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BGA soldering

Discussion in 'Electronic Design' started by Active8, Oct 12, 2003.

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  1. Active8

    Active8 Guest

    hi:

    i think win asked about using an oven. here's a snip from something that
    turned up in a search:

    ******************
    I recently came across and found the idea and the performances
    interesting
    as I'm balancing about using BGAs for homebrewing.
    They have "selected" 3 or 4 toaster ovens and as far as I understand the
    important thing is that they have the power resistors on the bottom side
    of
    the oven. This way the IR heat the PCB and then the solder past. This
    ensure
    a good temperature uniformity across all the PCB's surface (even under
    the
    component package) and a good temperature control.

    To ensure an optimum soldering quality, i.e. follow as closely as
    possible
    the recommended temperature profile, you could one or several
    thermocouple
    on a dummy board and make some measures to optimize the process.

    There's still the problem of solder past dispensing which is normally
    done
    by printing through a laser cut steal stencil. That's of course out of
    question for personnal use (cost).
    ******************

    note: in case anyone hasn't read the page on reflowing solder paste in
    an oven with SMD parts, the paste was just laid down as a bead across
    all the pads and the surface tension lined up the part and cleaned
    everything up when the paste reflowed.

    FWIW,

    brs,
    mike
     
  2. Terry

    Terry Guest

    Active8 threw some tea leaves on the floor
    and this is what they wrote:

    I can say from experience a that silk screened screen works almost as
    well and have produced around 2000 smt boards a day using this method.

    I've also used the laser cut stainless stencil method, both have their
    pros and cons.

    Don't be afraid to use a silk screen, it works. All you need do is save
    up for the solder paste ;-)
     
  3. Active8

    Active8 Guest

    i *know* you realize that stencil thing was a quote from someone else...

    none the less, thanks for confirming my suspicions. IIRC in "Printed
    Circuits Handbook" - Coombs goes to some length discussing the
    fabrication of those steel stencils and how undercutting, etc. can foul
    up the paste. who wants to screw around with steel and failure analysis
    when a silk screen will do?

    now since we have a flame war going elsewhere and this is a topic i've
    been researching, perhaps you could share some of your experiences with
    me. crap! i was hoping to find the latest file i found on making a
    screen. it mentioned three methods and parts were unclear.

    i'm really interested in knowing what works best for you and how to get
    started. why don't we start with the solder paste screen while i try to
    dig up that article on liguid imageable solder mask and etch resist
    screening.

    TIA,
    mike
     
  4. Garrett Mace

    Garrett Mace Guest

    I can say from experience a that silk screened screen works almost as
    I wonder if a silk screen for solder paste could be constructed with the
    toner-transfer method. Print a positive and iron it into the material, then
    use that to fill a negative on *another* piece of material.

    Nah, stuff would probably move around too much. Maybe you could use a
    positive to mask etch-resist onto a piece of thin metal shim stock, and then
    etch through the solder pads...
     
  5. Active8

    Active8 Guest

    On Sun, 12 Oct 2003 14:33:19 GMT, Active8, said...
    of course, with BGAs you don't need paste.

    mike
     
  6. Active8

    Active8 Guest

    On Sun, 12 Oct 2003 14:33:19 GMT, Active8, said...
    of course, with BGAs you don't need paste. te point is that the surface
    tension of the solder *may* pull the chip in as it does for solder
    paste. think of how you can pick up leads with an iron or get a chip cap
    to follow the iron around if you don't hold it down.

    mike
     
  7. Active8

    Active8 Guest

    i really think the normal sesitized emulsion method would easier
    check the links john gave in my toner xfer thread. they claim you can
    etch metal (at least brass - don't ask me why brass is speacial) with
    the film in the first link. i don't see why you couldn't do it with the
    PnP blue or wet, though. the dynaart/pulsar site doesn't mention
    anything special about their product that prevents undercutting of the
    metal. IIRC Coombs doesn't mention using a trapezoidally (is that even a
    word? :) ) undercut stencil, mirror imaged and flipped, so that it
    doesn't lift the paste off the board. he may have mentioned that it
    would be hard to squegee the paste in that way, though. i might play
    with it one day, i think i know the trick. i also found another link
    while checking out photoimagable solder mask. a company that etches
    steel and aluminum stiffeners. Coates or someone. it's all doable.

    but from what i've read, a silk screen will do just fine. i've seen
    elargements of screened images and for paste and soldermask, it's fine.
    i see resolutions that might work for etch-resist, also, but i'm
    skeptical as to whether this would be acceptable. the enlarged images
    i've seen were for traces and one of 'em looked, let's say "not too
    bad". another was almost perfect.

    let's see what Terry has to say about the pros and cons if he'll be so
    kind, please.

    crap! i can't find the link.

    now here's the real ball-buster if $150 ain't too much...

