As I remember, you need to plan ahead with a different via pad stack so
that some vias can have thermals (gnd & vcc Vias) and some that don't
have thermals.
It's a pain to do selective thermal reliefs on vias from a designers
view point, because it adds layout time and output complexities.
Lou Dallara CID
Johnson, Joseph SEA wrote:
> We have used via's for the last 10 years without any thermal relief on
> our boards. If someone does need to put a wire in it for some reason
> they still can and solder it.... But as what has been discussed here
> there is no reason to have a thermal relief on a via.
> If you have a lead in the hole then this is no longer considered a
> via, and you do need a thermal relief on it for proper soldering.
> Joe J.
>
> ------------------------------------------------------------------------
> *From:* DesignerCouncil [mailto:[log in to unmask]] *On Behalf Of
> *Karl Bates
> *Sent:* Monday, April 06, 2009 8:40 PM
> *To:* [log in to unmask]
> *Subject:* Re: [DC] when do vias require a thermal relief
>
> If you happen to have a via in pad (via in the pad of a SMT
> component), then you could need thermal relief.
> You would want to use a specific via padstack that is recognizable,
> and could have a unique thermal pattern.
>
> Karl
>
> please consider the environment before printing this email
>
>
>
>
> ------------------------------------------------------------------------
> Date: Mon, 6 Apr 2009 16:44:08 -0700
> From: [log in to unmask]
> Subject: Re: [DC] when do vias require a thermal relief
> To: [log in to unmask]
>
> There are no IPC restrictions on this topic that I am aware of… but I
> have some ‘old guy’ advice… for what it is worth.
>
> Plated through hole thermal relief structures are designed to create
> some thermal resistance between the outer pad and the plane that it
> connects to internal or external to the board...
>
> Why? Because the intended pad on the outer layers connected to the
> plated through hole needs to be able form a reliable solder joint with
> the lead of the thru hole component mounted on the surface of the
> board and soldered to that outer pad… the inner planes or outer planes
> act like heat sinks for the most part and lengthen the time required
> to heat up the pad to solder melting temps.
>
> How does having a thermal relief help? It allows the pad on the
> surface to heat up more quickly during the soldering process (reducing
> the heat sinking effect) so that the weld between the copper and the
> component lead all gets to the proper temperature at the proper time.
> A good solder joint happens when the lead and the pad and the solder
> all reach the proper melting point at the same time and then are
> allowed to cool together at the same rate. A full solid connection to
> the plane would sink the heat into the plane as fast or faster that
> you could heat the pad… and very likely you would end up with a cold
> solder joint which leads to premature failure of the assembly. This is
> true for surface mounted components too… surface mount pads that are
> soldered to pads in a copper plane need thermal relief connections to
> the plane to facilitate a good solder joint. It’s basically the same
> problem…
>
> My thoughts on a thermal relief pad as a VIA…
>
> WHY? You don’t solder to it… so what purpose could it possibly serve
> to reduce the thermal connection to the plane? Go ahead and connect
> them solidly to the plane they don’t need thermal relief.
>
> Most applications for vias have no component leads in the holes(hence
> the term via instead of thru hole pad)… and often they are used to
> transfer heat from one side of the board to the other to cool a
> component that gets hot in normal operation like the belly thermal
> pads you find under some hotter chips. A ‘thermal via’ with thermal
> relief connections would degrade the thermal conductivity of the via
> to the plane… making the situation worse and defeating the purpose of
> the thermal heat transfer purpose of the via. .
>
> So I can’t see any benefit from having thermal relief on a via… and in
> some cases it could be worse to do so.
>
> So I wouldn’t do it.
>
> Best regards,
>
> Bill Brooks
>
> Datron World Communications, Inc.
> (760)602-7004
>
> ------------------------------------------------------------------------
>
> *From:* DesignerCouncil [mailto:[log in to unmask]] *On Behalf Of
> *Allen Maddox
> *Sent:* Monday, April 06, 2009 12:07 PM
> *To:* [log in to unmask]
> *Subject:* Re: [DC] when do vias require a thermal relief
>
> Try IPC-2221A 9.1.3 The first sentance pretty much says what you said..
>
> Allen Maddox, CID+
>
> Allen Maddox Printed Circuit Board Designs and Services
>
> www.maddoxpcb.com <http://www.maddoxpcb.com/>
>
> http://www.linkedin.com/in/trailsitter
>
> ------------------------------------------------------------------------
>
> *From:* Dee Stover <[log in to unmask]>
> *To:* [log in to unmask]
> *Sent:* Monday, April 6, 2009 2:58:40 PM
> *Subject:* [DC] when do vias require a thermal relief
>
> Where in IPC does it specify when a via connected to a plane must have
> thermal relief as opposed to direct connect?
>
> I would think it would have more to do with the assembly process than
> the fabrication process. For instance - wave, dip or drag soldering.
>
> You input is appreciated.
>
> Thanks,
>
> :-) Dee Stover [log in to unmask] <mailto:[log in to unmask]> x8489
> Tech Associate II
> National Optical Astronomy Observatories
> www.noao.edu <http://www.noao.edu/>
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-- *
Louis Dallara Photography
Web Site: http://louisdallara.com <http://louisdallara.com>
Blog: http://louisdallara.com/wordpress2/*
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