleaning:
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Poor cleaning may lead to electro chemical failure mechanisms such as current leakage, corrosion, metal
migration or dendritic growth.
Through cleaning should remove both the ionic elements as well as the non-ionic elements
It is possible to minimise most corrosion effects by using an effective cleaning process.
Fluxes leave residues in general. High solid rosins & water-soluble fluxes are more aggressive in nature & are thus
more often subjected to cleaning prior to conformal coating.
The high solid rosins are cleaned using solvent, aqueous, semi aqueous process. Water soluble and OA fluxes are most
often cleaning using aqueous based regimes.
.
No clean flux may contribute to decreased adhesion if they are over applied. Low residue fluxes are benign in
nature and are intended for no-clean assembly process. As such the flux residue remain on hardware often
functioning as an intermediate layer between the hardware and the conformal coating. The amount of flux residue is
influenced by composition of the flux, heating process, environment in which reflow occurs.
solid content of the flux (most 1-5% solids) the pre-heat dynamics (hotter means less residue) and reflow
environment (air / nitrogen).
Proper parylene adhesion to a substrate can be significantly affected by cleanliness. Particulates, manufacturing
oils, human oils and other materials not considered a component of the substrate will impact parylene adhesion,
performance and reliability. The cleanliness of this surface has a great impact on the results of the conformal coating
process and the coatings durability.
Humidity and Dust Particles:
• At relative humidity (RH) above 60%, submicron dust particles can become acidic or basic, causing problems.
• At RH above 80%, ion flow begins, leading to leakage currents, dendritic growth, and corrosion.
Improper Cleaning:
• Essential to minimize adhesion problems due to residues.
• Even low-residue fluxes/no-clean processes must be free of flux residues to ensure good adhesion.
• Small traces of flux residues can lead to poor adhesion.
3. Baking
Preparation Prior to Conformal Coating: Baking the PCA (Printed Circuit Assemblies)
Conditioning: It is recommended that the Assembled test boards are be baked at 121 °C [249.8 °F] for minimum
one hour, but no more than four hours, prior to coating. It is also suggested that the environment in which the
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conformal coatings applied should be clean. Monitoring of relative humidity, temperature and particle counts ensures
a better coated assembly.
4. Masking
Masking materials are used to protect areas of the PCA during coating. Masking is necessary to prevent damage.
Masking is required to prevent unwanted removal and discoloration. Options include plastic boots, peelable masks,
or masking tape, often applied manually, though liquid masks can be applied automatically. Evaluate any residue
left by masks for potential harm to the process.
• Spraying: Use a shield approach to reduce masking time.
• Dipping: Use encapsulating masks.
• Vacuum Deposition: Use airtight masks that can withstand a vacuum environment.
Types of Masks
• Natural Latex Liquid Masks
These masks are made from natural latex rubber in water, stabilized by an alkaline component (often ammonia).
Apply thinly (less than 2-3 mm) to avoid prolonged liquid state, which can damage substrates like copper. The
alkaline component may inhibit the cure of some catalytic conformal coatings, so validate compatibility. Heating can
speed up curing but avoid temperatures above 158°F (70°C) to prevent degradation.
• Synthetic Latex Liquid Masks
These masks use synthetic polymers in water and are less alkaline than natural latex masks, making them suitable
when natural masks inhibit coating cure. They may have lower self-bond strength and odour. Heating can speed up
curing, but excessive heat can cause bubbles. Solvent-based coatings may react with these masks, affecting peel ability.
• Other Liquid Masks
Other options include heat-cure and water-soluble masks. Water-soluble masks are better for soldering, not coating.
Heat-cure masks can save process time. UV-curable removable acrylics can also be used.
Note: Use airtight masks that can withstand a vacuum environment – parylene C coating.
5. Application
Conformal coating is applied using various methods such as brushing, spraying, dipping and vapor deposition.
1.
Brushing
This is a simple process but requires a skilled operator to ensure the proper quality and finish of the coating. The
Factors affecting the brushing method quality are the type of brush, operator skill, coating viscosity, environment, and
coating material.
2.
