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Conformal Coating UK

Why Does Cleaning Improve the Adhesion of a Conformal Coating?

Technical Insights

Understanding how surface condition controls coating adhesion

For conformal coatings to perform effectively, good adhesion to the substrate is essential. Without it, coatings can delaminate, allow moisture ingress, or fail under thermal or environmental stress.Adhesion is not governed by a single mechanism. Instead, it is the result of several interacting effects at the interface between the coating and the substrate. Cleaning plays a critical role because it directly influences all of these mechanisms.

The three primary mechanisms that contribute to conformal coating adhesion are:

  1. Adsorption (wetting and surface contact)
  2. Chemical bonding
  3. Mechanical interlocking

Adsorption (Wetting and Surface Contact)

Adsorption occurs when the conformal coating wets the substrate surface and spreads to form intimate contact. At this interface, weak intermolecular forces (van der Waals forces) create adhesion.

This mechanism is highly sensitive to contamination. Even very thin films of residue can prevent proper wetting, leading to de-wetting, poor coverage and weak adhesion.

Cleaning removes these barriers, allowing the coating to spread uniformly and maximise contact with the substrate.

Chemical Bonding

Chemical bonding occurs when molecular interactions form at the interface between the coating and the substrate. These bonds provide stronger adhesion than adsorption alone.

If contaminants such as flux residues, oils or cleaning by-products remain on the surface, they can block or interfere with these reactions.

By removing contamination, cleaning enables the coating to interact directly with the substrate, improving the likelihood of effective chemical bonding.

Mechanical Interlocking

Mechanical interlocking occurs when the liquid coating flows into microscopic surface features and anchors itself as it cures.

Surface condition plays a key role. A completely smooth or contaminated surface reduces the effectiveness of this mechanism, while a clean surface with appropriate micro-roughness improves anchoring.

Cleaning ensures that surface features are accessible to the coating rather than being filled or masked by residues.

How cleaning improves conformal coating adhesion through wetting, chemical bonding and mechanical interlocking
Cleaning improves conformal coating adhesion by enCleaning enables wetting, chemical bonding and mechanical interlocking, improving conformal coating adhesion.

Why Cleaning Has Such a Strong Effect on Adhesion

All three adhesion mechanisms are influenced by surface cleanliness. Contamination can:

  • Prevent wetting and reduce surface contact
  • Block chemical interactions at the interface
  • Fill surface features, reducing mechanical anchoring

As a result, even surfaces that appear visually clean may still exhibit poor adhesion if invisible residues remain.

In most cases, adhesion failures are not coating problems โ€” they are surface preparation problems.

Achieving Reliable Conformal Coating Adhesion

Not all adhesion mechanisms need to be dominant in every system. Depending on the coating chemistry, substrate and application method, different mechanisms may contribute more strongly.

However, good wetting (adsorption) is almost always a prerequisite for effective adhesion.

For this reason, the most reliable approach is simple:

If in doubt, improve surface cleanliness before adjusting coating parameters.

Learn More About Surface Preparation and Adhesion

Effective surface preparation and cleanliness are critical for conformal coating reliability. Contaminants such as flux residues, oils and ionic salts can lead to adhesion loss, corrosion or electrical leakage.

For a detailed guide, see Surface Preparation & Cleanliness for Reliable Conformal Coating, covering cleaning methods, cleanliness testing, adhesion promoters and industry standards.

If you need support with coating adhesion or process development, contact us to discuss your application.

Cleaning No-Clean Flux Residues for Conformal Coating Reliability

Cleaning

Why incomplete cleaning can lead to adhesion loss, corrosion and long-term coating reliability problems

Cleaning the residues left behind by a no-clean flux process is one of the most difficult and misunderstood stages of PCB preparation. These residues are specifically designed to remain on the board, so they are not formulated to be removed easily.

