Design rules for selective robots 

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Conformal coating material applied to a circuit board using a selective robot

For selective robot conformal coating process the design rules become even more important.

After all, the process is normally used for medium or high volume processing and any improvements in the coating process can make a major difference to costs.

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

  1. Have a clear set of working drawings
  2. Designate three areas on a conformal coating diagram
  3. Design a keep out zone around components
  4. Avoid 3D Problems with components
  5. Avoid through hole component problems
  6. Do not specify the conformal coating as an under fill
  7. Fill the via’s to avoid capillary flow
  8. Add tooling holes for the circuit board
  9. Optimise the programming path for multiple PCBs
  10. Support the board if it is heavy and misshaped
  11. Protect connectors with gel to avoid capillary
  12. Make coating the edge of a PCB optional
  13. Make coating component packages optional

Have a clear set of working drawings

Clearly defined all points of interest for the conformal coating process on a set of drawings. This includes information such as:

  • Conformal coating material to be used, its viscosity and any other process details.
  • Application method to be used for applying the coating.
  • The masking details if required. Coating thickness requirements.
  • The inspection criteria that the circuit board is to be inspected to including any Standards to be referenced.

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.
 

Design a keep out zone around components

Maintain a minimum 2.5mm distance between MUST coat areas and MUST NOT coat areas.

If you do not define a sensible space then it may not be possible to prevent the liquid flowing into restricted areas.

Applying conformal coating to the base of connectors is difficult
Conformal coating right up to the connector base is difficult for a robotic selective coating system

Avoid 3D Problems with components

Try and group all components of similar heights within the same general area of the assembly.

This minimises the changes in dispensing height, enabling the machine to run efficiently at optimum speeds and give you the best coating coverage.

Applying conformal coating around 3D circuit component devices can by difficult
Applying conformal coating around 3D components can require careful programming to avoid valve crashes into the circuit board.

Avoid through hole component problems

Understand that through-hole components have a very wide range of placement orientations, and if used on a PCB, the drawing must cover all possible orientations of the devices, plus 2.5mm optional coating area.

This is to ensure you coat the component leads, wherever they may be positioned. 
 

Do not specify the conformal coating as an under fill

Its not recommended to use a conformal coating as an underfill
CTE mismatch issues could arise if a conformal coating is used as an underfill. Its good practice not to do this. Use a dedicated underfill material.

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.
 

Fill the via’s to avoid capillary flow 

Selective coating via's can be filled to avoid leaking coating through to the other side of the board.
Filling the via’s on a circuit board for conformal coating application and prevent coating going through to the other side and contaminating components.

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

If the via’s are not blinded (filled) then it is possible that the conformal coating can leak through and  result in coating restricted areas on the other side of the board.
 

Add tooling holes for the circuit board

If the board is not a standard shape then it may need to be supported on the fixture or conveyor.

If the board is not supported it may not be level or straight when the coating is applied which can lead to many different problems.
 

Optimise the programming path for multiple PCBs

When coating multiple PCBs in a biscuit configuration, ensure you seek input from production engineers regarding orientation to optimise robot path and valve operation.

Having to stop and start often and/or change dispense height reduces your throughput massively and the coating can be deposited poorly.
 

Support the board if it is heavy and misshaped

The board should be flat and sufficiently rigid to prevent sagging during dispensing or curing, otherwise coatings may flow and pool in unexpected fashions.

In particular, heavy boards may need to be palletized to provide sufficient rigidity.
 

Protect connectors with gel to avoid capillary

Use conformal coating gel around connectors if possible before applying the spray coating if you want to get close to connectors.

Conformal coating needle dispense can apply gel around components
Conformal coating needle dispensing can be used to provide a dam and fill process and apply gel coating around certain components

Conformal coating materials will flow freely into unsealed connectors and wick up connector leads.
 

Make 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.

Don't waste time conformal coating the edges of circuit boards unless you have a specific reason
Coating edges of a circuit with conformal coating is not easy. Decide if you actually need to conformal coat them completely?

 

Make coating component packages optional

Coating the sides of a 3D device is difficult for a robotic system, 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.


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