Of course it works. The only time it is an issue is when the buildup of
soldermask is so high that you get excess solder joints on the chip caps
and resistors. IC network resistors are not an issue. The issue on the
connectors can also be resolved by working with the PWB vendor to keep
the masking to a minimum over the vias.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Monday, September 28, 2009 1:29 PM
To: [log in to unmask]
Subject: Re: [TN] Plug vias first or second.
Thank you for the input. I see your point, but in our case we are
talking about resistor and capacitor networks. These have 10 mil pads
and 20 mil pitch. There are no vias under the component nor between
pads. It is not possible from a design perspective to move the vias far
enough from component to allow proper gasketing. We also have a fine
pitch SMT connector with 20 mil pitch. There are center pads on these
onnected to the ground plane. This is where the vias are that cause the
issue. I must have the vias here to connect to the plane. DFM.... I
know...We have other boards from other vendors with the same design so
this should work as long as the PCB vendor has a good process and is
given correct and manageable parameters.
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com
Stadem, Richard D. wrote:
> I posted a number of reasons that the vias should not be designed in
between the pads on this forum awhile back. Many responded with
"likewise" comments. Inability to gasket the stencil flat to the PWB was
just one of a list of issues it creates.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of stephen gregory
> Sent: Monday, September 28, 2009 12:45 PM
> To: [log in to unmask]
> Subject: Re: [TN] Plug vias first or second.
>
> Hi Leif,
>
> I've run across the same issue in the past. One way to get around the
bumps is to have your stencil vendor half etch all the vias into the
bottom of the stencil. I've had to do that a few times...
>
> Steve
>
> --- On Mon, 9/28/09, Leif Erik Laerum <[log in to unmask]>
wrote:
>
>
> From: Leif Erik Laerum <[log in to unmask]>
> Subject: [TN] Plug vias first or second.
> To: [log in to unmask]
> Date: Monday, September 28, 2009, 11:03 AM
>
>
> Technetters,
>
> We have a problem with certain PCBs where plugged vias have a solder
mask "mound" that creates a coplanarity issues and also causes the
screen to be elevated. The elevated screen results in excessive solder
deposit and massive amount of shorts for the finest pitch parts. We
believe the root cause is the practice of plugging the vias with solder
mask after they have been covered. This seems to result in these mounds
instead of a planar surface. From what we understand, this is the
preferred order of the two steps by PCB vendors.
>
> Why is this preferred?
> Is there a IPC spec that covers this issue?
> Is it reasonable to specify that the plugging must result in no mounds
and 0 elevation of the vias?
> Is there a particular process weakness from the PCB vendors that
causes this defect?
>
> Thank you.
>
> -- Leif Erik Laerum
>
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