Tom/All,
Unless I miss understood your email, it should not matter if you leave
or omit pads with regards to drill stack height, unless you are 2nd
drilling (which is scarce these days). Your primary drill is done
into a blanket of copper. Your copper weight is nominal across the
panel, so lands, conductors and pads are irrelevant to drill stack
height.
CONCLUSION:
What I am gathering from the responses is that the forum (industry),"
including myself, have always used pads with annular rings (the "we
always have" syndrome applies here). No one's at fault, we all have
established our own Fabrication and Assembly standards that are
specific to our Industry or product. Eventually the standards become
obsolete and fall to the way side. And after all the testing and
statistical data, we ask ourselves, "Why did we ever do that." The
answer is never as obvious as we want to be.
I initially theorized that the external pads (A/R) were necessary to
support the material expansion in the Z-axis. Doing so would minimize
the amount of possible circumferential voids. Some of this was proved
in past thermal shock testing (200-400hrs @85°C). As later explained,
this has some volatility.
Lyle Dove has another avenue of thought that makes sense. The
internal pads (A/R) minimized the amount of hole wall peel. You can
use the analogy, "nothing solders better than solder." Or,
electroless deposition would adhere better to the exposed copper
layers than to the catalyst/laminate.
Werners response is helpful, but my decision to omit lands from the
design are driven by real-estate constraints and not reliability. I'm
assuming the gain he speaks of is non-functional gain and omitting
lands causes potential problems. Could you further explain Werner?
And last, I spoke with Marshall Andrews at ITRI in Austin, Texas and
we reviewed the recent (Round Robin) Study entitled "PWB Hole to Land
Mis-Registration," (#97061201-G).
A synopsis of the results:
1. Leaving or omitting the land, internal and external, has no affect
on the end product reliability (there's a catch).
2. CATCH - The external pad does assist in minimizing the amount
Z-axis expansion due to the CTE mismatch of copper and laminate.
This failure was typically found at about 600 - 1000 hrs Thermal
Cycling. This is outside the scope of many products.
3. Internal lands did not provide more support to minimizing peeling
as with no lands.
4. Mis-registered drilled vias/pths (breakout) did not affect the
reliability any worse than dead centered or annular ring.
5. Drilled holes in the egress (conductor/pad junction) had no bearing
in the reliability.
So there you have it. Thank you all for your support.
If information on items 1-5 is discrepant with what was discussed and
read from the above study, I apologize to Marshall Andrews. Please
forward any direct.
John Gulley - Process Quality Engineer
Inet Inc.
1255 W. 15th Ste 600
Plano, TX 75075
972-578-3928
_____________________________ Reply Separator _________________________________
Subject: RE: FAB: Padless vs. Padded
Author: [log in to unmask] at Internet
Date: 7/16/97 9:56 PM
I agree wholeheartedly with Werner's statement and
conclusion that the possible gain in reliability is not
worth the loss in manufacturability. In some cases, if the
non-functional pads are required to be left on, drill stack
height is reduced and this impacts board cost. The round
robin study is also quite outdated (I believe the late
'80's), and was done at a time when small hole drilling was
new to everyone. There have been several process
enhancements since then. Hmmmmm....maybe it's time to do a
new study?!?
Tom Coyle
Field Services Engineer
HADCO Corporation
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]> *
* with <subject: subscribe/unsubscribe> and no text in the body. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]> *
* with <subject: subscribe/unsubscribe> and no text in the body. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
|
