footprints

Discussion in 'General Electronics Chat' started by deefactorial, Aug 19, 2008.

  1. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
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    When creating a footprint from the manufactures specification; They often specify a range +- 0.012 for example. What value do I choose for the pad. The center value the min or the max. I guess it depends on the design of the spec but should I create the pads to the max size of the spec? And should I add a little bit more around the edges for extra.

    For hand soldering surface mount devices with heat sink pads on the bottom of the surface mount component, do I extend the pad outside of the footprint so I can heat the pad with the soldering iron. What is the order of the process of soldering surface mount components with heat sink pads. Do you tin the heat sink, put the component on the heat sink, re-heat the heat sink pad/soldering the component to the heat sink, then solder the other pads?

    Thanks for the help.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    I'd go for the middle of the specification. If you're etching them at home, that gives you the maximum +/- tolerance. PCB manufacturers have it pretty much down to a science, but hobbyists don't have that kind of quality control. Over-etching will make the pads smaller; too much and you won't have enough pad material left. Under-etching will leave them larger; too large and ajacent pads will be shorting against each other either before or after solder reflow. By going with the middle of the specification, you get the widest margin for error.

    Usually, you would use a solder paste which is a mix of flux and tiny beads of solder. The solder paste is applied to the board pads, the IC's placed, and the board re-flowed (heated until the solder flows) in an oven. A number of hobbyists use toaster ovens. Beware that various ovens can reach very high temperatures during the preheat cycle.

    The oven is pre-heated to a moderate temperature, say 250°F. The board is then placed in the oven and allowed to come up to temperature for a number of minutes. Then the temperature is rapidly increased until the solder flows, and the heat is immediately turned down. It's important to let the board cool undisturbed, or "cold" solder joints may occur.

    Google is your friend here. Try searching for "Toaster oven PCB" or "Toaster oven reflow"
     
  3. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If a manufacturer specifies a footprint dimension I use their nominal dimensions.
    Most manufacturers have footprint documentation in the datasheet or an application
    note. If a manufacturer specifies lead spans then I make the pads protrude a little bit
    beyond the maximum lead span. I would not make the pads wider than the maximum
    width.

    Do a search for IPC-7351. IPC-7351 specifies three geometries for footprints.
    You pick the geometry that matches your processes. I believe that the "most"
    geometry will be the easiest to hand solder.

    I would extend the pad outside the footprint.

    You are better off soldering these on a hot plate or manually with solder paste.
    If you start tinning the heatsink you will end up with an uneven surface. If you
    end up with air bubbles between the tinned surface and the board pad you will
    increase your thermal resistance.

    For hand soldering a component with a heatsink I would place paste on the board heatsink pad, place the device on the board and solder one or two of the other pins (this will keep the device steady). I would then heat the paste with the iron. Then finish
    the soldering the other pins.

    I have a variety of footprint creation scripts for gEDA/PCB at http://www.luciani.org.

    (* jcl *)
     
    Last edited by a moderator: Aug 19, 2008
  4. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    I checked out their spec. I will have to switch to my windows machine to install the software. Looks like it could be a handy footprint tool. I found the part about the variations on component land patters to be useful. It looks like it is a trade off between large pads and small pads are solder joint stability and component density. The larger the pad the more robust the solder joint will be but I can pack less components closer together. The smaller the pad, the less robust the joint will be but more components can fit closer together.

    What about creating a Via in the middle of the heat sink pad. I could solder the two ends of the components then turn the board around and flow solder through the hole until it is full. kind of like what they do on this board.
    http://www.youtube.com/watch?v=E6WrzdgwyIw

    I've added the solder paste to the list of Items to order. Right now component space is not really an issue, I would rather have large footprints that are easier to hand solder than too small of a footprint that makes it really difficult to hand solder. Future modifications can decrease footprint size as my prototype develops and I can move on onto more advance reflow techniques. But in terms of creating larger pads as needed I feel I will only extend them outwards from the footprint creating space next to the component to place the soldering iron.

    But I don't know if I'm ready to start modifying a toaster oven for reflow. Although that sound like an excellent option for future production and I'm grateful for knowing that option exists.

    Thank you John Luciani and SgtWookie
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can pick up a brand new toaster oven for between $20 and $40.

    You can pick one up at a garage sale, swap meet or Goodwill store for practically nothing.

    With all the time and effort that goes into creating and making a PCB, and the expense of all of the components you'll have to purchase, it seems like one of these ovens is one of the better bargains for a hobbyists' PCB production.

    You don't have to take my word for it. I suggest it will be well worth your time to perform the searches I suggested, and read through the experiences of those who've done it themselves.
     
  6. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    I have done the plated through via. I didn't evaluate the thermal resistance. I think
    you are much better off with the paste. Reflow manually will work.

    A skillet is very easy to use as well. On the skillet when the solder color changes from dull to shiny gently push the board off the skillet. The skillet does not require
    temperature profiles or any complex control. $20 at Target.

    (* jcl *)
     
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