DAVY.J.G- wrote:
>
> 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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There seems to be a lot of confusion about "PLATED-THROUGH-HOLE" fill
requirements. The PTH solder joint is an interconnection that provides an
electrical path between (a) Component Lead and (b) PCB circuit. To
accomplish this solder is deposited either through:
1. Wave solder process
2. Paste print and reflow
3. Manual soldering with an iron
4. Solder fountain (similar to item #1)
Now the questions to be answered are:
a)When a joint is reliable?
b)When is the optimum process running?
c)When a hole does not fill, what is the reason?
Q(a) Under nominal conditions and normal hole quality, for a reliable
joint, it DOES NOT have to be filled 100 %.
Q(b) Since solder has affinity for clean copper surface at the soldering
temperature ( 200 deg. C or so), a copper barrel (ie PTH) should fill in
the presence of flux, solder and right temperature.
A normal process should, therefore, provide a hole filled with solder. In
the presence of a lead, hole filling is a little easier. So a PTH with a
lead, flux, right temperature and solder SHOULD BE FILLED. Right!
If the PTH does not fill, something is wrong either with the process
parameters or the PTH (and PCB) itself. An OPTIMUM runing process with
properly made PTH in a PCB should provide FILLED holes. A FILLED HOLE IS
AN EXCELLENT INDICATOR OF THE OPTIMUM PROCESS and OPTIMUM QUALITY PCB.
Q(c) Why a PTH is not filling?
Reasons are: -Outgassing, PCB moisture
-Poor copper, thin cracked plating with voids
-Laminate voids
-Poor Solderability (even Lonco flux does not help)
So when a hole does not fill, how does one ID the causes?
The main tool is the xsectioning and visual examination along with EDX or
ESCA etc to see what chemical species are on the copper surface etc.
When the hole does not fill due to BARREL COPPER CRACKS, the RELIABILITY
of the interconnection becomes suspectExcept in rare cases, solder does
not jump across the cracked barrel surface. A crack stops the solder dead
in its track.
Let say the PTH is half filled and there is solid connection between
bottom half of the barrel, and the lead. Crack in the barrel is just
above the 50% point. However, the internal siganl trace enters in the
top 50 % of the barrel above the crack. This joint is a failed joint.
A simialr scene can be repeated with a 75 % filled hole etc.
What is one to deduce from this? An UNFILLED hole is an indicator of a
CRACKED BARREL and demands investigation of the incoming PCB quality.
On the other hand a perfect barrel with a lead in it could be filled with
25% or 50% with solder and will work reliably.
Question is how does one know why the hole is not filled? If one accepts
the premise that full hole fill is not required for reliability, then one
may be shipping PCB with cracked barrels. May be a small percentage (x%)
of cracked open barrels are acceptable in the age of 5ppm defect rate.
My net is that when a PTH is filled (to the brim) as a quality person, I
know that both process and product are OK and running well. When there is
a series of unfilled PTHs on PCB it is time to look deeper into the
hole and find the cause. If the reason is cracked barrels change your PCB
supplier.
(Note: reworking the PTH with poor copper makes it worse)
-----
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]
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