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LPC2478 SBC and SDRAM question

Started by Unknown June 18, 2010
There used to be thermal relief issues with a via in the middle of a pad, and I recall wanting to use them on an RF board (to cut down inductance) but not allowed to at the time.
Something to do with the extra heat required to heat up the solder to the correct temperature with the extra thermal mass introduced by the via(s), but at the same time not overcooking the components on pads without vias.

Is thermal relief still a consideration ? (assuming you have taken precautions to solve the extra paste required for wicking).

-----Original Message-----
From: l... [mailto:l...] On Behalf Of "Pablo Nez Pcher"
Sent: Tuesday, 22 June 2010 1:34 AM
To: l...
Subject: Re: [lpc2000] Re: LPC2478 SBC and SDRAM question

In my case, assembly costs won't come into play, as I only need a couple of boards for my project, and I will assemble them myself. However, it *is* a good thing to know, just in case I find myself having to make more than a couple of boards.

As for the vias in the middle of the pads, I haven't placed any on top of them. I've spaced them a couple tens of mils, and I'm using a 208-LQFP package, so there will be no BGA solder balls that'll wick into the via. But then again, that's always a good thing to bear in mind.

I'm now reworking my layout with all the recommendations that you all have given me. I'll post the new work as soon as I get it done.

Again, thank you *very* much for your help and your comments!

Pablo

-------- Original-Nachricht --------
> Datum: Mon, 21 Jun 2010 11:00:41 -0000
> Von: "bigft_id"
> An: l...
> Betreff: [lpc2000] Re: LPC2478 SBC and SDRAM question

> Placing components on both sides of a board could nearly double automated
> assembly cost...
>
> Placing via in the middle of the pad is fine, as long as the pad is larger
> to compensate for the extra solder needed which will wick into the via.
>
> --- In l..., "murphy8674@..." wrote:
> >
> > DS board , put memory to other side. Shorter wires, smaller board.Allso
> easier to route out peripheral because LPC has die for BGA and pins are not
> in sequence with bus.
> > Placing via in middle of pad is not very smart idea if it is not blind
> via. Otherwise when in production, all solder will wick into via and no
> contact between component and pad.
> >
>
>

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An Engineer's Guide to the LPC2100 Series