Spray application
Aerosols: An aerosol consists of a spray gun. It is a solvent-based coating. Spraying aerosols within a spray booth is
a cost-effective method. The low setup cost, high process speed, simplicity, and quality of finish make this process
better than brushing.
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Batch spraying: Batch spraying uses a compressed air spray gun and gives an excellent quality finish to the coating.
The setup requires a spray gun, a spray booth, and an air compressor. Setting the spray gun with the correct
atomization pressure and material feed is critical. Blending the coating to the right viscosity is also important. It
offers advantages such as low setup cost, flexible process suitable for multiple coatings, etc.
Selective spraying: Selective spraying only coats the specific areas. The coating is not applied to the areas that
require masking, such as connectors and other components. A specialized robot system uses different spray patterns
for coating. The valve selection, material, and configuration of the board are critical factors. It is necessary to choose
the correct spray valve and coating viscosity.
3.
Dipping
This is a traditional method in which the PCAs are dipped into a coating liquid tank. The variables
controlling the process are the speed of immersion, dwell time of the coating, and the board’s
withdrawal speed. For dipping, solvent-based conformal coating like acrylics and urethanes is
preferred. It is a high-speed process that can be done in batch and inline productions.
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Vacuum Deposition
A different process deposits the coating in the form of gas on the board. Parylene (p-xylylene polymers) is a process
of chemical deposition of the polymers. It is a coating deposited as a gas in a vacuum chamber. Vapor deposition
gives excellent protection against moisture and superior electrical properties compared to other coatings.
6. Drying and Curing
•
•
Drying ensures that the PCA is coated and ready to be handled by the operators. It can take a few minutes to
several days.
Curing ensures that the coating reaches its desired properties and protect the board during the operation
Proper drying is crucial when using aqueous cleaning methods to avoid residual water affecting adhesion.
Curing methods:
•
•
•
Oven Cure: The PCA is cured in the oven for some time at a set temperature to cure the coating. This
is the most practiced process in epoxy and polyurethane coatings.
UV Curing: It is applied to apply UV-curable coatings. In it, the coated PCA is exposed to ultraviolet
light, and material curing happens nearly instantly. It is a fast and effective process. Hence, it is more
suitable for high-volume production.
Moisture Cure: Silicon-based coatings require moisture curing. They absorb the moisture supplied
by the ambient air encircling them. They are slower compared to heat and UV curing but very useful
when used in those coatings that should be flexible.
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UV Cure
Strengths:
•
Fast curing
Weaknesses:
•
Incomplete cure in shadowed areas
•
•
Pungent odour, potential skin irritant
Brittle at high temperatures
•
•
Difficult to rework
Cure affected by UV type and intensity
Catalysed
Strengths:
•
Fast curing
Weaknesses:
•
•
•
Cure inhibition
Short work life
Contamination sensitive
•
•
Difficult to rework
Pungent odour
For Parylene- c no specific curing like (Heat/ UV/ Catalysed/ Moisture) are required.
Date:
05/02/2025
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7. Demasking and Finishing
Masking removal is done after the coating. Next, finishing is implemented to ensure:
• Quality of coating
• Prevention of masking leakage
• Verification of standards
• Prevention of coating defects
The defects are identified, repaired and send for the next step.
8. Final Inspection
After the conformal coating process, it is crucial to perform a thorough through inspection to ensure the coating has
been applied correctly and that there are no defects that could compromise the integrity of the electronic assembly.
Here are some steps typically involved in the inspection after conformal coating:
Perform the following inspections post-curing:
• Visual inspection for uniformity, bubbles, pin holes, voids, or delamination.
• UV lamp inspection to identify coated and uncoated areas.
• Adhesion testing – On a test Coupon (2”x2” must be kept) and test may be performed
Inspect the quality of the coating under a UV “black” light. Since conformal coating is clear and colourless, it is difficult
to reliably see defects under normal light. UV tracer is added to most coatings, so it glows under a typical UV lamp
with a wavelength between 320 and 380 nm.
