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Chip Mounting

Discussion in 'Electronic Basics' started by [email protected], Feb 20, 2007.

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  1. Guest

    Hi all,

    I'm looking to get back into circuit design. I haven't messed with
    anything since my college days - about 15 years ago when I graduated
    with an EE. Since then all I've been doing is C/C++ programming work
    in embedded systems, but the bug has bit me to do some tinkering
    again.

    My current project idea has me using some parts I've never done much
    with - CPLD's, FLASH, etc. As far as CPLD's, I can get parts I'm
    interested in in PLCC packaging, which means I can solder or wirewrap
    a PLCC socket. The FLASH I'm looking at is available in TSOP style
    packaging.

    In general, though - how does a hobbyist go about dealing with these
    exotic forms of packaging? TSOP and QFP's are very dense and tiny
    surface mounted pins! Are we talking a very chisled point iron,
    controlled temps, and a very good magnifying glass? I've seen
    adapters that take such packaging out to DIP form, but you still have
    to mount the part to the adapter.

    What do you all suggest?
     
  2. DJ Delorie

    DJ Delorie Guest

    SMT soldering isn't that hard, even with a standard size iron tip.
    Google around a bit, you'll find plenty of tutorials. My way: apply
    solder paste to the board (I have a small syringe of it), place part,
    heat the paste to solder it, use copper braid to remove any shorts.
    It's quick and clean that way.

    Once you get used to it, SMD parts are easier to solder than
    through-hole parts.
    I do tssops (0.5mm pitch) and us-8 (0.4mm pitch) chips by hand
    (http://www.delorie.com/pcb/smd-challenge/mine-front.jpg), you can
    too. 0402s are easy, 0201s are harder, but even 01005's can be done
    by hand (http://www.delorie.com/pcb/first.html)
    For initial soldering, you're best off with a "hoof" tip, one that can
    touch 2-3 pins at once, so you can just swipe it over all the pins. I
    use a regular chisel tip for this, though.

    Besides that, I use a 0.020" conical (pointy) tip, a temp-controlled
    (600F) iron, and a 3.5x magnifying visor.
    It's not hard to make those at home with single-sided copper clad
    boards and some FeCl.
     
  3. Ron M.

    Ron M. Guest

    Yep. We have these SMT's where I currently work that we call dust
    specks. Takes a 20 power microscope to solder them. Their so small
    when you use your tweezers to pick them up you don't have to squeeze.
    They are so light just the capacitance of your body will pick em up.
     
  4. DJ Delorie

    DJ Delorie Guest

    We've named them "quantum capacitors" (the 01005's). One second
    they're there, the next second they've vanished. I usually hold my
    breath when I'm working with them. I use the 3.5x visor to place
    them, then a 60x scope to adjust and/or verify the placement. With
    care, though, they can still be soldered by hand.
     
  5. Guest

    You buy an eval board with the CPLD/FPGA or whatever on it first to
    develop your firmware.
    try http://www.digilentinc.com/ for some reasonable priced
    development boards.

    You must not be wearing an approved esd wrist-strap. Isn't that a
    firing offense at motorola? ;)

    <snip>
     
  6. DJ Delorie

    DJ Delorie Guest

    More likely, skin oils make your fingers sticky enough (or surface
    tension in action) to adhere the parts. The recommended way of
    picking up some small parts, for example, is to dip a toothpick in
    alcohol to make it just sticky enough to pick and place the parts.

    I've found that even the tiniest resdue of flux on my tweezers can
    make it very difficult to place the tiny parts, because they don't
    weigh enough to stay put when you let go of them - they'd rather stick
    to the tweezers.
     
  7. Eeyore

    Eeyore Guest

    Soldering a TSOP or QFP to an adapter isn't actually as bad as you might think.
    You'll need ultra fine solder and an ultra fine tip but it's certainly within
    the capability of a competent tech.

    Graham
     
  8. Eeyore

    Eeyore Guest

    Don't invest in too much solder paste though. It 'goes off' very quickly. I used
    barely a fraction of the syringe I once bought.

    It's worh adding that it doesn't matter if a few pins are 'shorted together' at
    this stage. You can clean up later with solder wick. The wick *with flux* works
    best ! I find the 'no-clean' wick utterly useless.

    I prefer to use HCl and H2O2 to etch actually. FeCl is nasty stuff.

    Graham
     
  9. Eeyore

    Eeyore Guest

    Don't sneeze even with 0603's !

    Graham
     
  10. DJ Delorie

    DJ Delorie Guest

    Mine is SynTECH from stencilsunlimited. The 35g syringe was only $20
    complete and it doesn't need to be refrigerated to get a reasonable
    shelf life. So far I'm happy with it, and if it goes bad, I don't
    have that much invested in it.

    http://www.stencilsunlimited.com/solder_products.php
    Yup, I mentioned that. In fact, it's *easier* to solder if you let it
    bridge the pins occasionally - it takes less time to fix with wick
    than to avoid the bridges in the first place, and the extra solder
    ensures a good joint.
    Wick with flux?
    Personal preference. I've never had a problem with FeCl, it's cheap
    and locally available, and only a single chemical is needed. If I
    stumble upon a local source of HCl and H2O2 I'll try them.
     
