Re: gEDA-user: size of 0402 footprint

[DJ Delorie <dj@xxxxxxxxxxx>]

> My next project will take into the realms of the barely visible: 

0402's are huge compared to 01005's.

http://www.delorie.com/pcb/smd-challenge/
http://www.delorie.com/pcb/smd-challenge/insanity_II.jpg

> The majority of parts will be populated by a third party in a reflow 
> oven. I guess, they are ok with the small size footprint. They even told 
> me not to choose large footprints as this would encourage tomb stoning. 
> However, I will need to manually rework some of the more sensitive 
> resistors. Would that be possible with the small size footprints?

There are three "standard" 0402 sizes - least copper, normal, and most
copper.  Non-standard sizes may be significantly bigger to allow for
manual soldering.  Look for the INDC1005[LNM] footprints in ~geda.
Choose the N size unless you have a good reason to do otherwise,
especially with third-party automated paste/place/soldering.

As for rework, get a hoof tip for your iron that's big enough to touch
both ends of the top of the 0402 at the same time.  The part should
just stick to the iron and you can lift it off.  Alternately, invest
in something like the Metcal Talon, which has two tips to act like
tweezers.  Metcal also has a "C" shaped tip that can heat both pads at
the same time.  For the really small parts, heating one end is enough
as conduction heats the other end.  Hard on the part, but if you're
going to toss it anyway...

You'll want needle-end tweezers too.  And a magnifying visor.

In any case, having a bigger pad will not really help you rework
these.  They're too small for the pad to really help; you'll always be
working from the top or sides of the part.  Heating both ends at the
same time is the key to reworking these small parts.

Even with bigger pads, you can control tombstoning (somewhat) by
controlling your paste mask.  PCB doesn't have an editable paste mask,
so you'll end up writing a script to go through one .pcb and twiddle
the pads to make a second .pcb from which you'll export the paste
mask.


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