X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f
X-Recipient: geda-user AT delorie DOT com
X-Envelope-From: paubert AT iram DOT es
Date: Fri, 28 Feb 2014 10:46:24 +0100
From: Gabriel Paubert <paubert AT iram DOT es>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Via under a pad?
Message-ID: <20140228094624.GA9706@visitor2.iram.es>
References: <CALSZ9gohqcqTT7y8x8x2wZdrcv8s2M-DhaN8vB5Zf6uvpRrCYA AT mail DOT gmail DOT com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALSZ9gohqcqTT7y8x8x2wZdrcv8s2M-DhaN8vB5Zf6uvpRrCYA@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-Spamina-Bogosity: Unsure
X-Spamina-Spam-Score: -0.2 (/)
X-Spamina-Spam-Report: Content analysis details:   (-0.2 points)
  pts rule name              description
 ---- ---------------------- --------------------------------------------------
 -1.0 ALL_TRUSTED            Passed through trusted hosts only via SMTP
  0.8 BAYES_50               BODY: Bayes spam probability is 40 to 60%
                             [score: 0.4862]
Reply-To: geda-user AT delorie DOT com
Errors-To: nobody AT delorie DOT com
X-Mailing-List: geda-user AT delorie DOT com
X-Unsubscribes-To: listserv AT delorie DOT com
Precedence: bulk

On Thu, Feb 27, 2014 at 03:01:21PM -0500, Rob Butts wrote:
> Is it bad practice to put vias under pads?  These pads are twice as big as
> the vias.

It can be done, and it is the only solution for very fine pitch (0.4mm) BGA,
but in this case you typically use filled vias, or microvias that are actually
blind vias (not supported  by PCB for the time being).

With standard (non filled vias), I have used it for the center pad of chips
which need a good thermal and electrical connection like LT3510 (actually
this is the also the only ground connection on that chip). In this case the
best is to have lots of small vias to maximize heat transfer. If it is only
for electrical contact (it happens, like LTC2636 in DFN package), a single
via is sufficient.

In case of many holes, it may be useful to fill the holes with high melting 
point solder (whether you use solder wire with an iron or paste and reflow
is up to you) or epoxy before proceeding to the last step.

If you have many vias to fill, ask your PCB house whether they can and how 
much they charge for filling vias.

	Gabriel