Hi Yehuda,
I have long pushed--unsuccessfully, I might add--for a different terminology
for plated-through holes [PTHs] for lead insertion, and plated through vias
[PTVs] too small for leads, for the very reasons you describe. PTVs primarliy
fail due to barrel-cracking, PTHs due to inner-layer separation [ILS] and
shoulder cracking.
For PTVs, the presence or absence of non-functional lands makes not a
significant difference.
However, for PTHs, lands, functional and otherwise, are stiffening ribs for
the Cu barrel and do anchor the Cu barrel to the epoxy matrix. Why is this
important? The epoxy on heating above Tg becomes a near-fluid and thus not only
expands in the z-direction, but radially into the PTV hole. That latter
expansion is resisted by thicker, stiffer Cu plating [or Ni-overplates] and further
significantly inhibited by the stiffening effect of the lands [think of a
submarine with its hull ribs externally]. Absent a sufficient capability to
withstand this inward pressure, the Cu barrel will plastically deform [this can
be seen by the so-called 'resin recession' where the epoxy returned to its
original geometry on cooling, but the Cu barrel could not, because of the plastic
deformation]. For PTVs the presence of lands in layers 1, 2, 3, n, n-1, and
n-2 are particularly important. Of, course all of this pertains to thicker
MLBs, not 2- or 4-layer PWBs.
Regards,
Werner Engelmaier
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
