Dee,

This is a somewhat old thread I am working my way through but let me exclaim a
resounding YES! to what you say here.  I'm sure my pads are way overkill (too
wide) but then the stuff we make is designed to last at least 30 years.  The
boards are very expensive to make and we can't easily toss them if something is
wrong with them when new or years from now.  They need to be REPAIRABLE  and of
course, with linewidths smaller than 12 mils if there is a bad short, that can
be a chore (get out the wirewrap wire and UV tack cure epoxy and a microscope!).
So we try to make the pads as large as possible so the desoldering tool lands
only on copper and not on FR4 which it will burn/scar.  The tool we use has a 1
mm tip (smaller tips plug up way too fast) so that means a 0.040" OD pad is
about the smallest that can be easily desoldered (vias of course don't count).
That has not caused us any hardships and we do get two 12 mil traces between a
pair of pads on 0.1" centers (everything we do so far is thru-hole).

This kind of design rule is not for everyone (sure would be difficult to make a
Pentium III board this way!

I would hope, Ray, that your new PadMaker program does allow one to continue to
make pads with these constraints.  I wish we were making PCBs like jelly beans
and used bed-of-nails testers and could simply toss in the dumpster anything
that doesn't test A-Ok but no can do.

Once in a great while we do get a stubborn board that refuses to give up the
secret of why it won't work and then we toss it aside for a rainy day but geez,
that hurts the bottom line either way!  9 times out of 10, it's a simple part
mix up (wrong part or inserted wrong).  Recently we had an assembly house do a
no-no.  They put indicator leds on the wrong side of the board (supposed to be
on the solder side AFTER wave soldering), then discovered the gaff, desoldered
and repaired them and shipped them to us.  Leds wouldn't light up.  Say what??
So we discovered that the person doing the repairs had a VERY heavy hand and suc
ked off a lot of the led pads and then put the leds back in the boards COVERING
UP THE DAMAGE!   You would have had to be blind not to see the damage BEFORE the
leds were soldered back into the board.  They had to go back to the assy house
to have rivets put in.  Those boards (assembled) cost ~$250 EACH!  No way I
wanted them (10 of them) tossed in the dumpster.

Regards,

Bob Landman
H&L

---- Original Message -----
From: Dee Stover <[log in to unmask]>
To: <[log in to unmask]>
Sent: September 24, 1999 1:18 PM
Subject: Re: [DC] Pad sizes for good soldered joints (Through Hole)


> Something else to consider - will the end product be fixed at the component
> level or board level.  Meaning, if the board fails in the field or back at
> the shop, will a tech be expected to fix it at the component level or just
> toss the entire board and put in a new one.  Maximum pad is necessary for
> desoldering and resoldering parts, reduces the chance of pads lifting off.
>
>
> Dee Stover  [log in to unmask]
> Design Drafting Technician
> National Optical Astronomy Observatory
>
>
>
> At 04:56 PM 9/22/1999 +0100, you wrote:
> >Dear all,
> >
> >All the data I have regarding pad sizes (e.g. IPC-2221 9.1.1) give examples
> >on how to calculate Minimum pad sizes, usually from a PCB fabrication
> >standpoint. IPC-2221 states that all lands and annular ring shall maximised
> >wherever feasible, consistent with good design requirements and electrical
> >clearance requirements.
> >
> >My questions are: How do I determine the pad size that will give me the best
> >soldered joint?
> >Would the minimum pad size calculated by the formula in section 9.1.1 still
> >provide a good soldered connection?
> >Are there upper limits to the size of a solderable surface of a pad?
> >Would the Ideal be to maximise all the pad sizes to one size that meets the
> >requirements of 9.1.1 for all holes?
> >
> >I am surprised there is no reference to soldering in the standard relating
> >to pad sizes, I am sure the pad size must have some effect on the solder
> >fillet.
> >
> >
> >Thanks in advance,
> >
> >Mark
> >
> >
>