    Ed (GPS) supplied this in this group. see:

    Re: Succes - first finepitch device soldered

    www.stencilsunlimited.com

    brs,
    mike
     
  8. Hal Murray

    Hal Murray Guest

    of course, with BGAs you don't need paste.

    I think they normally use paste to get the flux in there.
     
  9. Active8

    Active8 Guest

    depends on what you mean by paste.

    the "balls" in "Ball Grid Array" are made of solder and you add flux.

    brs,
    mike
     
  10. Georg Acher

    Georg Acher Guest

    |> depends on what you mean by paste.
    |>
    |> the "balls" in "Ball Grid Array" are made of solder and you add flux.

    This is not true for all packages. There are a few BGA-packages made of ceramic
    with so much weight, that they would squeeze out the solder when warmed up.
    Thus, their balls don't melt (non-eutectic) and you need solder paste for proper
    contact. Normal packages (ie. the cheap epoxy-based ones) have solder balls and
    only need some flux.
     
  11. I think I have missed that thread. Do you just use the silkscreen as the
    stencil for applying the paste? Could you direct me to the original thread,
    a link perhaps?

    Thanks

    Klaus
     
  12. Da Man

    Da Man Guest

    I think it was future active or digikey that sells little disposeable stick
    on stencils for BGA rework.
     
  13. Active8

    Active8 Guest

    it was archived at google IIRC. could have been an archive or search hit
    of a web forum. but there was no mention of silk screen AFAIK. it was
    about pcb fab and the poster just mentioned the stainless stencil.
    you'll find more stuff on this than you can shake a tree at via google.
    don't worry, i think i got all the relevant stuff as far as toasters are
    concerned so you didn't miss much.

    i just found that snip in my note. IIRC, win hill or someone was asking
    about BGAs in this group and the question came up about reflowing them
    in toasters.


    brs,
    mike
     
  14. Terry

    Terry Guest

    Active8 threw some tea leaves on the floor
    Opps, your marks for clairvoyance aren't too high ;-)
    I found the stainless stencil to be nice and accurate, but it had
    problems of paste leaking underneath.
    Eww, I'm not trying to start any flame wars :)
    I had my silk screens made at a local screen printer. I'd supply the
    artwork and they would make the screen.

    It all went together in a simple wooden jig, the pcb aligned by hand
    into two pcb thickness stops which were then fixed to the base of the
    jig.

    Apart from careful usage and cleaning the screen clean after each run
    they were fine for the task.

    Lets face it, IR reflow allows for a lot of errors anyway including
    pick and place machine alignment errors.
    My experience is getting a bit old now, and I imagine that there are
    many new techniques around.

    I used the standard solder paste that comes in a $300 tube, it was
    squeeged onto the silk screen carefully by a experienced operator
    and then the parts were placed by an Amistar SMT P&P machine with a
    rotary head.

    From there it was into the IR reflow oven and out the other end to
    inspection and testing.

    I guess like many things, when you work with them everyday, they soon
    seem very simple.

    I haven't made any SMT assemblies (professionally) for years now, but
    I'm thinking of setting up a small home system at the moment to compliment
    my little home development workshop.

    Pics of my setup and pcb efforts are at :-
    http://milkstone.d2.net.au/software/pcb.html
     
  15. Terry

    Terry Guest

    Active8 threw some tea leaves on the floor
    I do as well, tho I've never made a silk screen myself.

    Some kind of light sensitive emulsion would do, perhaps even PVC (the
    whitish clear liquid) ?

    A friend uses that as a photosensitive coating for etching aluminium
    labels, and it works nicely judging from the results.

    From my pov, and I'm not the expert here by any means:

    Stencil:
    Pros: Nice and accurate in terms of area to be covered and *depth* of
    solderpaste.
    Cons: Can get solder paste running underneath and smearing on the pcb.

    Plus hard to make at home without a 50W NC laser ;-)

    Silk screen:
    Pros: Accurate enough, the solder paste flows around the silk strands.
    easy to make compared to stencils, low in cost.
    Cons: depth of solder paste is hard to control, needs an experienced op
    to get repeatable results, must have good tension etc.
     
  16. Terry

    Terry Guest

    Klaus Vestergaard Kragelund threw some tea leaves on the floor
    Sorry Klaus I missed the whole thread as well, and just jumped in as is
    my way :)
    I used to be the technical manager of a small SMT assembly plant. We
    had one Amistar SMT P&P machine, a couple of Dynapert leaded p&p
    machine, a die bonder and a wire bonder, plus re-flow oven and solder
    bath etc.

    We used silk screens to apply the solder paste at that time. Later in
    another job, I came across stainless stencils and so I have had
    experience with both methods.

