I'm putting together a board and it's requiring a footprint for a 16 lead Plastic MSOP.
Usually I try to dig around and find a manufactures recommended footprint and try to stick with that. I figure they're building their parts and should generally know what will work best for a footprint (or Layout).
Anyway I came across a 16 MSOP package from Linear: data sheet (The package and associated footprint I wasn to use is on page 22)
Anyway as I'm sitting there crunching the numbers double checking the width on the part it gives 5.23mm. Now I always try to double check so I subtracted .889mm (twice for both the pads) so I should be left with something close to the 3.2-3.45mm dimension for the distacts between the inside edges of the pads.
Well usually when I do this things check out and I feel confident the footprint is correct. However in this circumstance I didn't end up with that dimension.
Now here's where all my confusion (OCD) kicks in.
I don't believe I can make the footprint work with it's given dimensions.
"5.23 mm MIN" is saying I have to make the outside of the pads be at least 5.23mm long.
"3.23-3.45 mm" is saying the inside spacing between the two rows of pads can very
".889 +/- .127mm" is saying the pads themselves can very that much but probably no more right?
However if I take the smallest inside spacing and add it to the smallest pads spacing I can't get 5.23 mm.
((smallest pad distance) *2 + (smallest inside diameter) = total width for part.
((.889 – .127)*2 + (3.2) ) = 4.724
Not 5.23mm!
(As a side note I also noticed that right above the recommended pad layout they point you to the packaging page for the latest version of their footprints and the footprint there is close. Except they've changed the 5.23mm to 5.1mm. Closer to that 4.724 mm I get when I calculate it but still not exactly there)
So which one of these rules should I break for this given footprint?
Best Answer
So if you make the outside 5.25mm (more than the minimum), make the pads 1.0 mm, which is 0.111 longer than nominal, you get an inside dimension of 3.25, which is within the allowable range.
I don't see the problem.