Inspection Guidelines
Refer to IPC/EIA J-STD-001, IPC-HDBK-001, and IPC-A-610 for detailed guidelines. Post-coating inspection typically
involves visual checks for:
• Coating coverage and integrity of masked areas
• Coating thickness and uniformity
• Voids or air entrapment
• Bridging
• Workmanship
Inspection can be done before or after curing has taken place. Many types of coatings have fluorescent tracers. This is
to make inspection much easier. Placing PCA under a black/UV causes the tracer to fluoresce so an operator can easily
detect coated and non-coated areas.
Magnification
Use 4x to 10x (preferably 10x) magnification for inspection, with higher magnification for suspected defects.
Instruments should have acceptable resolution, and aids for simultaneous viewing with both eyes are preferred.
UV Light Source
Use low-intensity UV or fluorescent, shadow-less light sources for inspection. Coatings with UV tracers fluoresce
under black light, aiding inspection. Place the PCA 50-152 mm (2-6 inches) from the light source for optimal
evaluation.
Workmanship
Workmanship of the conformal process needs to be evaluated to ensure that the applied coating fulfils the specified
requirements. Regardless of application method the applied film should not exhibit de-wetting behaviour. The cured
coating should be free of voids, bubbles, foreign material, no peeling, wrinkles or non-adherent areas. Masking residue
and degradation of the coating of the PCB should be minimized.
Example: Applicable accept/reject criteria see J-std-001 and IPC-A-610.
Coating Coverage:
The coating Coverage as per IPC-CC-830 are given below. Design guidelines for conformal coating usage should always
be included with a checklist of requirements that need to be examined. This checklist should consider all
conventionally used design criteria/guidelines considered to be typical engineering practice.
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To successfully apply conformal coating to PCA compatibility of the coating with various materials on the PCA during
application, curing need to be considered. This includes compatibility with board, component surface, solder masks,
common contaminants such as flux residue, chemicals such as plasticizer, defoamer, and Mold release agent.
Note: Aliena PCA was masked for connectors, PTH test points were not masked.
9. Tests:
Various tests measure the reliability of coated PCAs. Not all tests are necessary for every PCA.
Also, the suggested tests can be evaluated on the Test coupon but, vendor needs to submit the reports.
• Appearance - Visual
• UV Fluorescence
• Temperature and humidity
• Thermal cycling
• Vibration
• Moisture and insulation resistance
• Dielectric withstanding voltage
Music Engine ECU Unit:
Applied Coating Material – Parylene C coating
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Parylene C application: Vapour Deposition
Parylene C Thickness – 52.5mm (Measured and taken from test report)
Note: To Bake the PCB, it depends on the following parameters:
PCA: Max allowable Temperature of the PCB and the assembled/Mounted components.
As for Music Engine ECU PCB, the components max allowable temperature is 85 °C.
So, the preferred Bake Temp: 75°C±5°C.
Bake Duration: 2Hrs (preferred) – max (4 Hrs)
Note: Cleaning, Baking was not done prior to coating Application.
Coating Removal Methods
Conformal coatings can be removed using chemical solvent stripping, mechanical abrasion, micro-abrasive media,
dry ice abrasion, thermal degradation, excimer laser, and plasma stripping. Typically, only selective areas need
removal for component replacement
Note: OEM specification Detail Requirements sheet – Important, Needed for PCA’s in maintaining the performance
and reliability of the device throughout its lifecycle.
Rework of Parylenes
Due to their inherent resistance to chemicals and solvents, Parylene-coated components are not as easy as
other coatings to rework.
Parylene coatings tend to be more challenging to remove for board rework due to their insolubility to
chemicals. There are a couple of documented removal methods, however, so coating removal and rework of
Parylene-coated components is possible.
➢ Thermal Removal
➢ Mechanical Removal (micro-blaster or dry ice)
Causes of Corrosion in conformal coating (Parylene - C)
As Parylene C conformal coating (52.5mm) was applied on Music Engine ECU PCB’s we observed the
corrosion at test points overall the PCB. So material compatibility study was performed to understand
the cause.
Generic Material compatibility with PARYLENE-C
Electroless Nickel Immersion Gold (ENIG) is a two-layer metallic coating used in PCB manufacturing. It consists of:
1. Electroless Nickel Layer: Acts as a barrier to the PCB base material and provides a durable surface for
soldering.