That becomes important when conformal coating is planned. Surface contamination, partially removed residues and poorly matched cleaning chemistry can all affect wetting, adhesion and long-term reliability. The goal is not simply to make the board look cleaner. The goal is to avoid creating a worse failure mechanism than the original residue itself.

Cleaning PCB residues before conformal coating

How do you clean no-clean flux residues if you need to?

Whether a no-clean flux residue can be cleaned effectively depends on the cleaning chemistryโ€™s saponification factor and its compatibility with the residue chemistry present on the assembly.

Saponification is the ability of the cleaning chemistry to soften and break down the residue so it can be dissolved and removed. In simple terms, the more effectively the chemistry attacks the residue, the easier it becomes to clean the surface properly.

The key requirement is complete removal. If the cleaning chemistry does not fully dissolve and remove the residue, the process may create more risk rather than less.

Reality check: A partially cleaned no-clean residue may be more dangerous than a residue left untouched, because the protective resin matrix can be disturbed without fully removing the active chemistry beneath it.

What happens if the residues are only partially dissolved?

A no-clean residue that is only partly cleaned away may be far worse for a printed circuit board assembly than a no-clean residue left untouched. One reason is that lead-free flux activators are generally more active than those used in earlier leaded formulations.

When the residue is left in place, the activators are held within the carrier resin matrix. At normal operating temperatures, that matrix helps keep the residue stable and reduces the risk of corrosion-related problems.

However, if the protective matrix is only partly removed by an inadequate cleaning process, the active chemistry may become exposed. This can initiate corrosion on the circuit board and may be accelerated by heat, electrical bias in service or high relative humidity.

Why this matters for conformal coating

Conformal coating is often applied to improve environmental protection and long-term reliability. But coating over contamination or marginally cleaned residues can lock defects into the assembly rather than eliminate them.

If cleaning is incomplete, the result may include poor coating wetting, reduced adhesion, localised dewetting, corrosion risk and reliability problems that only appear later in service.

This is why cleaning should be treated as a controlled process step, not as a cosmetic operation. If you are reviewing coating failures, it is also worth understanding how poor preparation interacts with broader process issues covered in our article on why conformal coating fails in complex PCB assemblies.

So how should no-clean residues be assessed?

When considering whether to clean no-clean residues before coating, three questions matter:

  1. Can the residue actually be cleaned effectively on this assembly?
  2. Have you matched the cleaning chemistry to the degree of difficulty and the available cleaning process?
  3. Have you validated the overall process by testing, rather than assumption?

These questions matter because no-clean fluxes vary, assemblies vary and cleaning processes vary. A workable answer on one product may fail on another. Validation is essential.

Why Choose SCH Services?

SCH Services supports conformal coating users with practical process knowledge, coating services, training and engineering support. If you are reviewing cleaning, coating adhesion or broader process reliability, we can help assess the issue in the context of the full coating process rather than as an isolated symptom.

To discuss cleaning, coating preparation or process troubleshooting, contact us or call (+44) 1226 249019.

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This article is provided as general technical guidance only. Cleaning chemistry, residue behaviour and conformal coating performance vary by assembly, flux system and process conditions. Final process decisions should be validated through suitable trials, inspection and reliability testing.

FAQs Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD)

Atomic Layer Deposition (ALD) is an advanced thin-film coating technology used where extreme thickness control, conformality, and film integrity are required at the nanometre scale. It is increasingly specified for high-reliability electronics, semiconductor devices, optics, energy systems, and biomedical components where conventional coating methods reach their technical limits.

Unlike liquid-applied coatings or conventional vapour deposition processes, ALD builds coatings one atomic layer at a time through a self-limiting surface reaction. This allows engineers to precisely define film thickness, composition, and uniformityโ€”even on complex 3D structures, high-aspect-ratio features, and densely packed devices.

