[SI-LIST] Re: LDO Heat Dissipation with Vias
- From: Sol Tatlow <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
- To: STaylor5@xxxxxxxx
- Date: Wed, 17 Jun 2009 19:04:26 +0200
Shawn,
Short answer - talk to your assembly house to be 100% sure you make the
right choice: communication at an early stage is important in any case,
to help optimise the assembly and so avoid unnecessary costs.
Long answer - normally, I would recommend either:
a) plugged, capped, tented or flooded vias in the heatsink pad, or
b) open vias (as many as possible) directly on the edge of the pad, BUT
with a min. 0.1mm line of solder mask between the thermal pad copper and
exposed via hole (particularly if you're using the LDO well within spec).
The best solution is plugging as well as plating over the vias, if you
can afford it - then you can really fill the heatsink with vias, and get
100% direct contact between heatsink and board without running the risk
of voids (voids meaning less than 100% of the heatsink is in direct
contact with the board). Plugging is kind of overkill, however, if
you're just plugging the one LDO footprint on a big board! Talk to your
board manufacturer, and see what possibilities he has to offer, and what
the costs are: plugging can be surprisingly affordable, depending upon
the manufacturer.
At any rate, either solution will avoid solder running into the vias.
The problem in this case with solder run is, in comparison to a small
component, not so much that you run the risk of having a dry joint by
placing open vias in the heatsink pad; rather, you will almost certainly
have small 'pips' of solder sticking out the other side of the vias
which will impede solder paste application on the 2nd side (assuming the
board is reflow-soldered on both sides). This can be a very serious
problem if the board is full, especially in the case of fine pitch
components on both sides.
If, however:
a) the board is only assembled on 1 side (unlikely, right ;-)!?), OR
b) you can place the LDO on the 2nd side (it's generally a good idea
anyway to assemble all large, heavy components on one side, which will
then be the 2nd reflow side) AND the vias are small, OR
c) the 2nd side will be wave-soldered (likely if there are a number of
through-hole components present),
then you should be able to leave the vias open.
That said, keep the following in mind: with a lot of open vias in the
pad, even if it's not a problem for the assembly, you will get some
degree of voiding in the heatsink solder, which will, of course,
_increase_ thermal resistance, the exact opposite of what you are trying
to achieve with the vias! Voiding also decreases, to some degree,
mechanical stability, although this may not be a major concern.
My last comment: if you're using the LDO close to the limit, then IMHO
relying on the vias and board as a heatsink to keep the component
operating inside the spec, particularly without knowing precisely what
the thermal resistance of the mounted component on such an untested
footprint is, is a risky business - the worst case is, of course, that
the component DOESN'T burn out directly while you're testing, but after
a few weeks/months once the product is out in the field!!
Regards,
Sol
STaylor5@xxxxxxxx schrieb:
> All,
>
> We have a TI LDO (TI1963A-25KKT) in a KTT package (9.65mm x 10.67mm thermal
> pad), but we don't have the space on our (16 layer) board for the required
> 50mm x 50mm copper pours on the top and bottom. We're hoping to use the
> internal GND planes to help dissipate the heat away from this component, and
> have placed some vias *nearby* the LDO pad for this purpose.
>
> Wondering if placing vias right in the ground pad under a chip of this size
> is going to cause any issues with the soldering process? Certainly the
> contact with the ground planes in our stack is improved if the heat can
> travel direct instead of only through the neighboring vias.
>
> Any experience/insights would be appreciated.
>
> Kind Regards,
>
> Shawn
>
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