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cool surface mount technique

Discussion in 'General Electronics' started by Quack, Oct 13, 2003.

  1. Quack

    Quack Guest

    After trying lots of different ways to mount SM chips, i think i have
    found the easiest and fastest way, without applying to much heat to
    the chip, or fiddling around under microscopes etc.

    My boards are made using blue PNP on a 600dpi laser printer, and
    etched with ferric chloride chrystals desolved in warm water.

    First clean the board perfectly, using some cleaning alcohol (after a
    good scrub in the sink to remove any discoloration). Once is nice and
    shiny, apply a coating of 'flux 140' to the entire board.
    This protects it from oxidization and also contains some flux to help
    things along. (this can be purchased from RSWWW in UK, part#363-6539
    £7.29).

    Wait for this to dry completely (usually left overnight).

    Now for the actual soldering;

    Add a little solder to the iron, but only a LITTLE, just enough to
    paint it - no blobs hanging off.

    now use the iron to 'paint' the pads where the SM chip will sit - just
    make them go silvery. If you prepared your iron properly, not using
    too much solder, there will be no shorted pads, they should look just
    like they started, except silvery.

    now place the chip carefully (some people may need magnification here
    like me :), depending on the exact size of the chips) - now use
    something like a pen or small screwdriver to press lightly on the top
    of the chip holding it in place with one hand.

    lightly and quickly touch the top of each pin using a clean SOLDERLESS
    iron.

    and presto - done!.

    the solder on the pad itself heats fast enough to bond with the legs,
    no shorts, no messy flux or solder removing braid.
    and the chips dont even get warm! no damage.

    checkout some example pictures here of the latest board i made using
    this technique, works perfectly for me!

    (i use a WELLER standard tip, nothing special for my iron)

    www.quack.cc/board1.jpg
    www.quack.cc/board2.jpg
    www.quack.cc/board3.jpg
    www.quack.cc/chips.jpg

    I may take some snapshots and make a step-by-step html page for the
    next board i make. But i think its pretty self-explanatory, but
    perhaps for a begining to end kind of guide.

    Good luck :)
     
  2. Using your method:

    How do you work with the devices that use 1/100 of an inch spacing
    between the pins?

    How do you handle the new components that are 1/10 of in inch long and
    1/20 of an inch wide?

    For the 1/10 inch pin spaced IC's, I can see this being done, but very
    carefully.

    Jerry Greenberg
    http://www.zoom-one.com
     
  3. Quack

    Quack Guest

    Using your method:
    The smallest i have used was the above P15V 'video switch' chips, they
    are .025 spacing between the pins, and .008 pin width (not sure how
    that equates to inches).

    Actually i tried the other tricks (read lots of little tips and
    techniques by searching through google). And this was by far the
    easiest.

    I dont see why it cant be done with much smaller chips - given perhaps
    a better viewing arrangement..
    As the soldering iron contains no solder at the point of contact,
    there are no shorts that occur - and so the iron can touch multiple,
    if not all pins at once.
    as long as the tracks where prepared nicely - which is suprisingly
    easy todo once coating the 'flux 140', just a touch and they all go a
    nice silver.

    Actually it would be good to see someone else try it and give their
    opinion.. :)


    Alex.
     
  4. Charles Jean

    Charles Jean Guest


    Alex

    I saw your method and think its really neat. No excess solder to
    cause any bridging, just enough to connect the part to the board. I'm
    getting ready to learn SMT, and will probably start with this
    approach. Could I use a 600 dpi inkjet printer instead of the
    laserjet? Could you please post a USA source for the Flux 140?

    BTW, I did a search for SMT fluxes right after I read your post, and
    found a very interesting product put out by Kester-PN TSF6516-a "Tacky
    Solder Flux", designed to paint on SMT component leads and be tacky
    enough to hold them on the board prior to reflow soldering. I'm
    thinking one could prepare the board the way you do, apply ALL the
    SMTs to the board via the TSF compound, then reflow the whole board in
    a toaster oven(this has been done!). This would give you a little
    "wiggle room" when placing each device-you could always adjust it on
    the board with tweezers, etc., until it is positioned just right. Do
    you think the Flux 140 would be sticky enough to do the same thing?

    I also thought about tinning the copper traces, but have heard bad
    stories abut that stuff. Better to coat it with solder. How about
    getting to the point where the Flux 140 has been applied and dried,
    then dip the whole board in a solder bath? Wonder if bridging would
    cause a mess with this scenario?

    I'm looking around now for a good used 30X stereo zoom dissecting
    microscope. What next-die bonding and wire bonding on the PCB?

    Charlie

    If God hadn't intended us to eat animals,
    He wouldn't have made them out of MEAT! - John Cleese
     
  5. N. Thornton

    N. Thornton Guest


    I wonder if one mounted a short scalpel blade in an iron, and possibly
    turned the iron temp up, whether it would be possible to do much finer
    pitch work. Havent tried it.

    Regards, NT
     
  6. Quack

    Quack Guest

    Hi Charlie,
    I use "blue Press'n'Peel transfer paper" (www.techniks.com) which says
    'laser or photocopier' on the instructions. I think it specifically
    needs the toner to make the transfer - i have never tried anything
    with an inkjet.
    But a good photocopier (set to fairly dark) should produce quite well
    too, although i havent tried that either.

    I used to use kodak 'photo paper' (same laser printer) - cheaper than
    PnP, its okay for normal work but cant get the resolution for SM
    layouts.
    (usually using photo paper i always had to make some minor fixups with
    a pen afterwards - these fixups are pretty rare using the PnP film)
    Google doesnt return anything obviouse, im really not sure.
    its made by "seno", the bottle has german + english writing on it.
    Here's a pic; www.quack.cc/flux140.jpg

    sounds great!, i'll have to experiment with that if im going to make
    boards with a lot of SM components - could save lots of time.

    the flux140 is slightly sticky - although it does dry up after a full
    day or two. I guess it depends how much of a coating you slop on :)

    I doubt it would be too good for oven reflowing, but having never done
    any oven work - i wouldnt really know for sure.

    Yeah, i baught some chemical tinning mixture and tried that on a few
    boards. It made them go nice and silvery, but i was very disapointed
    when i went to solder it!

    I had to heat things up SO much to have the solder run nicely it was
    ridiculous.
    The solder just wouldnt touch the pads until the soldering iron was
    touching the pad for a good 5 seconds, then it would flow on (and
    unevenly, not very pretty at that).

    this could have been because after the tinning (then cleaning) process
    i coated the board in flux140 (which usually makes the boards really
    easy to work with).
    Pherhaps the flux140 is designed to work on copper, but not tin.
    strange effect anyway - i wont be doing that again :).


    I havent used a solder bath before - but it could be an interesting
    way of doing it - especially if you left the 'resist' on the tracks,
    and just removed it from the pads where the components would go ...
    (could be tricky).

    whats that ? i havnt heard of them :)

    Im not a PCB expert by any means, i have only been making boards for
    about a year and have settled nicely into my routine for making small
    batches. But as work increase i do need to look at new techniques to
    move things along a little quicker.

    But then again, if its something that will be made in larger qty's,
    there are always the profesionals todo it for you.
    (and in larger qty's the cost is still low per board, just saves a
    whole lot of time)

    Im glad you liked it, i think i will knock up that web page after all
    then.

    Alex.

    PS: i am not affiliated with SENO/flux140 - not trying to sell it or
    push it - i just find it really useful :), no reason this cant be done
    without it.
     
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