Hi Thad,
Your MLB is certainly a candidate for having reliability problems with the
PTVs, but it is not a given. A higher Tg certainly helps—and I have been told
that higher Tg materials do NOT carry a cost premium. A lot will depend on the
quality of your PTVs in terms of plating thickness, plating uniformity,
tensile strength and ductility of the Cu deposit inside the PTV. You also did
not say whether you have any manual soldering operations (they are worse than
HASL or reflow) as well as the severity of the use environment of your
product.
You may want to take a look at Appendix B in IPC-D-279, 'Design Guidelines for
Reliable Surface Mount Technology Printed Board Assemblies' for more
information. 
On hole breakout the necessary information is not yet in. some time ago, there
was an ITRI report and significant discussion on the TechNet on the subject of
non-functional landswhich is a related subjest matter and which I am sending
you below:
This is from the IPC News Releases dated 6/26/97:
ITRI STUDY: REQUIREMENT FOR ANNULAR RING WASTES MILLIONS
Northbrook, IL 6/26/97
The printed wiring board (PWB) industry loses more than $70 million annually
in boards discarded due to hole-to-land misregistration. The Interconnect
Technology Research Institute (ITRI) created an Improved Registration Project
Team to investigate the problem, and has just made the report on its findings
available for purchase.
In its report, PWB Hole-To-Land Misregistration: Causes and Reliability,
Project Final Report, the team concluded The requirement for an intact annular
ring at every innerlayer pad is excessively conservative. "This study
demonstrates that the requirement prohibiting breakout should be changed to
one that maintains a minimum spacing to adjacent circuits," says Marshall
Andrews, Chief Executive Officer of ITRI. "This change will allow PWB
fabricators and their customers to save millions of dollars while improving
board routability."
This conclusion culminates two years of research by ITRI. The project team
created a PWB test vehicle that was designed to have different degrees of pad
breakout from none to just a plated hole in a circuit line with no pad. Hole
reliability tests were conducted by thermal cycling the boards from -65 to
+125 degrees centigrade. No failures due to the line to barrel connection were
found even after 1000 cycles, 10 times the specification requirement.
For more information on the report, or ordering instructions, contact Diana
Hempstead, Administrator, at ITRI at (512) 833-9930, fax questions to (512)
833-9925, or e-mail [log in to unmask]
ITRI is an industry-led consortium for collaboration among the electronic
interconnection industry, government and academia. Its purpose is to enhance
the global competitiveness of the North America electronics interconnection
industry by enabling collaboration among its members to accelerate development
and application of revolutionary and evolutionary technology.
Seeking data/reports to address questions on the risks associated with Annular
Ring size.  In particular, as the mis-registration of the of the mechanical
drill to the PTH Pad moves towards a trace leading into the pad the potential
for an 'open' to occur increases.  Concerns center around 'thin' ring passing
ET, but in field stresses cause fracture and crack propagation to 'open'
state.
Thanks, Chris [[log in to unmask] (Chris Gardini-ECG005)]

Chris,	08/22/97
ITRI (The Interconnect Technology Research Institute) recently published
report that addresses your concerns. In essence they found that a PTH without
annular ring is as reliable as a pad with a large annular ring.  Samples were
submitted from several PWB manufacturers and the thermal cycle testing was
performed by NIST or Crane. Any way I'm sure you can get the report either
from ITRI in Austin, Texas or from the IPC
David Arivett [[log in to unmask] (David Arivett)]
Cuplex Inc.

Hi Chris,
If you get the ITRI Report 'PWB Hole-To-Land Misregistration: Causes and
Reliability, Project Final Report", you may be a little careful in applying
the conclusions. I am not saying that the conclusions are wrong, but that
based on the test performed, the conclusions may be a stretch. From a
reliability point-of-view of the PTH barrel-to-land interconnect, the most
severe loading occurs during excursions to soldering temperatures not thermal
cycling, even from -65 to +125C. That is because the largest thermal expansion
of the epoxy that causes the radial barrel compression and land rotations
occurs only at temperatures above Tg.
Werner Engelmaier

Hi Chris -- I agree with Werner.  The Z-axis stress is the concern which
normally occurs, above the 125C tg, at soldering temperatures, around 260C. A
second point to consider is the solderability issue. The annular ring aids in
the transfer of heat to the solder joint especially for hand soldering during
assembly or field repairs.
regards,
Arny Andrade [[log in to unmask]]
Sandia Labs/ret.	08/25/97

