Correct Wayne.  They also get used as a heat conductor. Micrel's switching regulators do that.  So we have to make a connection to some copper on a layer then isolate it.

Bottom line - I hate QFNs - wish they had never been invented.  Power supply parts often are blown when there's a short somewhere and then you have the Dickens of a time replacing them.  To make it easier to do, what I have done (when there's no center pad and the part is critical and likely to get blown occasionally is to extend the pads beyond the part far enough so I can apply the smallest Metcal soldering iron tip to them to solder down the replacement.  

I know some are good with solderpaste and a syringe but I've not gone that route "yet". 

Sigh.  And to think I started with vacuum tubes.  I'm getting more comfortable with SMT (reluctantly) but not happy that repair is very difficult thanks to QFNs and BGAs.  Board doesn't work?  Into the dumpster or off to the side for repair "some day".  Never thought I'd live to see the day that I'd need an X-ray machine to find a problem!

Bob

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW
Sent: Saturday, June 05, 2010 8:54 AM
To: [log in to unmask]
Subject: Re: [TN] Relying on soldermask as an insulator

That's exactly what makes these parts so frustrating--MUST be disconnect:  If you do ground this, power draw increases significantly.  So you can't even use the paddle connection as a joint ground node.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Saturday, June 05, 2010 8:46 AM
To: [log in to unmask]
Subject: Re: [TN] Relying on soldermask as an insulator

I have used quite a few QFN parts (analog) that use the paddle as an ground connection so check the datasheet very  carefully.  It's not entirely obvious, esp. with Maxim parts.

Bob Landman

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Thursday, June 03, 2010 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] Relying on soldermask as an insulator

routing should NEVER be placed on the top layer under a QFN.  That "big
paddle" is a heatsink, and should be soldered to a pad on the board.   And
even if you choose not to solder it, how can you inspect production quantity boards well enough to be sure the mask is providing adequate insulation?  It's just asking for trouble.

(but I have seen it done)



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