  11. Ben Jackson

    Ben Jackson Guest

    Any modern CPLD is programmable in-circuit, and anything available in
    PC44 is so simple and cheap that I wouldn't bother socketing it. A
    PC44 socket that brings the pins out to a PGA is more of a pain to work
    with than a straight PC package.

    I just made a CPLD board for a friend as a trial of gEDA, and I wrote it
    up here:

    http://www.ad7gd.net/xc9536/
    I've hand-soldered kynar (30ga solid) to adjacent pins on a QFP using a
    1/32" tip and no magnification. Of course it helps that I've done similar
    things under a stereo microscope so I knew what was happening without
    having to see it.
    Forget adapters. The key is flux. With enough flux, the solder wants to
    stick to the metal so badly that it's hard to bridge pins. Actually PLCC
    might be one of the worst SMT packages to deal with, just because the
    'hook' part under the chip can bridge and you have to suck that solder
    out. I'd rather do a TQFP given the choice.
     
  12. Rich Grise

    Rich Grise Guest

    .
    A socket is only $1.85 or so:
    http://rocky.digikey.com/WebLib/Amp/Web Photo/New Photos/1-822473-4.jpg

    :)

    Cheers!
    Rich
     
  13. TVisitor

    TVisitor Guest

    Hey all,

    Thanks for the comments on the Chip Mounting, it sounds like I have a
    lot of options to explore.

    FYI, the reason I was talking about adapters to bring it out to DIP
    form was so I can stick it in a breadboard and play with my circuit.
    Sure, if you had a "real" circuit board, you could do it that way
    right away, but I'm not sure how well it lends itself to experimenting
    (Unless you had it soldered to a board, and then brought wires off
    that board to your circuit...)

    After reading all this, a question or two comes to mind:

    Let's say I'm not going to use solder paste (which it sounds like most
    of you don't go for) but was going to use conventional solder with
    flux. If I were going to "slobber" solder all over the pins and then
    wick it up, wouldn't I first also have to tin the pads on the board
    (otherwise it sounds as if I've got metal pins to metal pads, with
    slobber solder all over, but none between the pins and pads!)

    Or the other option - conical tip and then carefully soldering each
    pin - still, you'd have to tin each pad, correct?

    Sorry for the ultra-basic questions, but I guess I've come to the
    right group for it :)
     
  14. DJ Delorie

    DJ Delorie Guest

    Done that: http://www.delorie.com/pcb/r8c-1b-adapter/

    Although that was sdip to dip adapter, the theory is similar. I used
    female headers as a "breadboard" (22ga wire sticks into them just
    fine) and male headers for my logic analyzer.
    If you've put flux on the pins and pads, the solder wicks between
    them. Still, if you're making your own board, it's a good idea to tin
    the exposed copper with solder (wiping off the excess with desoldering
    braid) to make it more solderable and to avoid corrosion.
    No, but you should flux each pad.
     
  15. Ben Jackson

    Ben Jackson Guest

    Keep in mind that a PC44 adapter is very wide (will span multiple
    breadboards) and not cheap. For the same price (but a bit of a wait)
    you could order a pcb from batchpcb with your own design.

    I know you are concerned about re-use and flexibility, but if you
    want to play with a CPLD, just make one board with some lights and
    buttons and play with it.
    No. If you're starting with a homemade PCB that's bare copper, it's
    easier to solder if you do wet all the pads first. You can do that
    with an iron or with tinning solution.
     
  16. Guest

    Assuming your pads are pre-tinned, put down a layer of flux. Too
    much is better than too little, assuming you can clean the board
    later. Tack down the chip, put a tiny bit of solder on the iron (use
    flat edged tip, NOT a tiny conical) and wipe across the pin-row semi-
    quickly near the bottom. The idea is to not try to solder each
    individually, not to depend on the flux in the solder rather than pre-
    added flux, and not to try to get a "good" visual looking solder
    joint. By this I mean some people see a lot of solder and think
    that's good. That's excessive, there should barely be any solder
    visible as it's mostly inbetween the lead and pad where it isn't
    seen. If there was plenty of flux and the pins weren't horribly
    dirty, they will all get coated with solder. If the part was tacked
    down flush before soldering you barely need any solder at all, having
    too high a solder-added-flux ratio then having to remove excess solder
    later is one of the most common mistakes, IMO.

    If it doesn't turn out good, put down more flux and reflow with just
    the tinned iron tip. Adding more solder is a last resort. A ball of
    solder on the iron is too much solder unless the board is pretty rough/
    crude/homemade/etc with large gaps. If the gaps are large you might
    just take a cold tool and press down the stay pins a little before
    soldering. Most people starting out may use 4X as much solder as
    needed, roughly. Ideally you get the technique down on scrap chips 'n
    boards beforehand.
     
  17. Ron M.

    Ron M. Guest

    Yep. We wear them faithfully. It doesn't matter with these things.
    They are really sensitive to body capacitance. I wish I worked for
    Motorola. That would be sweet.
     
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