What I have failed to mention is the process must be UL 
     recognized. That means not only the raw material, in this 
     case soldermask, but the process used to apply must be UL 
     recognized. In addition UL must also recognize the use of 
     soldermask as a dielectric insulating material. If anyone 
     knows of conformance of soldermask to those requirements, let 
     me know.


______________________________ Reply Separator _________________________________
Subject: INSULATING LAYER                                            
Author:  [log in to unmask] at Internet
Date:    11/1/96 2:53 PM


     We have an application on a 2 layer board with through hole 
     technology which requires an insulating material 
     laminated/applied to one surface/side. Initially we thought 
     of having a prepreg material laminated to one surface but are 
     unsure if this is the most cost effective method. Anyone out 
     there who knows of a method or material which would support 
     this requirement please respond.
     Some of our requirements are;
     
     
>>>>>>Have you considered solder mask!!!!
     
     1. Must be a UL recognized material
     
>>>>>>>>yes
     
     2. Must provide at least 500V of dielectric insulation
     
>>>>>>>>>solder mask is typically 40V per micron, so to 
provide 500V requires a minimum of 12.5um of resist, easily 
achievable by double coating wet on dry.
     
     3. Would preferably be thin, .001-.003 thick.
     
>>>>>>>>>see above
     
     Some questions we have are;
     
     1. What type of relief would be needed around PTH?
 >>>>>>>>photoimageable = print tolerance     2. How thick would the material 
 nee
d to be?
 >>>>>>>>see above     3. Who is doing this type of fabrication today? 
 >>>>>>>>everyone     4. Can it be applied selectively?
 >>>>>>>>photoimage     5. What is the typical cost impact to a double sided 
 boar
d?
 >>>>>>>>do you want % or in cents - or do you already have the
the answer and its on your board already?
     
Dougal Stewart
Exacta Circuits
Scotland 
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