2. Immersion Gold Layer: Protects the nickel from oxidation and ensures low-contact resistance.
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ENIG and Parylene C generally do not react chemically under normal conditions. However, if the ENIG layer has
defects or is improperly applied, the deposition of Parylene C can exacerbate these issues, leading to potential
corrosion or degradation.
Corrosion Concerns
ENIG is known for its excellent corrosion resistance but can suffer from nickel corrosion if the gold layer is too thin or
the nickel layer is exposed. This can lead to "black pad" syndrome, affecting solderability.
Preventing Corrosion
1. Proper ENIG Application: Ensure correct thicknesses for both nickel and gold layers.
2. Reduction-Assisted Immersion Gold (RAIG): Use a mixed reaction bath to deposit thicker gold layers.
3. Surface Preparation: Thoroughly clean and prepare the PCB surface before applying ENIG.
4. Regular Inspections: Conduct inspections and testing to ensure uniformity and detect issues early.
Gold is a noble metal and is highly resistant to corrosion and chemical reactions. Parylene C itself does not react with
gold to cause corrosion. However, there are a few important points to consider:
1. Contaminants: The presence of contaminants on the gold surface can lead to adhesion issues and potential
corrosion. Contaminants like ionic residues, oils, or other organic materials can interfere with the adhesion of
Parylene C and may cause corrosion over time. In harsh environments, any imperfections in the Parylene
coating, such as pinholes or cracks, can allow moisture to reach the underlying metal, potentially causing
corrosion if contaminants are present.
2. To make better Adhesion, Promoters are to used like thiol-based adhesion promoters can significantly
improve the bonding between Parylene C and gold, reducing the risk of delamination and subsequent
corrosion.
3. Proper surface preparation, including thorough cleaning and possibly plasma treatment, is essential to
remove contaminants and ensure good adhesion.
In summary, while gold itself does not react with Parylene C to cause corrosion, contaminants on the gold surface
can lead to adhesion issues and potential corrosion over time. Ensuring proper surface preparation and using
adhesion promoters can help mitigate these risks.
Summary:
The closest/nearest root cause explanation was given below.
Deviation: For Aliena PCB, prior to conformal coating (Parylene C) Baking, cleaning was not done.
By addressing the closest root cause point, you can significantly reduce the risk of corrosion in your PCAs.
Based on the observations and analysis, it is justified to conclude that the corrosion observed on the test points (PTH)
of your parylene-C-coated PCB after six months is likely due to No cleaning, which left flux residues on the board.
Because of vulnerability to contaminants, cleaning the surface of the substrate prior to coating is crucial to achieving
Parylene adhesion. Trace contaminants disrupt the bond between Parylene film and underlying surfaces (ENIG) at
test Points.
Here is the closest root cause point that supports to the conclusion:
1.
Ionic Residues: These residues can lead to corrosion and insulation breakdown, compromising the integrity
of the PCB over period. These contaminants prevent proper adhesion of the conformal coating, causing it to
peel and exposing the underlying metal to corrosion.
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Given these factors, the presence of flux residues / contaminants due to No cleaning and Baking prior to
conformal coating is the closest cause of the corrosion. Ensuring thorough cleaning and baking processes are
essential to prevent such issues and maintain the reliability of the conformal coating.
Corrective Actions / Best Practices:
Plan for coating requirements during the PCB design phase to avoid conflicts during manufacturing and strictly follow
the handling and storage guidelines throughout fabrication, assembly and testing.
1. Baking the PCA: Ensure the PCA is baked prior to coating as per IPC-CC-830 guidelines.
2. Cleaning the PCA: Ensure that the PCB is clean & free of contaminants before applying the conformal
coating.
3. Conformal Coating Checklist: Need to give checklist along with the PI and strictly needs to adhere
to the standard conformal process.
4. Best Practices for the PTH design: To prevent or resolve issues, follow these guidelines for PTH
design, specifically for PTH filling.
PTH Best Practices: A separate document is prepared and strictly followed during design time.
As a preventive measure, via filling and protection techniques should be implemented during the PCB
fabrication process.
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Conformal Coating Checklist:
S. No
Parameter/Designation/ Check
1.
