Gerry,
Everyone has a different opinion on these issues.
We allow routing under 1206 or larger parts as it does not seem to affect assembly.
0805 and smaller parts can be slightly lifted from solder pads if routed beneath.
That said, you still need to look at the component in question and determine if the body is in question of causing shorts or SI issues.
If in any way there can be shorting caused then the practice should not be followed.
Soldermask should NEVER be used as an insulator as it's thickness is variable and coverage is not fully guaranteed.
I don't know of any specs determining this but good practice should be followed and due diligence is critical.
All said, if the Engineer requires it, all we as Designers can do is caution against and then do as asked (it is the Engineers bottom line responsibility for the PCB functionality).
Hope it helps,
FNK

Frank N Kimmey CID+
Manager - PCB Design
Powerwave Technologies Inc
Mobile - 916-670-0645
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Gagnon
Sent: Wednesday, June 02, 2010 8:21 AM
To: [log in to unmask]
Subject: [TN] Relying on soldermask as an insulator

Hello Technetters,

 

From time to time we all probably get asked if it OK to route non-common etch runs underneath components where the only "insulation" preventing etch shorting to the component body is soldermask.

 

I don't typically advise people to do this type of thing, although I know it is done in practice...usually as a last resort.

 

Is there any specification out there that either disallows the practice OR allows the practice with certain caveats?

 

Also, how do the product safety agencies view the practice?

 

Thanks in advance.

 

Gerry
 		 	   		  
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