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Are there design rules for applying conformal coatings?


Conformal coating is not simply a consumable material. Unfortunately, for too many designers, conformal coating is simply a part number, to be applied to circuit boards.

However, this can be a major problem especially in the conformal coating production stage of the process.

There are guidelines in the IPC standards that may help with Design for Manufacture (DFM) principles. These are worth considering.

Unfortunately, there are no official design guidelines that will help directly with the application process and conformal coating.

Unfortunately, for too many designers, conformal coating is simply a part number, to be applied to circuit boards. However, this can be a major problem especially in the conformal coating production stage of the process.
Unfortunately, for too many designers, conformal coating is simply a part number, to be applied to circuit boards. However, this can be a major problem especially in the conformal coating production stage of the process.

Nexus Design Rules

Nexus, the independent conformal coating knowledge base, has developed both guidelines for conformal coating reliability and conformal coating process.

Their philosophy is that for companies embracing lean philosophies and applying conformal coatings, a failure to appreciate the subtleties of the application process can result in an uncoatable (at least as specified) assembly process.

They also state that if the rules are not followed, the resultant circuit board design can challenge even the most sophisticated conformal coating system and its operator to achieve the finish desired.

Nexus have design rules for both general processing and for specific application processes such as dipping, selective coating and batch spraying.

For further information visit conformal coating design rules to learn more.


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Do you need MiL spec qualification for your conformal coating?


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Normally, customers know if they require MIL-I-46058C qualification for their conformal coating. It normally is required if it is a military product.

However, caution should be shown when examining conformal coating datasheets that state MEET the requirements of MIL-I-46058C since the conformal coating will likely not be on the Qualified Product List (QPL).

What is the Qualified Product List (QPL)?

The Mil Standard for conformal coating has been inactive for new designs since November 1998. However, the standard is still widely used for independent certification of conformal coatings.

All companies tested to the MIL-I-46058C standard are listed on the QPL. It is still possible to register the coating on the list.

Conformal coatings listed on the QPL will have been through rigorous 3rd party testing to confirm they meet the standard. They are not self-certified.

So, if you require a conformal coating material that is Mil-spec approved then it will have to be on the QPL and it will have been independently tested.


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The ABCs of Atomic Layer Deposition (ALD)


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
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
  • Sulfides

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

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