I work for an assembly house in Phoenix Arizona and we have alot of problems with the situation you have
presented.  Believe me when I tell you there isn't anything that hasn't been tried!  It's popular for some
reason in the industry to place vias on SMD land patterns, and as you can imagine when the board comes out
after reflow the solder gets pulled right down into the via usually leaving the device unsoldered or in
some state of half soldered.  I would advice against it, but understand we know in some situations it may
be necessary.  What I usually advise our customers who require this type of situation is:  If it's an
exception and not a rule and you point this out as a precaution on your assembly drawing specifically
showing the areas that will have to be checked its a situation that can be managed.  Yes it will cost you
alittle more then your standard board assembly but pointing it out on the assembly drawing enables the
assembly house to add it as part of their line process!  In fact that goes for anything special about your
board assembly.  Designer think that assembly drawings are for showing component location and that it.
Assembly drawings, and the bill of materials is what we use to assemble your product.  If your job has
really thick ground planes and not thermal relief or you have vias in smd pads then point that out on your
assembly drawing!

I hope this helps

Mike Gish
Sr. PCB Designer
Ditron Manufacturing, Inc.


Barb Zaepfel wrote:

> I have a situation where the engineer has placed via holes  within SMD
> pads, on a double sided PCBoard.
> IPC-SM-782 (3.6.3.1 & 2)  discusses geometries used to channel or "neck
> down" land areas going to a via to prevent solder migration.
>  I am familiar with blind/ buried via technology (IPC-275 5.4.8.4) used for
> multilayer printed circuits.  Can anyone give me information on
> placing vias within smd pads and does it fall within a Class -2
> specification?  Please send any info on Assembly processes used
> to  prevent solder migration /insufficient solder fillets, and size
> recommendations of the via/technology used. (IPC does not seem to
> clearly state that vias cannot be placed within smd pads).
> Thanks!
>
> ################################################################
> DesignerCouncil E-Mail Forum 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 DesignerCouncil <your full name>
> To unsubscribe:   SIGNOFF DesignerCouncil
> ################################################################
> Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
> For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
> ################################################################

################################################################
DesignerCouncil E-Mail Forum 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 DesignerCouncil <your full name>
To unsubscribe:   SIGNOFF DesignerCouncil 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################