Phil,
We have gone through several of the exercises you mention for probably the
same reasons. We ended up not tenting any vias due to outgas and fluid
entrapment problems, and under BGAs we also found having mask as close to
the hole as possible was preferred.
One item that made the most difference for 1.0 mm BGAs was Hi TG FR4 (170C).
So far this has greatly reduced the solder errors for BGAs and fine pitch QFPs.
We don't design any boards that have either of those components without
hi TG FR4. It seems to be much better that way.
Good Luck........DT
At 10:30 AM 3/17/00 +1030, Phil Dutton wrote:
>Hello TechNet gurus,
>
>The discussion on whether to plug or not has been going on for a long time.
>There appears to be many things to consider.
>We've been recently having poor yields with 19x19 BGAs (1mm pitch) and are
>trying to find the reason for this.
>We have partially tented vias under the BGAs.(soldermask over the via pads,
>but only up to the holes, leaving the holes open).
>Via size is 26/12 and 24/12
>Pads have been gold plated, but I'm not sure if the gold is heavy enough to
>produce soldering problems.
>Could we be having CTE mismatch problems, as we are using regular FR4
>materials?
>
>A summary of information that I have found, mainly on technet is as follows;
>
>LPI soldermask does not tent well, and cannot be relied upon to adequately
>fill a via.
>Tenting both sides can result in soldermask voids within the vias, that can
>outgas during reflow.
>Tenting the top only, still allows wave solder to travel up the via, so
>possibly partially reflowing the solder balls, as with open vias.
>Tenting bottom only, will stop the wave solder travelling, but we have a
>contaminant trapping void on the top side that can outgas again.
>
> From what I can gather, possible solutions are;
>
>1. To epoxy plug the vias. - Is this a common practice?
>2. Make use of blind vias, even micro vias May be worth investigating the
>cost penalty for this. Especially compared to low PWA yields.
> Are we kidding ourselves, thinking that we can use fine pitch BGAs
>and uBGAs without going to this technology?
>3. Something that I've only heard about, and may be a bit exotic, is to
>epoxy plug the vias and overplate copper (with this we can have vias in pad)
> Is this process a reality, or theoretical and perhaps impractical?
>4. Avoid wave soldering (I understand that this may be unavoidable due to
>thru-hole comps)
>5. If there is a choice between BGA and say, PGA, it would be better to use
>the PGA device unless absolutely necessary.
> Although BGA use will only increase, and we should learn to work
>with the technology.
>
>Any discussion, real experiences, ideas, and general enlightenment would be
>appreciated.
>Good forum, (even if occasionally off the rails. Green rectangles can get
>stale)
>
>Many thanks,
>
>Phil.
>
>
>
>
>
>Phil Dutton C.I.D.
>Senior CAD Technician
>IPC Certified Interconnect Designer
>
>Vision Abell Pty Ltd
>Second Avenue, Technology Park,
>Mawson Lakes. SOUTH AUSTRALIA 5095
>================================
>Phone : (08) 8300 4400 (reception)
>Fax : (08) 8349 7420
>email: [log in to unmask]
>Internet Page: http://www.vsl.com.au
>================================
>
>##############################################################
>TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
>##############################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with
>following text in
>the body:
>To subscribe: SUBSCRIBE TECHNET <your full name>
>To unsubscribe: SIGNOFF TECHNET
>##############################################################
>Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
>information.
>If you need assistance - contact Keach Sasamori at [log in to unmask] or
>847-509-9700 ext.5315
>##############################################################
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################
|
