How are double sided pcb's soldered in a reflow oven?

Discussion in 'The Projects Forum' started by danforth, Jun 2, 2011.

  1. danforth

    Thread Starter Member

    May 8, 2011
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    So you apply solder paste to one side, place your components, heat and reflow in oven. When you do the same to the other side, won't the components fall off the opposite side from gravity?

    What is a good way to do this?
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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  3. danforth

    Thread Starter Member

    May 8, 2011
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    thanks .
     
  4. #12

    Expert

    Nov 30, 2010
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    I keep finding glue dots under the components I remove from circuit boards. Or, If I may quote the above link, "a teeny dot of epoxy".
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Well, I have two ways of doing it.

    The second way was made necessary when my company shipped my beautiful reflow over an 8 hour drive away. I have here a small desktop "spinner" as I call it, 6 arms that rotate, half the base is formica and the other half is three sections of "hot plates," pre-heat, reflow, cool-down. You place a part before an arm and it is swept through the 3 zones and gets left on the formica all nicely reflowed.

    Of course it is useless for double sided boards as it will wipe off all the parts on the "other side." So I use it for one side, then use a heat gun for the other.

    My preferred method is a chain belt reflow oven. You put parts in one end and they come out the other. Generally you place parts on a "boat," some sort of solid surface to keep them off the belt. We have lots of ceramic substrates lying about so I use them, but even a bare PC board would do.

    Now the thing about double sided boards is... I don't do anything special. Just put the parts in with the side already reflowed down, and reflow the top. Small parts get held in place by the surface tension of the solder. I've never had a part heavy enough to fall off, but if they do then those would get glued down.

    One reflow oven I saw only had a heating element on the top side in the reflow section, this would keep the bottom a little cooler and perhaps not go to reflow.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The chain ovens normally use IR radiating elements on the top side, so as speed and temps are controlled quite precisely the top only gets hot enough to flow for a few seconds and the bottom always remains quite a few degrees cooler (doesn't melt bottom solder).

    You can do similar thing at home by using a electric frypan on low heat to preheat the PCB and bring it near the solder flow temp then a few seconds of hot air gun on top to flow the top of PCB, and it's actually a pretty good way to do SMD for home use in general.
     
  7. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Hola RB,

    Could you elaborate? I hate the idea to build it (buying locally would be impossible because $$ and Customs regulations about anything using above 24V supplies).

    I mean, how to control (kitchen-table environment is possible?l) the temperatures and all that. Gracias.
     
    Last edited: Jun 3, 2011
  8. russpatterson

    Member

    Feb 1, 2010
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    I've had good luck just using a toaster oven with a temp sensor inside. I put a blob of solder paste on some copper right in front by the window. When the blob turns silver and spreads out... count to ten then turn the temp down on the toaster and let it cool. Then do the other side of the board. The parts on the bottom stay on. There's some good articles and products here but I think the controller is over-kill.

    http://www.sparkfun.com/search/results?term=oven&what=tutorials

    http://www.sparkfun.com/datasheets/Widgets/ToasterOven-UserManual.pdf
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I used to use a toaster oven and still have my little oven in the workshop. But the trouble is the "hot spots" near elements and cooler areas elsewhere (normally the element is like a square loop). There is also the hassle of visibility as it's all inside a dark metal box, and hassle to open/close doors etc.

    If you use an electric frypan it has a lot of metal mass that gets a pretty even temperature, and access, lighting and visibility is very good. You can also put a piece of thermal mass in there like a 6" square ceramic tile or piece of flat metal. This evens out all the hot spots and gives a constant "plate temperature" to sit the PCB on.

    Then you use the frypan for the preheat, usually 120'c or so for a minute (see temperature charts on google etc) so the whole PCB is hot but well below melt point. This dries moisture and reduces thermal shock.

    Then use a fan heat gun or hair dryer (reduce fan air flow if needed with restrictions so it doesn't blow stuff around) to heat the top of the boards. With the good lighting and top access you can get your face right over it and easily see the melt flow occur after a few seconds (maybe 10 seconds to get to melt point?), then a few seconds after melt just turn the heat gun off.

    The frypan has a heat adjust knob to set it's preheat temperature and the heat gun air heat is controlled by how close you put it over the top of the PCBs. I works very well, I probably won't use my toaster oven again...
     
  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Encouraging to say the least. Gracias RB!
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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