[SOLVED] Solder mask, same as permanent polymer coating?

[mmitchell]

Hi,

In IPC-2221, 6.3, Table 6-1 Electrical Conductor Spacing I encountered the concept of conformal coating. In this table, conductor coating are classified into two types:

1. Permanent polymer coating
2. Conformal coating

After reading materials at
[1] Conformal coating - Wikipedia, the free encyclopedia
[2] Dow Corning - What are Conformal Coatings?

It seems that the key characteristics of conforming is that

1. It is applied after all components have been soldered.
2. Its primary purpose is for providing an additional covering layer to withstand harsh environments, such as high humidity.

Is this understanding correct?

I have only used solder mask before. Solder mask is for preventing solder from bridging pins, and it is applied before soldering, so it seems that it can easily be distinguished from conformal coating.

But is solder mask the same concept as (1) permanent polymer coating? Should I use columns corresponding to (1) for determining conductor spacing?


Matt

 

[yura717]

Areas that should not be soldered may be covered with a polymer solder resist (solder mask) coating. The solder resist prevents solder from bridging between conductors and creating short circuits. Solder resist also provides some protection from the environment. Solder resist is typically 20–30 micrometres thick.
Solder mask comes in different media depending upon the demands of the application. The lowest-cost solder mask is epoxy liquid that is silkscreened through the pattern onto the PCB. Other types are the liquid photoimageable solder mask (LPSM) inks and dry film photoimageable solder mask (DFSM). LPSM can be silkscreened or sprayed on the PCB, exposed to the pattern and developed to provide openings in the pattern for parts to be soldered to the copper pads. DFSM is vacuum laminated on the PCB then exposed and developed. All three processes go through a thermal cure of some type after the pattern is defined.

 

[marce]

I am not at work so cant find the link easily but look up creepage and clearance:
Solder resist cannot be classed as a conform coating, for reduction of creepage and clearances distances you have to apply a specified conformal coating, the whole process has to be controlled and monitered. You also have to categorise where a product is going to be used and the pollution level it is likelt to encounter.

 

[mmitchell]

marce,

Does solder resist belong to "permanent polymer coating"?

Matt

 

[marce]

For purposes of this specification, the term ‘‘solder mask’’
is used herein when referring to any type of permanent
polymer coating material applied prior to assembly, but
excluding marking (legend) inks and temporary hole plugging
materials.

From IPC-SM-840c
So solder resist is a permenant polymer coating, as it leaves the metal pads exposed you cannot count it for creepage and clearance distances though.

 

[mstrghettorigg]

So is it ok to route solder mask covered traces closer as long as the exposed pads meet the required distance?

For example, I am required to have 3mm of gap with exposed copper, but only 1mm if it's covered.

Would I be able to route the traces under solder mask at 1 mm spacing as long as keep 3 mm distance from the pins?

 

[marce]

No, from memory for high voltage and SELV regulations it has to be a conformal coating. I have always worked on the safe side and used conformal coatings and strict inspection of such, and when only solder mask was used I have always used the worse case (largest gaps).
Agian this has always been my preference and the preference of the companies where I have done these sort of designs, always work to worse case, ie bare PCB, for creapage and clearence spacing then you are covered for any eventuality. If you have the room use the biggest gap possible. The way the rules and regulations are written is rather ambigous and open to interpetation, hence my preference to err on the side of caution.

 

[mstrghettorigg]

Thanks for the feedback. That's what I've been soon also.

I just thought I was making my life more difficult than It needed to be.