While agreeing with you both in general cases, there is one case where
the finished via hole size CAN have a detrimental effect on the board -
small vias for high aspect ratio boards in the area of skip plating on
ENIG boards.
At a 10 mil drilled hole size on a 96+ mil thick board, you are getting
towards the limits of the ability to effectively both changeover plating
solution in the hole, and for stripping systems to do their job in terms
of tin strip.
Unless you specify such holes as a finished size with this particular
metallurgy, you will "get what you get" and depending on the fluid
dynamics of the particular plating line you might not like the result.
The skip plating while often identified and labeled erroneously as
"black pad" (same chemical composition different root cause) takes the
form of hyper corroded nickel caused by insufficient nickel deposition
which has then been corroded in the gold bath with gold deposited over
the top OR a condition where the nickel has failed to plate completely
and the gold has deposited onto the copper. This second condition
rapidly turns into a discolored pad as the gold dissolves into the
copper.
This is normally found in the BGA ball field on BGA pads connected by a
"dog bone" to a small via. It is only found here more often since the
BGA pad is a critical interconnect area which gives a very positive
indication of a soldering issue in most cases, and obviously there tend
to be a lot more small vias in a BGA field.
This is generally a random effect and is due to the finished hole being
too small/too high an aspect ratio for the plating/stripping systems OR
a small hole being blocked with debris giving the same effect. Generally
the root cause is little or no nickel caused by residual tin left in the
hole during the stripping process which then "poisons" the subsequent
enig process.
John
John Burke
Senior Manager - Operations , Optichron
[log in to unmask]
W: 510 249 5233
M: 408 515 4992
http://www.optichron.com
Profile: https://www.linkedin.com/e/fps/2665502/
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeffrey Bush
Sent: Monday, April 24, 2006 11:43 AM
To: [log in to unmask]
Subject: Re: [TN] Drill specification
Agreed. For vias we look for a callout of a maximum number, like .010"
max. This give the fabricator latitude on what drill size to use based
on their specific processes and capabilities. This also provides a
safeguard for designs that need a larger via for thermal or electrical
capacity.
Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302
Voice - 802.257.4571 ext 21
Fax - 802.257.0011
<http://www.vtcircuits.com/>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Monday, April 24, 2006 2:21 PM
To: [log in to unmask]
Subject: Re: [TN] Drill specification
Hi Jeffrey,
Specifying finished hole sizes made sense when we needed clearance for
pin
insertion--for vias the drilled hole diameter is important and the
finished hole
size not at all.
Werner
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