General conformal coating design rules

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Applying conformal coating to the base of connectors is difficult

There are many general rules that apply to nearly all of the conformal coating processes that could be extremely useful in saving both time and money in production if used.

Here are a list of rules and guidelines that could be considered.

  1. Have a clear set of working drawings
  2. Understand your inspection criteria
  3. Apply the holistic approach to conformal coating
  4. Designate three areas on a conformal coating diagram
  5. Do not specify the use of conformal coating as an under fill
  6. Fill the via’s to avoid capillary flow
  7. Make coating the edge of a PCB optional
  8. Make coating component packages optional
  9. Apply silicone RTV after conformal coating
  10. Provide a repeatable method of masking
  11. Allow liquid latex to dry
  12. Use liquid latex with care
  13. Match the coating thickness with the process
  14. Allow a variable coating thickness
  15. Check conformal coating compatibility
  16. Match the application process with the circuit board

 

Have a clear set of working drawings

Clearly defined all points of interest for the coating process on a set of drawings.

This includes masking details, conformal coating material, process to be used, inspection criteria to be used or referenced and any key information.

Clearly defining all the key points of interest for the conformal coating process on a set of drawings is critical for good conformal coating production.
A correct set of instructions can help in the conformal coating process

Understand your inspection criteria

Ensure that the Standards for inspection and acceptance are understood by all team members.

People will interpret individually so ensure the instruction and criteria are explicit.

The conformal coating inspection criteria and guidelines are important to be understood by everyone
Ensure all of the team understand the inspection criteria for the conformal coating process

 

Apply the holistic approach to conformal coating

Ensure the method of application, coating material and board design can be combined to meet the Standards for inspection and acceptance.

This is the holistic approach to the conformal coating process.
 

Designate three areas on a conformal coating diagram

The three different areas are:

  • Areas that MUST be coated
  • Areas that MUST NOT be coated
  • Areas where it doesn’t matter (coating is optional).

Having the don’t care area allows flexibility in the coating process.

Do not specify the use of conformal coating as an under fill

If a device needs to be under filled, specify a formulated under fill.

Or, consider the difference in CTE mismatch may lead to the component lifting off the PCB in the long term.

Use an underfill under a BGA. A conformal coating underfill is not a good idea due to CTE mismatch problems.
Conformal coating underfill is not a good idea

 

Fill the via’s to avoid capillary flow 

Specify and use tented via’s to prevent the capillary flow of material from one side of the assembly to another.

Via’s that allow coating through the holes can result in coating going through to restricted areas on the other side of the board.

Conformal coated via's (electrical connection holes from one side of a circuit board to the other) can cause problems if they are open (not sealed) by allowing conformal coating to leak through to the other side of the board into components that should not be coated.
Conformal coated via’s can cause problems by leaking through to the other side of the board

 

Make conformal coating the edge of a PCB optional

it is tricky and messy, especially if there is no frame or breakout around the board, and is of questionable efficacy in improving coating or reliability performance.
 

Make conformal coating component packages optional

Coating the sides of a 3D device is difficult, especially since the coating is subject to gravity, as well as de-wetting due to mould release agents used in the component fabrication.

The plastic or metal package mouldings are almost certainly more resistant to humidity or other forms of water than any conformal coating.

Applying conformal coating to the sides of tall components may not be required and can make the production process difficult. Don’t do it if you don’t need to.
Conformal coating sides of components may not be required and can make processing difficult

 

Apply silicone RTV after conformal coating

If you require a silicone staking materials for ruggedisation purposes, then apply it after conformal coating.

If this is not possible, use a silicone conformal coating for compatibility.

If you don’t want to use a silicone conformal coating, then use a urethane or epoxy staking so that is compatible with the coating.
 

Provide a repeatable method of masking

If masking by hand ensure the method of masking is understood by all technicians.

Avoid operators interpreting the masking method and using their own methods.

The secret to a good conformal coating masking process is a repeatable process that all operators can achieve each time 100% perfect.
The conformal coating masking process should be repeatable by all operators to ensure quality is maintained

 

Allow liquid latex to dry

If using liquid latex as a masking material ensure that enough time is allowed in the production process for the latex to be fully cured before the coating application is started.

Not drying latex correctly before applying conformal coating can mean the latex does not remove easily
Ensuring liquid latex is completely dry before conformal coating can be crucial to its successful removal

 

Use liquid latex with care in masking

If a temporary liquid latex is used to mask a connector ensure that the latex can be easily removed from under the device and not get broken off and entrapped underneath.

Latex trapped inside a component when it breaks off is a disaster to remove and can cause a lot of problems in long term reliability
Ensure the latex used in the masking process cannot go into the device and get entrapped

 

Match the conformal coating thickness with the process

Ensure the conformal coating thickness specified can actually be achieved by the coating process and within the tolerance defined.

Some processes such as dipping struggle to build greater than 25um coating thickness in a single coat.

Ensure the conformal coating process chosen can achieve the conformal coating thickness required
Dipped PCBs can struggle to achieve high build coating thickness

 

Allow a variable conformal coating thickness

Conversely, do not define a tight coating thickness tolerance if a wide tolerance is allowed.

Achieving 30±5µm dry coating thickness across a circuit board is an order of magnitude harder (or more!) than 50±25µm!

Ensure the conformal coating thickness tolerance is as large as possible to help the production process
Conformal coating thickness tolerance needs to be as wide as possible to avoid difficult production problems

 

Check conformal coating compatibility

Ensure the conformal coating specified is compatible with the circuit board. Some coatings can attack components, markings and inks.

Match the application process with the circuit board

Ensure the coating application process is suitable for the circuit board design. This is the holistic approach to the conformal coating process.


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