PCB Visual Inspection
1. PCB Surface condition
verification
2.
Environmental Condition
1. Temperature
2. Relative Humidity
3.
PCB Cleaning
1. Cleaning agents
2. Cleanliness Verify Through
Post Cleaning
Cleaning Standard Guidelines
a) IPC-CH-65
b) ECSS
c) MIL
Specify the standard
4.
Pre-conditioning
1. Baking – PCA
A) Bake Temperature
B) Bake Duration
C) Chamber – RH%
Post Bake - PCB Visual Checks
5.
6.
PCA Priming, Masking
Conformal Coating Material Selection
Type:
1. Acrylic (AR)
2. Epoxy (ER)
3. Silicone (SR)
4. Urethane (UR) and
5. Poly-para-xylylene (XY)
Coating Thickness
Coating Temperature
Coating Shelf Life
7.
Conformal Coating Application
1. Dipping
2. Brushing
3. Spraying
4. Vacuum Deposition
Safety Section & Guidelines Followed
1. IPC-CC-830
2. ECSS-Q-ST-70-28
3. If Any, Specify
8.
9.
10.
Drying, Curing
1. Oven Cure
2. UV Cure
3. Moisture Cure
De-Masking
11.
Final Inspection & Testing
Specification/ Requirement
Observation
Remarks (Ok/Not)
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12.
Package and Storage
13.
Tools Used
Conformal Coating Process Doc for Music Engine ECU Unit – Mark 2
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Annexure: A
Why is a conformal coating used?
1.
2.
3.
4.
5.
6.
It acts as a resistant barrier to moisture and humidity, reducing leakage current, crosstalk, and
electrochemical migration across the board.
The coating provides a barrier against particulate contaminants reaching the board surface.
The coating offers high insulation protection, resulting in closer conductor spacing.
It protects the board from chemical and corrosive attacks.
It prevents mechanical damage due to mechanical shock and vibration.
It improves solder joint fatigue life and providing support for small parts.
Conformal Coating Advantages?
1.
2.
3.
4.
Environmental Protection
Increased Durability
Improved Performance
Extended PCB Life span
Conformal Coating Types:
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❖ Acrylics are easy to apply, dry rapidly, and can be removed using solvents, making them ideal for quick
repairs. They are fungus-resistant, provide long pot life, and do not shrink during curing. However, they
soften at elevated temperatures.
Acrylics Strengths:
•
•
•
•
•
Easy rework
Simple drying process
Good moisture resistance
High fluorescence
Easy viscosity adjustment
Acrylics Weaknesses:
•
•
•
•
•
High VOC potential
Difficult viscosity maintenance
Requires solvent concentration monitoring
Flammable
High reversion probability under stress
❖ Epoxies offer good humidity, abrasive, and chemical resistance but are difficult to remove for rework.
Silicones are excellent for extreme temperature cycling environments, providing high humidity resistance
and good thermal endurance.
Epoxy Strengths:
• Useful up to 150°C (302°F)
• Harder durometer
Epoxy Weaknesses:
• High chloride contamination potential
• Process-intensive, complex mix ratios
• CTE close to epoxy PCB substrate
• Good dielectric properties
• High stress during temperature cycling
• Difficult to rework
• Abrasion resistance
• High reversion probability under stress
• Priming for Silicones
For optimal adhesion of silicone conformal coatings, a thin, uniform coat of primer is applied. The steps are:
1. Preparing the surface: Ensure it is clean, dry, and reactive if needed.
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2. Applying the primer: Apply a very thin, uniform coat.
3. Curing the primer: Cure to create an ideal bonding surface for the silicone.
4.
❖ Polyurethanes offer good humidity and chemical resistance but can be challenging to rework.
❖ Poly-para-xylylene (parylene) coatings provide excellent protection against harsh environments but are
difficult to repair and require precise masking during application.
Advantages- Polyurethane and Parylene:
PCB packages to provide protection against degradation of electronic assemblies by environmental influences & to
provide electrical insulation, the most preferrable coating material is polyurethane and para xylylene.
