I think that the theory behind this is that there is another layer under the
component, and when the solder paste
melts, it could possibly push the component body upwards, and when that
happens you get the likelihood of Tombstoning.

If you look at a side on drawing of board/pads/resist, you will se that the
resist usually makes the copper & resist to
very similar levels, if you then add a legend under the component the part
can possibly see-saw on it, despite the solder paste.
Even worse would be when the legend is not exactly aligned, and the line is
further towards one pad than the other.
I cant find anything to hand that supports this theory, so if I'm just
talking codswallop someone will undoubtedly say so...;-))

Myself I do not bother with silk screen for SMT components at all, for the
most part there is no need to provide a
picture/outline of a resistor and component density usually means that there
is no room for the reference designator.
After all, they get covered by the part.

So the boards do not get any SMT reference, unless it is specifically
required to identify site changeable parts.

Instead a SMT component only drawing is made that the production and
engineers can use for reference.


> -----Original Message-----
> From: Paul Noonan [SMTP:[log in to unmask]]
> Sent: Wednesday, June 26, 2002 10:09 AM
> To:   [log in to unmask]
> Subject:      [DC] 0603 LEGEND & REPORTED TOMB STONING
>
> Hi All
> I have had a query from someone who has a very dense board with lots of
> 0603
> passives ,
> for legend to mark these 0603's instead of an ink box drawn around the
> component they used an ink dot or line drawn between the pads this then
> leaves more room for the Ref Des.
> The board shop has reported that placing any ink between the pads may
> cause
> tombstoning has anyone had any experience with this.
>
> Paul Noonan
>
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