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Discussion in 'Electronic Design' started by John Larkin, May 27, 2006.

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  1. John  Larkin

    John Larkin Guest

    We sent out a batch of 35 VME module kits to be built by a contract
    assembly house. There are lots of high-value resistors on this board

    http://www.highlandtechnology.com/DSS/V360DS.html

    in the filters and such. Boards started failing in test and it seems
    to be caused by ionic contamination trapped under parts. The stuffers
    used water-soluble flux (which is contrary to our rules) and obviously
    didn't clean the boards enough. They claim to use a super
    high-pressure conveyerized spray cleaner with super-clean water. I'm
    skeptical about the cleanliness of their system, and they just told us
    that the cleaning line "just broke" so now they can't rerun the
    boards.

    So, what's your experience? Can a water-soluble flux be reliably
    cleaned off to decent leakage levels? Can they really clean under
    surface-mount parts?

    Last time this happened, some years ago with another assembler, we
    nabbed a sample of their wash water, and it was 20x as conductive as
    tap water.

    I'm thinking in terms of slowly hand-scanning each board with a
    water-pic sort of high-pressure wand, with single-use distilled water,
    or something like that. It looks tricky to clean under surfmount
    parts, especially with water. Our normal process is RMA flux followed
    by solvent wash in a vapor degreaser.

    Is there anything that can be added to the wash water to reduce its
    surface tension, to get under parts better, but that isn't itself a
    source of leakage? Maybe an alcohol/water mix?

    Are there any lead-free implications as regards leakage? This board
    uses regular pb/sn solder, but it may become a concerm some day.

    John
     
  2. Ken Smith

    Ken Smith Guest

    Did this "just broke" happen right after you complained about the PCBs adn
    asked that they be rewashed?

    If so, I'd start to suspect everything they've told you. Chances are it
    really failed before your PCBs. It wouldn't suprise me to dicover that
    they cleaned them with a crub brush and a bucket of water.

    As far as I know the board house that did the last PCBs for me used what
    is really a dishwasher with a different trim option. The trick seem to be
    just to throw enough water at it for long enough.

    The only flux I've really had trouble with was the "no clean" type. I
    don't think it should ever be used.

    Using dirty solvent is trouble, whether or not it is water.

    .... or ...
    You could attach the PCBs to the wall and get 20 guys with fire hoses to
    blast away at them for a day. If one squirt with water only lowers the
    flux level by 10%, 1000 squirts will get the board clean.
    I believe that there is a detergent of some sort added to the first wash.
    After that it is washed with pure water.

    A lead free soldered PCB will short out due to tin whiskers, so the ionic
    stuff won't matter at all. :)

    The unobtainium in the lead free flux may be harder to move.
     
  3. Jim Thompson

    Jim Thompson Guest

    [snip]

    Due to allowing ESD devices to be used as clamps ?:)

    ...Jim Thompson
     
  4. In one case, I successfully improved the leakage resistance of a board
    with surface mount parts, but it didn't include a battery or any
    electrolytic capacitors. I first soaked the board in warm 99% pure
    isopropyl alcohol for an hour or so, to remove any covering of rosin
    flux (some parts had been hand soldered), and then cooked the boards
    for an hour or so in near boiling (perhaps 85 C) distilled water,
    followed by a couple rinses in cold, clean distilled water.

    I then baked the boards in a vacuum oven at 50 C, overnight, to make
    sure all the surface bound water escaped.
     
  5. John Larkin

    John Larkin Guest

    My real concern is leaving hygroscopic crud under the parts. If we
    bake them dry, they'll pass test, but may well later suck moisture out
    of the air and fail in the field.

    We've never had any leakage problems with rosin flux. It's not
    conductive whether you clean it or not.

    John
     
  6. Pooh Bear

    Pooh Bear Guest

    Snort !!!

    Graham
     
  7. qrk

    qrk Guest

    Had this sort of thing happen in the mid 90's on a board with a hi-Z
    (10Meg) node. We got on the assembly house's case and made sure they
    gave the boards a final wash in a fresh batch of cleaner. I believe
    they were using water soluble cleaner. Problem was contamination under
    a SOT23 Schottky diode. In retrospect, I wonder if it would have
    helped if the soldermask was removed under the part to give more
    clearance between the underbelly of the part and the board?
     
  8. Phil Hobbs

    Phil Hobbs Guest


    Don't knock dishwashers. They work great. The problem is getting flux
    out from under those high-value parts--it should happen eventually, but
    you'll have to calibrate it. Try bringing a board home and running it
    through your DW three or four times, first with detergent and then with
    ordinary tap water.