Dis-advantages- Polyurethane and Parylene:
Among the coatings mentioned, epoxy and polyurethane systems are the two-part coatings. These coatings require
mixing two components before application, which can complicate the process due to their short pot life and specific
curing requirements and not suitable for high temperatures. It does not bond well with the existing surface, leading to
discrete lamination. Their reliability is questionable at temperatures below -55°C or above +130°C due to brittleness
and reduced flexibility. Fluorocarbon coatings are hydrophobic and oleophobic, reducing surface energy and allowing
liquids to bead and drain off.
Desired Properties of Conformal Coatings:
Conformal coating should have following properties:
1. Good adhering & wetting to most of the surfaces
2. Easly curable
3. High Mechanical strength after curing
Parylene C Key Properties:
• Excellent moisture and gas barrier
• Good dielectric properties
• Better thermal stability than Parylene N
• Biocompatible and biostable
Best Suited For:
•
•
•
Medical devices
Moisture-sensitive electronics
Corrosion protection
Limitations:
•
Slightly lower penetration ability compared to Parylene N
How to Apply Conformal Coating to PCBAs: Steps
Step 1: Material Selection and Planning
• Assess Application Requirements: understanding the specific environmental and operational requirements of
the PCBA like exposure to chemicals, temperature variations, and mechanical stresses should guide the
material selection.
• Select the Coating Material: The choice depends on factors like cost, environmental resistance, and the
desired longevity of the PCBA.
Step 2: PCB Preparation
• Thorough Cleaning: The PCBA must be meticulously cleaned to remove all contaminants, including dust, oils,
and flux residues.
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Inspection and Repair: Inspect the PCB for any defects or loose components. Make necessary repairs before
proceeding. Automated optical inspection (AOI) can be used to ensure the board is in perfect condition.
Masking Critical Areas: Protect areas that should not be coated, such as connectors, switches, and test points,
ensuring only the intended areas receive the coating.
Step 3: Application of Conformal Coating
• Method1: Automated Spray Coating: Described below.
• Method 2: Dipping: Described below.
• Method 3: Brushing: Described below.
• Method 4: Vapor Deposition (Parylene Coating): Described below.
Step 4: Curing
• Room Temperature Cure: Allow the coating to cure naturally in a controlled environment.
• Heat Cure: Use ovens to accelerate the curing process, ensuring consistent results.
• UV Cure: For UV-curable coatings, expose the PCB to UV light to cure the coating quickly and efficiently.
Step 5: Final Inspection and Testing
• Visual and Functional Inspection: After curing, perform a comprehensive visual inspection to ensure the
coating is uniform and defect-free, check for coverage accuracy, coating thickness, and overall quality.
• Electrical Testing: Verify that the conformal coating has not interfered with the PCB’s electrical functionality,
ensuring that all components operate as intended.
Step 6: De-Masking
• Carefully remove any masking materials from the PCB, ensuring that the protected areas remain clean and
free from coating residues.
• Perform any required touch-ups to address areas that may need additional coating.
Step 7: Packaging and Storage
• Protective Packaging: Once the coating process is complete, the PCBs are packaged using protective materials
to prevent damage during storage or transit.
Problems in PCB Conformal Coating Process
1. De-wetting
This issue is typically caused by residual contaminants like flux, grease, oil, mold release agents or fingerprints on the
substrate, which hinder the coating’s ability to adhere properly. De-wetting is when a conformal coating will not
evenly coat the surface to which it is being applied. De-wetting usually happens because of non-ionic contamination,
often from the manufacturing, transport, or handling process.
Many things Cause De-wetting:
• An uneven coating application
• An improper mixture of two-part materials
• Residue on the coating surface
• Variations in Surface Tension and Surface Energy
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Solutions:
• Thorough Cleaning: Ensure the substrate is meticulously cleaned to remove all contaminants. Depending on
the residue type and substrate material, methods such as ultrasonic cleaning, solvent cleaning, or chemical
cleaning may be employed.
• Preventive Measures: Control environmental factors such as temperature and humidity and ensure proper
handling and storage of substrates and coatings to prevent contamination.
