Re: gEDA-user: OT: soldering QFN packages with exposed bottom pad?

[joe tarantino <joeft@xxxxxxxxxxxxx>]

   I think a few small holes are better than one large one.  What you are
   trying to achieve is a path for heat and maybe flux, but not solder.
   To get solder to the part, either pre-tin the pad or use paste.  Then
   you are just heating the pad to re-flow the solder.  Trying to apply
   solder from the back risks shorting other pins.  You should be able to
   come up with a footprint (w/vias) that works for hand soldering and
   doesn't annoy the production soldering process too much.
   Joe T

   On Tue, Apr 28, 2009 at 5:33 AM, Stefan Salewski
   <[1]mail@xxxxxxxxxxxx> wrote:

   On Mon, 2009-04-27 at 22:55 -0500, Bill Gatliff wrote:
   > Guys:
   >
   >
   > Got any recommendations for how to hand-solder a QFN package that
   has an
   > exposed bottom pad?  At the moment, all I have is a soldering iron.
   >
   > Do you think I could just put a plated-through via underneath the
   pad,
   > and then flow solder through that from the back side?
   >
   >
   > b.g.
   >

     Hello,
     I will have the same problem soon, when the layout of my DSO board
     is
     finished. Your suggestion "plated-through via underneath the pad"
     should
     work (indeed is does, some people did it). I am not sure if I
     should
     make a large hole so that the tip of the soldering fits in it, or
     only a
     smaller, which I may fill with a piece of copper wire to conduct
     heat.
     The problem is to control/see when the solder flows -- to prevent
     heating to long. And one problem may be that too much solder may
     flow in
     making connections to other pins. And of course fixing the device
     at the
     right position is difficult when soldering from the backside.
     For cheap, small PCB boards heating the whole PCB with a heating
     plate
     may be fine, but I do not like to do this with my expensive board.
     I think I will have to do some experiments...
     Best regards
     Stefan Salewski

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