Have you micro-sectioned a board to ensure that it complies with spec? Is
the plating in the through-holes of the correct thickness and quality?
i.e. there are no 'smears' left after the board drilling, as these would
cause plating defects.
a. Insufficient wetting in PTH's can be caused by poor plating, boards
are too cool, wrong flux (what are you using?), contamination of hole
walls, to name but some.
Poor plating - talk to your supplier. Boards too cool - heat soak the
boards at about 100 deg C for about an hour (or two hours if you an
afford the time). You may find, if the boards aren't kept heated durig
the soldering operation, that you will have to return the boards to the
oven every half hour or so to re-heat them. This should not be done more
than twice, or the board will suffer too much thermal stress. Wrong
flux - I don't know what you're using, but presume it is an RMA flux
whose residues will have to be washed off after soldering. Contamination
- depends on what it is. Either clean the boards thoroughly before
soldering and then bake to dry them out, or talk again to your PCB
supplier about supplying cleaner boards.
If there are no other problems with the boards, heating them will
probably be the answer, as it would prevent the solder from solidifying
too soon. If soldering to a cold board, the solder will freeze on the
surface of the hole walls, while the hotter core will travel a bit
further before it, too, solidifies. This will give the appearance of
non-wetting. See above.
Flux, of course has an effect on the wetting action, but try heating the
boards first.
Hole size won't affect wetting. If it is too large, though, it may not
hold the solder. This will be noticeable either because the solder has
fallen out completely, or there will be a 'bump' or stalagtite at the
solder side of the hole and a well at the component side. If you have
been following board design guidelines, though, for the types of
components you are using, you shouldn't be having this problem.
Hope this helps a bit.
Pete Duncan
"S. Miller"
<CircuitekAsi To: [log in to unmask]
[log in to unmask]> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Sent by: Subject: [TN] Insufficient Solder Fillet in a PTH
TechNet
<[log in to unmask]
ORG>
05/30/01
07:39 PM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
CircuitekAsia
Dear Technetters,
My company has been conducting studies on implementing the use of robotic
soldering in our assembly process.
Whilst experimenting with such equipment, we found our product from our PCB
source to have problems; namely insufficient wetting in the PTH or
(insufficient solder fillet in the PTH).
As we have never seen this problem before, we are no evaluating the root
cause to rule out whether or not it is equipment related.
Your advise on the following matters would be truly appreciated:
a. What causes insufficient wetting/solder fillet in a PTH?
b. What are the corrective actions for insufficient wetting/solder fillet
in
a PTH?
c. Would preheating the PCB's eliminate this problem?
d. Does the selection of flux have an influence on proper wetting/solder
fillet of a PTH?
e. Does the size of a hole influence whether or not proper wetting/solder
fillet in a PTH is achieved?
Any and all responses would be truly appreciated.
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