Hi Werner,	08/27/97
Your comments to Chris were interesting and I agree for the most part, but I
have a question with regard to the comment about radial barrel compression
during elevated temperatures. It is my understanding that as the board expands
in X & Y that the hole will also increase in diameter. Due to the
disproportionate difference in CTE I would expect that the barrel would be in
tension and not in compression (a hoop stress). This phenomena has been
demonstrated in the Physics classroom on many occasions with Ring and Ball
apparatus. It's good to see you responding to some of these interesting
problems showing up on Technet. So what do you think about this analysis?
Les Connally [[log in to unmask]]

Hi Les,
I don't know about the 'Ring and Ball' experiment, but I do know what happens
inside a PWB around the PTHs during heating to solder temperatures.
(1) At those temperatures the epoxy is way above its Tg, is very soft, and
should be considered a high-viscosity liquid as far as behavior is concerned.
(2) For analysis as to what is going on around a PTH, the PWB can be
considered an infinite plane in the x- and y-directions.
(3) While the epoxy tries to expand in all directions, it is constrained in
the x,y-directions by the glass reinforcements.
(4) Thus, the primary expansion is in the z-direction at roughly 3 times the
linear thermal expansion, because all the volumetric expansion is funneled
into the z-direction.
(5) The PTH hole, however, allows the epoxy to also expand into the space
occupied by the PTH if the PTH barrel wall is thin enough to by compressed.
That is why in cross-sections of PTHs with 1 mil of copper plating and less,
one sees the PTH barrel bowed inward between innerlayer lands, particularly in
prepreg layers (higher resin content, less cure) and when there are larger
distances between lands (no non-functional lands), and sometimes a space
between the barrel wall and the receded epoxy. If there were no copper
plating, the hole diameter would decrease on heating.
(6) Thus, the PTH barrel in tension in the z-direction, with the largest
stresses near the PWB center, the PTH barrel experiences compressive hoop
stresses, innerlayer lands experience tensile stresses at their attachment to
the PTH barrel, and innerlayer lands near the PWB surface experience bending
moments due to land rotation resulting from the z-direction expansion of the
epoxy surrounding the PTH barrel.
Werner Engelmaier

Hi Ken & Richard,	08/29/97
This has been a long ongoing debate. 
The work done for IPC-TR-579, 'Round Robin Reliability Evaluation of Small
Diameter Plated Through Holes in Printed Wiring Boards', indicated a small
improvement in reliability with the presence of functional and non-functional
innerlayer lands. This was NOT a major finding of this study, nor was it a
recommendation for the use of non-functional innerlayer lands. 
The results of the just released ITRI Report, 'PWB Hole-to-Land
Miregistration: Causes and Reliability', give the same indication (Note:
essentially all the failures come from one PWB manufacturer, primarily from
PTV barrel cracking, before the test was stopped at about 1100 cycles; the
other manufacturers show some interconnect separations of the innerlayer trace
from the barrel). The 1997 ITRI report also shows the vast improvement in the
quality of PTV structures since the 1988 IPC-TR-579 study. 
The ITRI study was done with temperature cycling from -65<->+125C; thus, the
reliability results directly, on an accelerated basis, relate to the thermal
cycling PWBs see in product operation. However, because no excursions were
taken to soldering temperatures, where because of being above Tg of the epoxy
significantly more thermal expansion mismatch occurs and the epoxy is much
softer, the results can not be assumed to be representative of what might
happen during PWB assembly. It is during solder temperature excursions that
the compressive hydro-static hoop stresses act on the PTV barrel and that
surface and innerlayer land rotations occur. These represent quite different
loading conditions than -65<->+125C temperature cycling, and may produce
different conclusions.
CONCLUSIONS: Non-functional lands are not likely worth having, because the
small potential reliability gain for high-reliability PTV structures for most
applications is not worth the manufacturing difficulties and costs that have
been encountered. Misregistration and the absence of annular rings are no
significant reliability threat to PTV barrels. Assuming, that the behavior
during solder temperature excursions is not dramatically different and the PTV
interconnect structure survives assembly, misregistration and the absence of
annular rings are no significant reliability threat to PTV interconnects
(Interconnects can be protected from large loads by having a 90 degree trace
jog just before the PTV, i.e. do not approach the PTV with a long straight
trace).
Werner Engelmaier

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]


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