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Subject:	Re: Low Aspect Via definition
From:	Werner Engelmaier <[log in to unmask]>
Reply To:	TechNet E-Mail Forum.
Date:	Tue, 3 Apr 2001 16:59:02 EDT
Content-Type:	text/plain
Parts/Attachments:	text/plain (39 lines)

Hi,
You have to remember--or maybe recognize--that the aspect ratio of a via does
not have a physical meaning by itself. By definition it is the dielectric PCB
thickness through which the via goes divided by the drilled hole diameter.
The aspect ratio was defined (not physically, but as a terminology) to be
able to raise a generalized 'red flag' in the course of the work done for
IPC-TR-579 "Round Robin Reliability Evaluation of Small Diameter Plated
Through Holes in Printed Wiring Boards."
Increasing PCB thicknesses have 2 effects: (1) they increase the stress on
the PTH barrel, and (2) they make it more difficult to plate good Cu into the
hole; decreasing the hole diameter has the effect of making it more difficult
to plate good Cu into the hole, but does not appreciable increase the stress
on the Cu-barrel. Improvements in plating technology and chemistry make the
difficulty of "plating good Cu into the hole" a moving target.
Therefore, there can be no meaningfull fixed definition of what constitutes ei
ther "high" or "low" aspect ratios.
If you want to know more about this, get yourself a copy of IPC-TR-579, or
better yet attend my workshop "Design for Reliability and Quality
Manufacturing
of Plated-Through-Holes and Vias."

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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