On Nov 26, 2010, at 5:06 AM, Rick Collins <gnuarm.2...@arius.com> wrote:
> At 05:05 PM 11/25/2010, you wrote:
>> On Nov 25, 2010, at 4:05 PM, Rick Collins <gnuarm.2...@arius.com> wrote:
>>
>> > At 05:01 AM 11/25/2010, you wrote:
>> >>> I am missing the reason you must mirror the footprints, however.
>> >>> Aren't the pins still in the same orientation they would be with the
>> >>> standard footprint? Since your DIP packages are mounted in the
>> >>> "normal-side-up" orientation, it seems the pins should be in the right
>> >>> order, unless you have placed the IC on the "component" side of the
>> >>> board in pcb... ?
>> >>
>> >> Yes, the component is at the same position, but traces are on the
>> >> opposite side of board so something must be mirrored. SMD traces are at
>> >> component layer. Through hole components have traces on solder layer. Or
>> >> both of them? Well, you confused me now :-) Maybe I did not have to
>> >> mirror the footprints.
>> >>
>> >> regards,
>> >> Jan
>> >
>> >
>> > Yes, of course you have to mirror the ICs that you are changing from
>> > through hole mount to surface mount. As you say they are on the opposite
>> > side of the board from the pads, so now the pads need to be mirrored,
>> > unless the software is capable of moving the pads from one layer to the
>> > other without changing how they look on the screen. But normally it
>> > treats this as moving the part from one side to the other and you have to
>> > mirror the footprint to keep pin one oriented correctly.
>> >
>> > Heck, if it wasn't needed to mirror the footprint, it wouldn't work
>> > correctly after you do mirror it.
>> >
>> > Rick
>>
>> Rick,
>> Carefully look at an so8 and a dip 8
>>
>> And really explain why you need to mirror a footprint.
>>
>> Because the top side of a dip footprint is identical pinout as so8. (At half
>> of the pitch). Just remove the bottom pads.
>>
>>
>> Another way to think of it, if you took an dip package and made it into a
>> gullwing and soldered it to the dip footprint. You would have vias through
>> the board to that "mirrored" footprint so that mirrored footprint was put
>> on the wrong side of the board and you had to move it through the layers.
>
> Ok, I see that. I was thinking that if you designed a single sided board the
> component footprint pads would only be on the bottom so that when you flipped
> them to the top for the surface mount part, they would now be the wrong
> orientation. I see what you are saying. However, if you use an auto router,
> how do you tell it to ignore the bottom pads?
>
> I guess you can edit the footprint to remove the bottom pads, just leaving
> the top pads, but I wouldn't find it any harder to design a footprint from
> scratch that is actually intended for surface mount work. I think the round
> pad of a through hole part is not the best design for a surface mount part
> which should have oblong pads. But I guess this is all hand soldered and is
> likely hobby stuff, so it doesn't matter as much since it is labor intensive
> anyway.
>
It was a thought experiment not an implementation. If your doing a single
sided board put the surface mount components on the same side as the copper
traces.
In pcb you can flip the board to place components on the other side of the
board.
Steve
> Rick
>
>
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