Matt, If I understand your question correctly, the answer is quite elementary.  Generally, a four layer board has circuitry on the two outerlayers and the internal cores are a power plane and a ground plane.  To pass a conductor from the top to the bottom, the internal layers have a clearance etched in the copper larger than the 
hole that is going through.  When you laminate the layers together the clearance area fills in with epoxy.

At locations where you want a conductor to connect to the ground plane (or power plane) there is no clearance on the plane.  

You can subscribe to the IPC technet by sending mail to the same address you sent your question to with the word subscribe in the subject field and no text in the body.  I doubt you want an average twenty five or thirty messages per day from people who build or use boards listing all their travails.

----------
From:  Matthew Leary[SMTP:[log in to unmask]]
Sent:  Monday, January 13, 1997 4:51 PM
To:  [log in to unmask]
Subject:  vias

Hi guys,
	I wound up here thanks to Yahoo and I do have aquestion for you, 
although I do not know if I am in any position to ask you to answer me.  
If I need to sign up for something, could you fill me in.  Anyway my 
question is this: 

 A via passes through a four layer board, is a hole 
drilled in the ground and the power layers.  What is used to insulate the 
ground and power layers from the solder that eventually will fill this 
hole?  Is it a form of mask?  I thought mask prevented it from the top of 
a layer (i.e. the top of the board), but not the width if a board is cut. 
 Anyway, please get back to me on this.  Thanks.
					Matt
					[log in to unmask]

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