Jack and guys.
Jack is very correct in recommending a thorough look
at the design is necessary before determining
a sufficient pad, but....
2221's AR specifications are..how to say this
with out offending... not very good for soldered
pads.
Talking with old fart that teaches Design, I was
given a value that I've heard for years regarding
a SOLDERED pad. Not a via or mounting hole, but a
Soldered pad. The value of 2 over the hole size
was given.
The Min AR specification in IPC spec wasn't designed
to provide a solderable pad, but only to say "this
is the bottom line". Our industry has a history of
providing limits but not working values. This is due
to the fact that many working values are APPLICATION
SPECIFIC and what worked for one doesn't work for others.
The values in IPC will actually solder ok, but the
some assembly (not all) people will not like it and the
chances of component replacement are low. Thicker board
are more apt to re-soldering since there is enough in the
hole to keep the plating from ripping out.
These are the questions to ask...
*Could the component be replaced - or would the board just
be discarded.
*Is space in demand on this board.
CR
>
> The difficult part of your question is the phrase
> "proper value". In my opinion a good designer should
> do two things:
> 1) determine the MINIMUM acceptable pad diameter based
> on some logical scientific approach.
> 2) ADJUST the minimum (IF NEEDED) to match your
> products to particular manufacturing needs
>
> As for the MINIMUM, the approach that makes the most
> sense to me is the one suggested in IPC-2221. since
> the MINIMUM goal is to have a good plated hole with at
> least a little bit of copper pad around it (no
> breakout), take the drill size, and a minimum amount
> of copper all around (called annular ring, at least
> one mil? so add 2) and then add enough to allow your
> fabrication vendor some "wiggle room" since
> manufacturing processes aren't perfect (fabrication
> tolerance).
>
> As for the ADJUSTMENT, some designers need more space
> so they will REDUCE the minimum but add a teardrop to
> protect against breakout. Others will ADD to the
> minimum to allow for rework or hand soldering. The
> result is not the issue, the designer taking into
> account the needs of others down the line is what is
> crucial.
>
> The tricky part is the fabrication tolerance. You may
> want to speak with your fabrication/assembly partners,
> but in your case I would think you are going for high
> volume and cheap price (consumer products), so I would
> give the vendors as much leeway as possible (add more
> copper) if the boards aren't too dense.
>
> Just don't go below the minimum unless you are
> prepared to defend your decisions!
> (especially if reliability problems arise or vendors
> are upset with you for making their lives difficult)!
>
> Jack
>
> p.s. Just for the record, I believe that adding a
> generic multiplier to a hole size is bogus, unless you
> have studied your design needs and manufacturing
> processes enough to develop your own "rule of thumb"
> (and be warned that explaining a board problem by
> saying "I did it that way because some old fart did it
> that way" usually doesn't make you look so good)
> And from my experience, pads aren't used for "heat
> dissipation".
>
> onward thru the fog...
>
>
> --- Ahmed El-Sanhoury <[log in to unmask]>
> wrote:
> > All
> > When we have a passive component used in consumer
> > electronics application
> > (TV application single layer),which has a hole
> > diameter equals to 1.2mm,what
> > is the proper value of pad diameter to satisfy
> > maintenance and reliability consideration.
> >
> > Thanks
> > A.sunhoury
> >
> >
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