An aspect ratio is the size of the hole to the board thickness or the 
     finished plated through hole to board thickness ratio.  The aspect 
     ratio is an indicator for the degree of difficulty for plating the 
     hole.  In the electroless process, the deposit of copper in the hole 
     is a chemical process and depends on the "throwing power" of the 
     electoplating equipment.  The ability to deposit a complete and 
     continuous film of copper on the hole wall.  As the aspect ratio 
     increases, the difficulty increases.  IPC/DOD-D-275 lists the 
     following producibility levels for plated throug hole aspect ratio.
     
     Preferred  1:5 or less
     Standard 1:6 to 1:8
     Reduced producibility 1:9 or >
     
      If you have a thick board, (.093) and use a small hole .008, it is 
     hard to get uniform plating.  The plating can go through the hole and 
     create a "dog bone" effect.  
     
     There is a cost adder when you go greater than a 1 to 8 ratio.
     
     Char Dwyer
     US Robotics


______________________________ Reply Separator _________________________________
Subject: Aspect Ratios
Author:  [log in to unmask] (Isaac Stringer) at Internet
Date:    12/31/96 1:18 PM


Greetings:
     
Would someone be so kind as to explain 'aspect ratios' in a way 
that will make sense to someone who understands hole sizes and 
trace width/space?
     
Thank you!
     
Isaac Stringer
KML Engineering Corp.
[log in to unmask]
     
     
     
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