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August 2008

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Subject:
From:
Bob Willis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bob Willis <[log in to unmask]>
Date:
Fri, 8 Aug 2008 13:05:06 +0100
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Like to see any feedback from anyone that has tried the blind via test I
reported in IPC Review a few months back as a alternative or support to the
information seen during sectioning or IST due to the recent debate

I hope that the updates to IPC 600 cover via inspection in more detail.

See link to IPC Review Paper www.bobwillis.co.uk/pdf/viatest.pdf 


Bob Willis
2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
Tel: (44) 1245 351502
Fax: (44) 1245 496123
Mobile: 07860 775858
 
www.ASKbobwillis.com
 
New Bob Willis "Printed Board Inspection & Quality Control Workshop" 3rd
September www.ASKbobwillis.com/PCBworkshop.pdf 
 
Book your place on Bob's "Step by Step Failure Analysis Workshop" 11th
September  www.ASKbobwillis.com/faworkshops.pdf
 
 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Gagnon
Sent: 07 August 2008 19:14
To: [log in to unmask]
Subject: Re: [TN] measurement and acceptance of microvias

Hi Paul,
 
Can I have a copy of the Curtis Wright, Coretec and PWB Inc paper to take a
look at?
 
You state some interesting results.
 
Gerry
 
> Date: Thu, 7 Aug 2008 13:25:19 -0400> From: [log in to unmask]>
Subject: Re: [TN] measurement and acceptance of microvias> To:
[log in to unmask]> > Hi Louis,> > We do a lot of microvia reliability testing
here at PWB Inc. We have tested .002" to .006" microvias; drilled, plasma
etched (flex circuits) and laser ablated; with and without fill, stacked;
staggered etc.> > An aspect ratio of 1:1 or less is easier to clean,
metalize and electro-plate, and, therefore, produces more reliable
microvias. Many companies are producing reliable microvias with higher
aspect ratios. You just have to tweak the process using an objective
measurement like reliability testing to rank methods, processes or
procedures.> > The shape of the microvia is critical. Flask shaped and
perpendicular side walls (with right angles at the base) microvias are
usually less reliable. A "bowl" shape microvia is most reliable but this has
less surface area at the base than a straight hole wall. Out gassing
chemistries, surface tension, and the lack of solution movement all conspire
to inhibit cleaning and plating in corners at the base of the microvia. It
appears liquid can circulate better in a "bowl" shaped microvia than one
with straight side walls. So I would say the hole diameter to base surface
has an inverse relation to reliability until you get below some undefined
minimum hole or base size. > > Aggressive desmear, desmear that makes the
hole wall rough, is less reliable. At the same time, any debris on the
internal pad will induce early failures. The presents of a smooth, slightly
angled hole wall and a slight microetch in the internal pad is optimal.> >
Metrology counts in evaluating microvias. Contrary to what you may have
heard, you must have a slight microetch to evaluate the integrity of the
microvia in a microsection.> > We feel that you must test microvias at 190°C
to fail weak microvia within 500 thermal cycles. Tested at 150°C bad
microvias, ones that will fail in assembly, will survive thousands of
thermal cycles.> > There is an article available. A joint study was
undertaken between Curtis Wright, Coretec and PWB Inc. It covers much of
what I have described.> > Contact me off line for more on microvias.> > By
the way, I refrain from using the terms target and capture pad. What does
IPC-T_50 call the external and internal pads in a microvia structure? > >
Sincerely,> > Paul Reid> > Program Coordinator> > PWB Interconnect Solutions
Inc.> 235 Stafford Rd., West, Unit 103> Nepean, Ontario> Canada, K2H 9C1> >
613 596 4244 ext. 229> Skype paul_reid_pwb> [log in to unmask]
<mailto:[log in to unmask]>> > > -----Original Message-----> From:
TechNet [mailto:[log in to unmask]] On Behalf Of Louis Hart> Sent: Wednesday,
August 06, 2008 5:12 PM> To: [log in to unmask]> Subject: [TN] measurement and
acceptance of microvias> > TechNetters, I'm trying to plan a designed
experiment for optimization of a laser drilling operation, for microvias. I
need to identify a variable to measure..> > Is the ratio of the diameter of
the exposed copper at the bottom of a microvia to the diameter of the
microvia at the surface of the board a suitable measurement? (The jargon may
be 'ratio of target land diameter to capture land diameter'.)> > What is the
target value for this ratio? I'm assuming there is one such number - please
correct me if I am wrong.> > Thanks for any comments or guidance.> > Louis>
> Louis Hart> Compunetics> Monroeville, PA> USA>
[log in to unmask]<mailto:[log in to unmask]>> 412-858-1272> > >
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