The Atomic Layer Deposition FAQs below provide a practical overview of:

  • What ALD is and how it differs from other CVD-based coating processes
  • The types of materials that can be deposited using ALD
  • How the ALD process works in practice
  • Where ALD is typically used across different industries
  • The key advantages and limitations of ALD compared with alternative coating technologies

This section is intended to give engineers, designers, and procurement teams a clear understanding of when ALD is technically justified and how it fits alongside other advanced coating solutions such as Parylene and liquid conformal coatings.

What is ALD?

Atomic Layer Deposition (ALD) belongs to the family of Chemical Vapour Deposition methods (CVD).

  • It is a deposition process at a nano-scale level within an enclosed vacuum chamber.
  • The deposition process forms ultra-thin films (atomic layers) with extremely reliable film thickness control.
  • This provides for highly conformal and dense films at extremely thin layers (1-100nm).

What coatings are deposited in ALD?

ALD principally deposits metal oxide ceramic films. These films range in composition from the most basic and widely used aluminum oxide (Al2O3) and titanium oxide (TiO2) up to mixed metal oxide multilayered or doped systems.

How does ALD work in practice?

The ALD deposition technique is based upon the sequential use of a gas phase chemical process.

  • Gases are used to grow the films onto the substrate within a vacuum chamber.
  • The majority of ALD reactions use two chemicals called precursors.
  • These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner.
  • Through the repeated exposure to alternating gases there is a build up of a thin coating film.

Where is ALD used?

ALD is used in many different areas including:

  • Micro-electronics
  • Semiconductors
  • Photovoltaics
  • Biotechnology
  • biomedical
  • LEDs
  • Optics
  • Fuel cell systems

What are the Advantages and disadvantages of ALD

Advantages

  • Self-Limiting. The ALD process limits the film thickness. Many other processes like Parylene are dependent upon amount of dimer and will continue to deposit successive polymer layers until it is completely used up.
  • Conformal films. ALD film thickness can be uniform from end to end throughout the chamber. Other coatings like Parylene can have a varied coating thickness across the chamber and the devices being coated.
  • Pinhole free. ALD films can be pinhole-free at a sub-nanometer thickness. Parylene and some other materials are only pinhole-free at micron levels.
  • ALD allows layers or laminates. Most other films including Parylene are single component layers.

Disadvantages

  • High purity substrate. This is very important to the quality of the finish similar to many other vapour deposition processes.
  • ALD Systems can range anywhere from $200,000 to $800,000 based on the quality and efficiency of the instrument. This tends to be 3-4 times the prices of a Parylene system.
  • Reaction time. Traditionally, the process of ALD is very slow and this is known to be its major limitation.
  • Masking challenges. The ALD masking process must be perfect. Any pinhole in the masking process will allow deposition beyond the masking barrier.

What are some of the ALD coatings that can be deposited?

A wide variety of chemistries are possible with Atomic Layer Deposition. They include oxides, nitrides, metals, carbides and sulfides.

Want to know more about Atomic Layer Deposition (ALD) coatings?

Contact us now, call us on +44 (0) 1226 249019 or email your requirements on sales@schservices.com

Hybrid ALD/CVD Coatings for LED Protection โ€“ Where Do They Really Fit?

Technical Insights

Understanding the role of ultra-thin coatings in LED protection without the hype

Protecting LEDs from long-term exposure to harsh environments is becoming increasingly critical, particularly for outdoor and high-reliability applications. Moisture, salt, UV exposure and thermal cycling all create failure risks that must be managed through coating selection.

There are already multiple established protection strategies including Parylene, liquid conformal coatings, ultra-thin fluoropolymers and encapsulation. Each offers advantages, but all involve trade-offs between protection level, process complexity, optical performance and cost.

Hybrid ALD (Atomic Layer Deposition) / CVD (Chemical Vapour Deposition) coatings are often presented as a new alternative. The key question is not whether they are interesting, but where they realistically fit alongside existing coating technologies.

What is a Hybrid ALD/CVD Coating?

