Several thoughts, Moonbase 1, but mostly distracted by the one thought of
your balls being shorted out. What would Kathy, Genny and Mary Jane have to
say about that as some of your future ex's, I ponder? Better not ponder too
long ....
I don't like tenting either, especially double sided tenting since there's
then no chance of cleaning anything out that might be imprisoned inside the
hole. I also have to say I don't use blind vias (dogbone or in-pad) either
at the moment, as they add complexity to the board and therefore processing
cost, unless you happen to have a fab house that's very good at laser
drilling. So far, our one attempt (or rather one of our fab house's
attempt) to laser drill was an unmitigated disaster, and there seems to be
some considerable doubt about the reliability of the process as far as
military boards are concerned anyway. So we continue to use up real estate
with through holes.
I would prefer to use blind dogbones in some ways, since they get rid of
the physical weakness inherent in having a closely spaced pattern of
through-holes right where you also have a lot of heat, especially if having
to replace a BGA. The downside, though, is you've then got nowhere to put a
test probe when carrying out fault diagnosis, and boundary scanning with
ICT doesn't always cut it unless you have some very slick techniques to
hand.
Double sided reflow means gluing components to at least one side of the
board (doesn't it?) - something we try to avoid to facilitate repairs and
mods. I used to hate having to break all those adhesive bonds. It does mean
two thermal excursions to solder, though, which may be the greater of the
two evils, or not depending on your point of view. Going back to your short
balls, for a moment, wouldn't this only happen with wave soldering? Or a
serious over-abundance of solder paste if reflowing? My BGA boards have all
been print one side, mount, reflow, print side 2, mount, reflow. No solder
has ever gotten near the via holes to well out and cause shorts.
I've no experience of using pallets at all, as we opted to hand solder
connectors after CR soldering the components. Ditto with shields, except
years ago when we had to use some heat shunts on Tant Caps to stop them
overheating and blowing up. As for a 0.200" keep-out for rework, I can just
hear the consternation from the Dream Team about loss of real estate for no
good purpose - such is the lack of respect we spanner monkeys have to
endure out here in the tropics where monkeys are common. But, yeah, I would
guess shields would require that sort of keep-out.
Aye, it's a sair fecht, as they say where I came from.
Peter
Earl Moon
<[log in to unmask] To: [log in to unmask], DUNCAN
M.COM> Peter/Asst Prin Engr/ST Aero/ST Group@ST Domain
cc:
03/08/02 Subject: Re: Fab Process question related to
01:41 PM ENIG
Peter,
You bring up an interesting point. I got involved in an almost hopeless
situation, as usual and love so much, involving incredibly poorly designed
MLB's with interesting BGA's. We've gone over most of this drama before.
First, I don't like tenting/masking over vias of any kind. Second, I like
using blind dogbone vias, temp mask, selective solder pallets, double sided
reflow when possible, and/or solder shields beneath BGA's to prevent solder
welling/wetting up BGA dogbone vias and shorting out my balls - all this
not
necessarily in that order.
Question is, what are some good design guidelines so solder shields can be
used under BGA's on the back side if one has to use through hole dogbone
vias? It must be kinda like the .200" keep out for the rework process but
on
the back side?
Do you have any thoughts?
Earl
[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
person. Thank you.]
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