    The silk screen method is really simple. The screen has been coated
    with a photosensitive paint and exposed thru the CAD generated artwork
    which contains the images of all the SMT footprints (or the inverse).

    So where a footprint exists on the pcb, the that area of the silk is
    clear and will allow solderpaste to be squeegeed thru onto the pcb.

    The operator makes a thin line of solderpaste at the top of the screen
    after the pcb has been placed below into its alignment holders and one
    pass of the squeegee is done. The screen is then lifted off (its hinged
    at the top or side) and the pcb removed. The SMT parts are then placed,
    naturally they stick to the solderpaste and even a decent knock against
    a wall fails to move them!

    If too little paste is placed, a second squeegee can be done, but this
    isn't a great idea.

    Oh, one more thing, the silk screen sits *above* the pcb by a few
    millimeters and in theory only the area below the squeegee comes into
    contact with the pcb, unlike a stencil which lays flat on the pcb.
     
  17. Terry

    Terry Guest

    Active8 threw some tea leaves on the floor
    I found that this is true, however one needs a little more paste for
    the reflow to move the SMT parts into place, and this makes the final
    job a little less neat due to the extra solder and resin present.

    With perfect alignment of the smt part, less solder paste can be used
    and the board is ready to go without any cleanup, however too little
    solder paste and some parts may "toombstone" or stand up slightly with
    one end not touching the pcb.

    This naturally is a "bad thing" (tm) :)
     
  18. Terry

    Terry Guest

    Active8 threw some tea leaves on the floor
    and this is what they wrote:

    <deletia>
    I agree that the temperature of the "oven" should be well regulated
    and follow the accepted profile of reflowing solderpaste.

    Our IR commercial reflow oven was a bit like a pizza makers oven and
    had about 5 heat plates above and below a metal mesh conveyor belt.

    Each bank had its own temperature controller and the settings could be
    altered, as could the speed of the conveyor belt.

    The unit was about 10 feet long and the boards went thru in about 3
    minutes (from memory).
    This makes me cringe because too much solder paste can be a real
    problem and wicking *can* occur.

    If that technique works, it's got to be more luck than anything else
    imho.

    I know that working with SMT is a hassle in many ways (especially at my
    age and eyesight) but I don't think that trying to make it easy by just
    placing solder paste across all the pins of a part is a process that
    will be very reliable.

    I imagine most of us here just make a few prototypes at a time?

    In that case we need something that's as simple and cost effective as
    possible, but what's the answer ?

    I think that a syringe and a good eyeglass may allow the dispensing of
    solder onto each pad of small prototype pcb with reasonable results ?

    Or some easy way to produce a silk screen, but I haven't looked into
    this yet.

    Can some laser printer paper manufacturer please develop a semi stiff
    silk A4 sheet please ? ;-)

    Hmm, whats to stop someone using a a4 sticky backed paper sheet and
    sticking some silk screen to it, then running thru a laser ?

    If the silk melts, its *your problem*, but then again what about a fine
    brass mesh instead ?

    If the brass or the cut ends of the brass screen damage the laser toner
    roller, once again its *your problem* not mine!

    I think there are a few workable ideas here ?

    I've got an expendable laser and nearly empty cart, so if I can find the
    materials, I'll give it a go as soon as I can
     
  19. Active8

    Active8 Guest

    oops. it was clearly mentioned and delineated within asterisks ;-)
    and i'd rather have a discussion with you than read that bs. it looks
    like normal posting is back up. weekend's over and all. and my friggin'
    news client didn't alert me to your reply. either it's reboot time
    (hmm... how many days has it been) or the damn think takes naps.
    ok, i'm planning on making my own for now. done a little research.

    i noticed you mentioned PVC as a photosensitive emulsion. i didn't know
    that it was used like that. anything else you can add to that. i know
    nothing of PVC other than pipe and it's various uses.<snip>

    brs,
    mike
     
  20. Active8

    Active8 Guest

    profiles have been run on 4 toasters, but the pics can't be read and i'm
    not convinced it's right because you have to tailor the profile for
    certain situations.
    and those toasters tested had only top heaters which was claimed to be a
    good thing. that's something else with which i don't neccessarily agree.
    the pic of the beads of paste looked like it was a sloppy way to do
    things but the result looked real good. i think the soldermask helped.
    it was the pads. still, i'd just as soon hand solder than do that.
    yeah, but my time is valuable and i'd like to have little robotic
    foreigners doing my light work for me.
    ouch!! silk screen it.
    the best screen turns out to not be silk. it's polyester and that's the
    one most commonly used.
    what about getting some screen from a graphic arts supply biz?
    great! if you don't **** it up and it's expendable. send it to me. i
    need one.

    well, you may never have made your own screen before, but this thread
    has been a lot better than that flame war.

    talk to ya soon.

    brs,
    mike
     
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