Minimise the contaminants that are on the component parts before assembly. This includes laminate & component
cleanliness control before assembly and selection of low residue process materials including fluxes and pastes. Clean
the boards before coating to ensure cleanliness.
2. Scaly Skin
This problem typically arises from improper spray application, such as low spray pressure leading to inadequate
atomization, or the use of an incorrect diluent.
Solutions:
• Correct Spray Settings: Follow the manufacturer’s recommendations in the Technical Data Sheet (TDS)
regarding optimal spray equipment setup, including pressure, nozzle size, and spray pattern.
• Proper Diluent Selection: Use the correct diluent or solvent as specified in the TDS to maintain the desired
viscosity and flow properties of the coating material.
• Thorough Mixing: Ensure the coating material is properly mixed before application, using techniques like
mechanical mixing or gentle agitation to achieve uniform distribution of additives and pigments.
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3. Cracking
Cracking happens when a smooth surface of coating fractures into sections. The cracks between the sections leave the
area below exposed to potential contaminants. Because there is a higher risk of moisture, water, and debris reaching
the board level, cracking could lead to other conformal coating defects.
Common Cause for cracking:
• High temperature during curing
• Coating curing too quickly
• Coating applied too thick to the substrate
• Insufficient drying time between coats
• Operating temperature too high or too low for coating specifications
Solutions:
• Lower the cure temperature
• Allow additional drying time at room temperature
• Apply the coating to specified thickness levels
• Choose a coating with a wider effective temperature range
• Choose a more flexible coating
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To
prevent
cracks,
staged
use a
evaporation rate. If cracks persist, cure at a lower temperature for longer. Some coatings benefit from a two-step
accelerated cure: first, cure volatile solvents at a controlled rate and lower temperature, then flash off slower solvents
at a higher temperature.
4. Corrosion:
Corrosion causes due to:
Surface cracking, thermal degradation/oxidation effects, electrical leakage, surface resistance, electrical
tracking/erosion, contamination/moisture, decreased strength, incomplete cure.
Solutions:
• Use lower cure temperature
• Use longer cure time or heat cure
• Select product with higher arc/track
• Select product with higher elongation
• Select product with higher useful temperature range
5. Delamination
Delamination occurs when a coating lifts from the substrate, often due to surface contamination or insufficient tack
time between coats. Proper cleaning and ensuring adequate time between coats can prevent this issue. Delamination
is usually noticed only after the part is in use, making prevention crucial.
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Common Cause for Delamination:
• Contaminants on the surface of the substrate
• Lack of compatibility between the coating and the substrate material
• Coating applied too thick to the substrate
• Moisture between the coating and the laminate
• Improper curing of the conformal coating
• Insufficient drying time between coats
Solutions /prevent delamination:
• Clean the board thoroughly before coating it
• Choose a different coating material
• Reduce the coating thickness
• Reduce force drying
• Choose a less permeable coating material
• Allow adequate time between coats
• Apply a “primer” material known to bond with the substrate and the conformal coating material
6. Orange Peel
Orange peel happens when the coating is uneven and textured, often appearing dull and very similar to the skin of an
orange. Mottled textures in the conformal coating may prove to be cosmetic and non-critical. However, they usually
indicate a process flaw that must be modified.
Common Cause for Orange peel:
• Improper application of the coating materials
• High viscosity of the coating material from incorrect thinners
• Insufficient coating applied to the substrate
• Coating applied too thick to the substrate
• Not enough time curing
• Low air pressure leading to uneven atomization
Solutions /prevent orange peel:
• Optimize the spraying technique to manufacturer specifications
• Choose a different thinner to reduce the viscosity
• Apply the coating to the recommended thickness
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Increase the “flash-off time” for the solvents to evaporate before turning up the temperature to speed the
curing process
7. Bubbles, Pinholes and Foam
Bubbles happen when pockets of air become trapped under a layer of conformal coating. They commonly occur when
the coating fails to level and adhere to the substrate. Applying conformal coating with a brush may also create bubbles
in the hardening surface.
Pinholes occur when a bubble bursts through the coating layer. Meanwhile, foam is a form of extreme bubbling. While
bubbles and foam may be non-critical, they could exacerbate deterioration of the coating. These effects will also reduce
the level of protection provided to the circuit board and may indicate problems with the process.
