Hi Jack,
It sounds to me if you want repeatable results with stacking heights you
would have to fixture the parts when they are soldered wouldn't you?
Besides... if they are concerned about the solder thickness, the tolerances
must be awfully tight... that would make anybody a little nervous about it.
A good tolerance study would show the variances in the extremes... then you
would need to compensate for that in the design or use a through hole
connector... Obviously, I am guessing because I have not seen the design.
The surface mount part will float in the x,y and z to some extent... I have
not seen any studies done, but it could probably be modeled if you knew the
viscosity of the molten solder it's worst case quantity on the worst case
pad and the worst case weight of the connector on the worst case board
thickness and plating, etc... The float obviously can't be worse than the
thickness of the paste... so you could use the max thickness of the solder
paste as a worst case height off the surface of the board to the bottom
surface of the connector pins even though the part will most likely settle
some.
I use through hole parts when stacking boards for this very reason... they
are more reliably located by the hole pattern and structurally much
stronger. In the rare occasion where I was forced to use a surface mount
connector, I used one with mounting holes and hardware and some fixturing to
make sure the part ended up where it needed to be. Surface mount connectors
are good where tolerances are more relaxed and the mating is not critical
like say with a cable or something...
Wish I could be of more help... I expect all this stuff I said doesn't help
at all. But let me know what you decide to do... it's an interesting
problem.
Best regards,
Bill Brooks - KG6VVP
PCB Design Engineer , C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
e-mail:[log in to unmask]
http://www.dtwc.com
http://pcbwizards.com
-----Original Message-----
From: Jack C. Olson [mailto:[log in to unmask]]
Sent: Friday, March 18, 2005 9:09 AM
To: [log in to unmask]
Subject: [DC] Floating SMT components?
I have been asked a question that I've never read anything about before, so
I'm hoping someone will know this...
We have a mechanical engineer who is trying to predict the distance between
boards after soldering SMT board-to-board connectors. We know the connector
mating height, but the question is:
"If the connector is sitting in solder-paste, how high will it stay above
the surface of the board?"
Common sense tells me the connector will not be flush to the surface, there
will be some solder under there, and I think we are using 6mil paste
screens. Has anyone had to document something like this before?
Jack
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