On January 9 Thad McMillan posted eight questions on TechNet related
to filling through-holes with solder paste and reflowing it (instead
of filling by wave soldering), which is what a subcontractor does on
high-density double-side SMT/through-hole assemblies. The resulting
connections typically have solder in the middle of the board, with
each end of the hole unfilled. Among the questions he asks are whether
such connections are reliable, what standards exist that describe a
"good" or "bad" connection, and how such a board might be designed to
get better-looking connections.
I have noticed two other recent postings to TechNet that also ask
about hole fill. One questioner was having difficulty filling holes
because of the heat capacity of the board (very thick) and the other
was losing solder due to blow-out from water vapor evolution. Here is
a modification of a response I posted previously, starting with two
questions of my own:
1. How hard it is worth trying to get a through-hole filled with
solder?
2. What is the requirement?
When the word "requirement" is used, it is used in one of two ways: ·
a "real" requirement - necessary to ensure reliability, and a "paper"
requirement - necessary to comply with the contract.
Studies have shown that filled holes are not a real requirement. The
best that can be said for filling the holes was alluded to by Jim
Moffitt in TechNet recently: to compensate for brittle copper plating.
If the copper has cracks in it, solder can bridge them and help ensure
electrical continuity. There was a time, about a generation ago, when
the plating process was not as controlled as it is today, and perhaps
there was some merit in using the solder as a Band-aid to save the
board and meet the schedule. Perhaps.
But what would be a reason for insisting that solder fill the hole
today? Some would call incompletely filled holes a process indicator,
and in some cases it is. It may also be a design indicator, with the
poor manufacturing engineer being asked to cover for the design
engineer who caused the condition, to save the board and meet the
schedule. In the case of screening solder paste to fill the hole, it
can be thought of as a process indicator in the sense that it indi-
cates what process was used, but it doesn't indicate a _deficiency_ in
the process unless it is assumed that the process must fill the hole,
which is exactly what is at issue. In other words, if a condition
doesn't indicate that something went wrong, then it shouldn't be
called an indicator, because there is no preventive action that is to
be taken in response to it.
The study which showed, to my satisfaction at least, that holes need
not be filled was paid for by the US Army (Product Assurance Directo-
rate, Picatinny Arsenal-Dover, NJ, Contract No. DAA21-76-C-0100) and
performed by the Lockheed Electronics Company in Plainfield, New
Jersey over twenty years ago:
Gangemi, R. and P. Cipolleti, "The Dynamic Measurement and Functional
Inspection of Solder Joints", Picatinny Arsenal Technical Report No.
5055, December 1976. Available as AD-A034852 from National Technical
Information Service, Springfield, Virginia, 703-487-4650 ($39).
This study showed that connections between leads and plated-through
holes do not have to be very good, by prevailing visual inspection
standards, to be as reliable as, or even more reliable than, connec-
tions judged to be acceptable. Only two causes of premature failure
were found:
· Less than 55 percent of the area of land and lead on the solder side
showed good wetting
· Plated-through holes were less than 25 percent filled with solder.
All other visible "defects" studied were found to be unrelated to
reliability.
These results, though carefully researched and documented, fly in the
face of conventional wisdom almost as much today as they did then, and
seem to have been completely ignored. They have never been challenged
with conflicting data (at least in print), but neither have they been
accepted. More recently, Colin Lea of the National Physical
Laboratory in England has also shown that hole fill is not a real
requirement. He found that filling the hole actually slightly reduced
reliability (as judged by thermal cycles to failure), perhaps because
of the reduced mechanical compliance of the leads.
In 1989 a government-industry team visited a number of military repair
depots to look at solder connections on boards that had been returned
for repair. Out of over one hundred thousand connections that the
team examined, a huge number of connections were found which were
"bad" by anyone's solder acceptance criterion, but only four were
found that had actually caused a failure, all due to terrible solder-
ability. Ten unsoldered through-hole connections were found. Not one
of them had caused an electrical problem due to opening the circuit.
(The report was published as EMPF-TR-008 in 1989 with the title of
"Tri-Service Evaluation of Field Electronic Hardware" and is still
available from http://empf.arl.psu.edu/library/.)
The question "What is a 'good' solder connection?" has a simple
answer: A "good" solder connection is one that is unlikely to fail in
service. Of course, this answer doesn't tell how to recognize what is
likely to fail, but based on the above it seems that a connection can
look pretty bad and still not fail. As for what the standards say is
"good", acceptance requirements are arrived at by people voting. Some
vote on the basis of data, some vote on the basis of belief, some vote
on the basis of what can be sold to the customer, and some vote on the
basis of the probability of a condition that bad or worse actually
occurring.
The conclusion that I draw is that a connection which includes solder
that wets the adjoining surfaces is unlikely to fail in service, and
this would be my description of a "good" solder connection. If you
have a connection that you're not sure about, it's probably OK, since
one that is likely to fail will look so bad, you'll not think twice
about reworking it. So my advice (based on the data and reasoning
I've presented) is, unless you are required by contract to fill it, or
unless you have reason to doubt the integrity of the copper plating,
if there is solder in the hole, leave it alone and let it go.
Gordon Davy
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