Graham,
Could have happened on a grid tester. Some people also call this a "Bed of
Nails" test. The head of the pins come in every shape imaginable so it does
not surprise me to hear about the shape you are seeing. Typically when
building a fixture for this kind of test a pin size(actually head size) is
chosen for the through hole that will not allow the head of the pin to go
through the center of the hole. Therefore, they could be large and
potentially damaging. You may not see the same on the SMDs' because a much
smaller pin would be used for two reasons, one you are not worried about
falling through the hole and two the test point density may force you to use
a smaller pin. When the fixture and board is put on the tester, pins that
reside within the machine itself are pushed up against the pins in the
fixture and then against your board to perform the test. If either the
board is a little warped or if the fixture is not good and flat the pressure
can be adjusted to make the proper connection. Too much of this pressure
could damage the board. Hope this helps.
Jim
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Friday, March 09, 2007 10:40 AM
To: [log in to unmask]
Subject: [TN] whodunit?
Hello Technet!
I've got a mystery today. A colleague showed me a board, it's got some
unique damage on it. It is the only one of maybe a thousand identical
assemblies, the rest do not have this damage.
Throughout the bottom side of the board there are damaged through-hole
pads. The damage is an impact hole, where something triangular in shape
has in some cases literally punched through the pad. In some cases
there is a lighter mark on the pad and/or on adjacent solder mask. The
damaged pads are scattered throughout the board, they aren't restricted
to one area. In some cases where there is damage there is visible
exposed copper.
Some facts
1) the board went through our assembly process without incident until
inspection
2) no unusual equipment malfunctions have been reported recently here -
so I don't think it is something we did.
3) The board is received with HASL finish on the pads, and is wave
soldered.
4) In all cases the damage is at a through-hole pad, some overlaps the
pad onto bare FR4/solder mask, but in all cases the damage at least
touches a pad.
5) the board also has SMT parts on the bottom side, we glue parts on
and wave solder attach them. No damage has been identified on these
pads.
I'm concluding that:
- we don't have any equipment capable of doing this type of damage to
a PWB with such precision.
- because there is exposed copper, whatever happened likely took place
after HASL coating.
At this point my theory is that the electrical test at the PWB shop did
this, but it's a theory. Can you PWB guys advise, is this possible?
Where else would something triangular have the potential to whack the
pads?
As I said already, it's only one - but I hate an unsolved mystery!
Please help.
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
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