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Date: | Fri, 17 May 1996 23:12:30 -0400 (EDT) |
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I couldn't agree more with Tom. We have established via reliability testing
(air-to-air thermocycling) that solder plugged holes are MORE reliable than
the unfilled yet solder coated holes.
Dave Rooke
Circo Craft
>
> As I read some of the comments regarding the open vias
> problem, I think there is some confusion which I'd like to
> clear up. Regarding tolerancing microvias with no bottom
> tolerance (i.e., they may plug shut), the plugging generally
> occurs during the solder levelling operation, not plating.
> Board fabricators must walk the tightrope between keeping
> holes open and not blowing too much solder off of surface
> mount pads when adjusting air knife placement and pressure.
> Another issue is using a small enogh drill diameter so that
> hole breakout is minimized. Board fabricators generally
> like some slack with vias in using a smaller drill size than
> the nominal callout of the via hole diameter in order to
> keep the hole in the pad. Aspect ratios (bd. thickness to
> dill size) of 7:1 or greater are not uncommon in volume
> environments these days. My company has just installed new
> PAL plating lines in our volume shops to meet such
> challenges. The PAL folks tell us that we should be able to
> plate boards with aspect ratios of 10:1!
> The fallout rate experienced by the original author
> strikes me as unusually high. One possible way that they
> might have escaped the E.T. net is if they were intermittent
> opens which may show up only when the board is flexed or
> stressed. Gary Ferrari's comments about supplying and
> requesting IPC-D-356 net lists should be well taken. This
> is an extra measure which should guarantee 100% electrical
> test on the part of the board fabricator. At the very
> least, it assures that the design intent and the Gerber
> supplied match.
> The bottom line is that I'd like to dispel any notion
> that allowing vias to plug shut causes a less reliable
> connect. Hole sections should be done by the board
> fabricator to get to the root of the real problem.
> Regards,
> Tom Coyle
> Field Services Engineer
> HADCO Corporation
> [log in to unmask]
>
>
>
D. Rooke
([log in to unmask])
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