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47310Re: [SeattleRobotics] SMT toaster oven problems

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  • Peter Balch
    Oct 25, 2012
      From: "Tony Mactutis"
      > Peter, I don't think the solder paste would go bad in a few months if it
      > has been in the fridge. I've used much older paste than that.

      That's good to know.

      > There may be scattered beads when you are done;

      Yes, that's how it used to be. But now there's too much grey goo.

      Same solder, same oven. And I think the same profile.

      > I would make sure to leave the board in the oven until all the solder is
      > melted. Don't worry about the profile. I just turn my oven up all the
      > way (the dial goes up to 500 F) and wait for it to hit temperature. I
      > then put the board in and wait for the paste to melt. When it has all
      > melted (maybe 4 to 5 minutes), I give it perhaps 20-30 seconds and then
      > take the board out to cool. No timing required.

      That's pretty much what I do except that I _do_ worry about the profile.

      The oven has an aluminium tray that I sit the board on, a bottom element
      below the tray and a top element above the tray. I turn on the lower element
      for a couple of minutes to pre-heat the board. The aluminium tray nicely
      spreads the heat evenly. Then I turn on the upper element as well until the
      solder melts. Then leave it another 20 sec. Then turn off and open the door.

      It used to work just fine.


      From: "Randy Carter"
      > The aging factors in solder paste are the solvent in the flux evaporates
      > and the solder corrodes. The solvent evaporating is slowed by
      > chilling the container.

      It was sold as a 50ml syringe of solder. So it's always completely sealed
      apart from the very tip of the needle. I don't see how it can evaporate or
      oxidise other than _through_ the plastic barrel. Which is unlikely. Right?

      > The corrosion of the solder is going to make it a little more difficult to
      > solder. Requiring a little more heat and time.

      Well, I tried more time. All that happened was that I over-cooked some of
      the chips.

      (I can't turn the temperature up any further.)

      I wonder it I'm not ramping up the heat fast enough or I pre-heat for too
      long. Maybe the flux has all evaporated before I turn the oven up to
      solder-melting temperature. That way, the solder can flow together into a
      single blob.

      > Also you may be using too much paste.

      Could be but I don't think I'm using more that I used to.

      On the pads with too much paste, the solder used to ball-up. Now it doesn't.

      > The process I use is to heat the board to 95C for 30 minutes
      > to chase out any moisture

      I don't think that's a problem with the chips I've been using.

      We just got some accelerometer and RTC chips which came with big warnings
      about "use within 160 hours of opening the package". I haven't tried
      soldering them yet.

      > Then I crank it all the way up and wait until 220C is reached then I turn
      > off and open the door,

      Aha. So it's a quick up and down. Not a long pre-heat that might destroy the
      flux.

      > I do have a fair amount of solder balls stuck to the board
      > that are easily knocked off with a pick.

      Yes, I used to get those. As you say, they're no big deal.

      Peter
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