On December 20 Kong Hui Poh in Singapore asked for suggestions about
how to get solder to fill the plated-through holes on a six-layer
board. He said that he couldn't get hole fill above 75%. Several
people have already responded with suggestions about how to increase
the amount of fill, but I want to comment in a little different way,
namely to raise the question of how hard it is worth trying to
accomplish this task.
When the word "requirement" is used, it is used in one of two ways:
· a "real" requirement - necessary to ensure reliability
· a "paper" requirement - necessary to comply with the contract
In this case, studies have shown that filled holes are not a real
requirement. The best that can be said for it was alluded to by Jim
Moffitt in TechNet recently: to compensate for improper 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 is the reason for insisting that the solder fill the hole
today? Some would call incompletely filled holes a process indicator,
and in some cases it is. But 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.
The study which showed, to my satisfaction at least, that holes need
not be filled was paid for by the US Army and performed by Lockheed in
New Jersey:
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.
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,
connections 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 twenty
years ago, 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 actually found that filling the hole slightly
reduced reliability (as judged by thermal cycles to failure), perhaps
because of the reduced mechanical compliance of the leads.
So my advice to Mr. Kong is, unless you are required by contract to
fill them, or unless you have reason to doubt the integrity of the
copper plating, leave 'em alone and let 'em go.
Gordon Davy
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