Hybrid coatings combine two thin-film deposition techniques into a layered structure.

  • CVD (used in Parylene) deposits a conformal coating in a vacuum environment
  • ALD deposits extremely thin, controlled layers at atomic scale

In hybrid systems, these layers are applied sequentially to build a multi-layer film. The structure is fundamentally different from traditional coatings, as properties can be engineered layer-by-layer rather than relying on a single material.

The result is an ultra-thin coating system, typically in the nanometre range, with tailored barrier, adhesion and surface properties.

Compare this with traditional coating approaches โ†’

Why is this approach relevant for LEDs?

LED protection introduces constraints that are not always present in standard PCB coating.

  • Optical clarity โ€“ coatings must not reduce light output
  • UV stability โ€“ long-term outdoor exposure
  • Moisture resistance โ€“ prevention of corrosion and failure
  • Thermal stability โ€“ cycling and elevated temperatures

Hybrid coatings are often positioned as suitable because they are extremely thin, highly transparent and can provide good barrier performance relative to thickness.

In applications where traditional coatings create optical or masking challenges, this type of approach becomes more attractive.

Masking Reduction โ€“ Not Elimination

One of the most common claims is that hybrid coatings do not require masking due to their extremely low thickness.

Reality check: Ultra-thin coatings can reduce masking requirements, but they do not remove interface risks completely. Connectors, contact surfaces and critical electrical interfaces still require validation.

Whether masking can be reduced depends on:

  • Connector design and contact force
  • Electrical sensitivity of interfaces
  • Long-term wear and fretting behaviour
  • Customer acceptance criteria

In practice, masking strategy becomes an engineering decision rather than being eliminated entirely.

Performance Compared to Established Coatings

Hybrid coatings are often compared with Parylene and liquid conformal coatings. The comparison is not simply performance-based, but application-dependent.

  • Hybrid coatings โ€“ ultra-thin, optically clear, engineered film structure
  • Parylene โ€“ proven barrier performance and long-term reliability
  • Liquid coatings โ€“ scalable, robust and well understood processes

Hybrid coatings can offer advantages in specific LED applications, particularly where optical performance is critical. However, established coatings still dominate in many applications due to proven reliability and process maturity.

The decision is not which coating is โ€œbestโ€, but which is most appropriate for the application and risk profile.

Process and Cost Considerations

Hybrid ALD/CVD processes are often described as low cost due to reduced masking and simple operation. However, real cost depends on the full system.

  • Equipment investment and process control requirements
  • Throughput and batch size limitations
  • Cycle time and scalability
  • Validation and qualification requirements

While operator interaction may be simple, the overall process must be evaluated at production scale rather than individual step level.

For surface preparation prior to coating, see plasma cleaning for conformal coating, which explains how plasma is used to improve adhesion and surface energy.

Where Hybrid Coatings Actually Fit

From a practical engineering perspective, hybrid ALD/CVD coatings are best positioned as a specialist solution rather than a universal replacement.

  • Suitable for optically sensitive applications such as LEDs
  • Useful where ultra-thin coatings provide a design advantage
  • Complementary to existing coating technologies

For most applications, coating selection remains driven by environment, geometry, process capability and reliability requirements.

In many cases, structured coating strategies using established materials remain the most robust approach.

Final Perspective

Hybrid ALD/CVD coatings represent a technically interesting development, particularly for LED protection where optical and environmental requirements must be balanced.

However, they should be viewed as part of a broader coating strategy rather than a direct replacement for Parylene or conformal coatings.

The key is selecting the right coating approach for the application, not chasing a single โ€œbestโ€ material.

Need support selecting the right coating approach?

SCH supports coating selection, process design and validation across conformal coatings, Parylene and hybrid strategies.

Contact us to discuss your application.

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Note: This article provides general technical guidance only. Final design, safety and compliance decisions must be validated against application requirements and relevant standards.
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