SMD Design Considerations?

Started by FunkyGibbon, January 05, 2011, 09:29:26 AM

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I've not used SMD much, but have a Big Muff project with quite a few CMOS switching options in mind to try to cram into the enclosure, so am thinking SMD might help out here.

When designing a PCB for SMD ICs, e.g. SOIC14, SOIC16, can one place traces under the IC? The only SMD part I've used was an SOIC8 opamp, and that had a thermal pad underneath. Do they all have such a pad? If so, I presume any unmasked traces underneath will be bridged.




Look at the foot print in the data sheet.  Some have the pad underneath, some do not. 


whether it's good practise or not - I don't know - (though I suspect the purists would frown upon it)  but I'm forever running traces under the ICs (sometimes I'm so pushed for space there's nowhere else to go! And all that available 'under IC' land, is just soooo tempting when filling  a 24" monitor!).

I've even created my own 1206 library resistor parts (so the pads are smaller and at the extreme ends of the resistors) so I can also run traces through/in between resistor pads). This actually works better than it sounds - because there's little spatter with SMD, so the possibility of briging is reduced.

You just gotta go with what works with the constraints you're working to.

Most SMD parts I work with have no thermal pad (it tends to be power hungry devices - output ICs, regulators etc that do)....also - don't shoot me for this - even if they do have a thermal pad, quite often, there's nothing to stop you just going ahead and running a track through/over it - it's just a bit of extra copper to help disspate IC heat....the same copper that your track is made from! (obviously check the data sheet as some thermal pads are connected electrically to the IC)


The thermal pads are not present in most common IC's, some FETs will have them though. If there is a thermal pad on the part, it should have it's own pad on the pcb to be soldered to. Otherwise, it is very common with SMD work to run traces underneath parts, be they ICs or resistors, etc. .

Just keep in mind that you should have a minimum clearance (usually set in your pcb layout software) appropriate for the size traces and size of parts you are using to keep from accidentally bridging them.
"Sometimes it takes a thousand notes to make one sound"


Thanks for all the help.