Common causes of bubbles and foam are:
• Contaminants on the surface of the substrate
• Coating applied too thick to the substrate
• High viscosity of the coating material
• High temperature during curing
• Coating cured too quickly
• Coating applied with incorrect equipment settings or pressure
Solutions/ prevent these problems:
• Ensure the coating is applied only to the recommended thickness
• Apply several thin coats, allowing bubbles to dissipate between layers
• Use a lower viscosity version of the conformal coating
• Blend coats applied with a brush so they flow easily into all areas of the substrate
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Parylene Failures:
Inappropriate Parylene application can result when contaminants such as stray particles, oil, metal shavings, dust and
dirt remain on assemblies during the deposition process.
The resulting weakened Parylene film-to-surface bond can lead to causes of Parylene failure, including moisture
diffusion and pinholes, corrosion, cracking, delamination, and problems from substrate outgassing.
1.
Moisture Diffusion and Surface Pinholes
Parylene coatings can fail if substrate surfaces are not properly prepared, as contaminants can cause pinholes and
reduce protective capacity. In MEMS technology, thin Parylene films (less than 5 μm) can corrode in saline
environments, but increasing the film thickness improves moisture barrier performance.
2.
Corrosion
Although Parylene is highly corrosion resistant, problems can emerge when Parylene coatings apply on metallic
medical implants can degrade due to H2O2 in the body, which generates OH-dot radicals. This degradation starts at
the metal/polymer interface and progresses outward, compromising the protective coating.
3.
Cracking
Parylene can experience environmental stress cracking (ESC) due to extreme operating temperatures, causing the film
to flex and crack. Overly thick coatings also lead to surface cracking. Proper management of the CVD process and
reducing coating thickness can help prevent these issues.
4.
Delamination
Parylene's chemical structure can limit adhesion, causing delamination and poor finishes with torn, non-conformal
coatings. Issues like coating porosity, demasking, material incompatibility, or unclean surfaces can trigger
delamination. Ensuring substrate compatibility, moisture impermeability, and surface cleanliness is crucial for
effective adhesion.
5.
Outgassing
Parylene outgassing is minimal compared to wet conformal coatings but can be problematic in high-vacuum
environments like medical implants and aerospace equipment. Substrate components may outgas at high
temperatures (150°C – 220°C and 400°C – 510°C), cross-contaminating surfaces and reducing the film's effectiveness,
potentially rendering PCBs inoperative.
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1.
What is via?
A plated through hole (PTH) in a Printed Circuit Board that is used to provide electrical connection between a
trace on one layer of the Printed Circuit Board to a trace on another layer. Since it is not used to mount
component leads, it is generally a small hole and pad diameter.
Advantages of Via Protection:
Prevent corrosion caused by PWB fabrication processes such as plating or flux residue, from either fabrication or
assembly, from passing through or becoming trapped in plated-through holes (PTHs).
Manufacturing Steps of Via in Pad Process
•
Via is drilled (through hole).
•
Fill via with conductive material.
•
Surface of plugged via is plated with copper.
•
Newly plated surface is planarized (i.e. flatten and smoothen) so that it becomes even with surrounding
copper surface.
•
Final surface-finish applied i.e. ENIG or ENEPIG or Immersion Silver etc.
•
Solderable surface mount pad is ready that can also pass signal to inner layers eliminating the need to rout
the signal through the Via.
2.
Via Hole Plugging:
• Involves filling the via holes with a conductive or non-conductive material and then capping them
with copper. This process ensures that the vias are completely sealed.
• Purpose: Provides a flat, planar surface which is essential for certain assembly processes, such as
when vias are placed directly under components (via-in-pad). It also enhances the mechanical
strength and reliability of the vias.
• Applications: Often used in HDI PCBs and in designs requiring a flat surface for component
placement.
In summary, while both processes involve filling via holes, via hole plugging goes a step further by capping the filled
vias with copper, providing additional benefits in terms of surface flatness and mechanical strength
As a preventive step – via filling, via protection techniques must be implemented as per IPC 4761 guidelines during
the PCB fabrication process.
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