Dennis wrote:

>jami,
>re your 25 mil hi silkscreen
>do the fab shops complain?
>can you actually read it or does it all glob together?
>when it is that tight and when they can't be charted off to the side
>somewhere we usually move them to another non fab layer and move them
>right on top of the pads and present those on paper or PDF
>
>of course the pick and place machine doesn't care

It is pretty small, but is usually legible, and I have been lucky most
of the time in that I could either run a line (5 mil track) on the
silkscreen layer out to a point that was open enough for the ref
designator, or in instances where I have 4 channels, they have identical
layouts, and I can usually identify each component in in at least one of
the 4 channels, and I usually then identify those parts in common
channels as RX-1, RX-2, RX-3, and RX-4, and so on.

As you say, the pick and place machine doesn't care, but the girl who
does most of our prototype assembly does want a ref designation
somewhere, since I usually just kick out a composite Print of MECHANICAL
4 (which I use for board outline), TOP (or BOTTOM) PASTE (which shows
the pads), and TOP (or BOTTOM) OVERLAY (the Reference designators) at
about 2X size, which fills in as a makeshift Assembly Drawing. Once we
start shipping product, then I can worry about real Assembly Drawings.

We actually setteled on the 5 mil line thickness for the text after
talking with our board house, who said that that was the smallest they
were comfortable with, and the 25 mil height came about since it was
about the smallest we could go and still understand with that line
thickness.

I will admit though, on a few of these designs I have had to go in and
apologize to our assembly girl in advance, since some of the things I
have packaged needed to be assembled in a specific order, such as an
0402 cap down in between a tantalum cap and a molded inductor. Once
again, the pick and place didn't care, and there was enough room for
either IR or Hot Air reflow,  but it makes a big difference when your
00SA Tweezers can't get down in to a hole at the same time as your
smallest tip for the Metcal.

JaMi

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