    Cheers,

    Phil Hobbs
     
  9. Yes, you would want to soak them in the highest specified humidity
    before testing, so the thorough dry out is probably pointless.
    But ionic crud could have contaminated the surfaces before the hand
    soldering added a layer of rosin. I had to remove the rosin before
    the crud could be dissolved off with water.
     
  10. John Larkin

    John Larkin Guest

    I'll second that. It's nasty, greasy, dirty stuff, more like "can't
    clean" flux. But the no-clean crud I've used doesn't seem to conduct.
    Organic solvent loaded with a bit of rosin flux seems to be safe, if
    not very cosmetic. Rosin residues don't seem to conduct. In our
    in-house vapor degreaser, we submerge the board for a while in a
    boiling solvent that has a deflux agent in it, then spray it down with
    a wand that pumps clean, freshly-distilled solvent. The solvent wets
    the pcb and parts, unlike water which beads.
    We'd just need a distilled-water fire hydrant. I considered using my
    big ole 1500 psi pressure washer, but it would use tap water, too.
    Hence the dental water-pic idea, fillable with distilled or deionized
    water, whichever ohms highest.

    I just filled my red plastic Official Presidential Drinking Cup with
    local tap water, and poked in a pair of standard Fluke probes 1"
    apart, and got about 1.2 Mohms, not too bad. Same test with water from
    the office cooler, Alhambra Mountain Spring Water, is about half that.


    John
     
  11. mc

    mc Guest

    The only flux I've really had trouble with was the "no clean" type. I
    What kind of trouble?
     
  12. Rich Grise

    Rich Grise Guest


    Dude, man up! If the boards don't meet your spec, they're rejects. Send
    the boards back and tell the stuffer they don't get paid until the boards
    are right, or make them pay for the rework.

    Good Luck!
    Rich
     
  13. John Larkin

    John Larkin Guest

    We own the boards and the parts, and customers are waiting for
    delivery. The only thing we can do to the vendor is not pay them for
    assembly and never use them again. Meanwhile, the problem has to be
    fixed.

    John
     
  14. Last (and only) time I had this problem, cleaning the boards using
    solvent scrub in the critical area worked. The crud was quite
    difficult to remove, a simply solvent wash was not enough. But in our
    case, I think it was on the surface-- a nasty hydrophilic (?) layer of
    something, rather than underneath the parts. Try axing the chemical
    manufacturers, they are generally pretty good (at least I've had good
    luck with Kester).


    Best regards,
    Spehro Pefhany
     
  15. You need distilled water only for the final spray rinse to displace
    the film of tap water. Tap water is fine to remove the vast majority
    of the contamination.
     
  16. I've had reasonable luck with an ultrasonic tank filled with the appropriate
    solvent. I'd probably wash in a mild soap and water solution followed by a
    rinse in distilled.

    Tequila would probably work also {;-)

    Jim
     
  17. Mike Monett

    Mike Monett Guest

    John,

    Here are some posts I collected from Bob Wilson on cleaning flux
    from pcb's. His comment on activator salts leaving a white residue
    being dynamite is absolutely correct. I ruined a perfectly good dvm
    by trying to clean it after it started acting up in high humididy.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Subject: Re: Flux cleaning ?
    Date: 2001-10-09 16:16:48 PST

    Never mind fooling around with stuff whose solvency you don't
    understand. Partial or incorrect removal of resin based flux is FAR
    worse than doing nothing at all.

    Resin fluxes require both polar and nonpolar solvents to remove all
    components completely. Using a bit of solvent (even the correct
    blend) and a brush, is REALLY bad news. What is mainly does is just
    spread the flux around all over the place.

    There are 2 main ingredients in resin fluxes. First, there is the
    resin itself. The active ingredient in resin is Abetic Acid. Resin
    requires a nonpolar sovent to remove it (such as trichloroethane, or
    even toluene).

    The other ingredient is the activator salts (typically chlorides and
    fluorides). These are soluable in a polar solvent ONLY (such as
    isopropanol, or even water). They are ABSOLUTELY UNAFFECTED by the
    nonpolar solvents that will dissolve the resin component.

    Attempting to use Disc Brake cleaner, or any other aggressive
    NON-polar solvent will simply remove the protective resin (that
    previously and harmlessly encapsulated the hygroscopic and
    conductive activator salts), and expose these salts to the air. This
    results in high impedance conductive paths all over the PCB, and
    possible corrosion.

    You cannot see the activator salts, in most cases, but in higher
    concentrations they do appear as a slight white residue. This is
    dynamite!

    So before anyone uses whatever snake oil thay happen to find laying
    around, it is wise to understand the chemistry that underlies this
    operation.

    For years an excellent flux remover was a 30% mixture of isopropyl
    alcohol, with the remaining 70% being trichloroethane or freon.
    Since both trichloroethane and freon are no longer easily available,
    a good substitute is plain ordinary "lacquer thinner" (mainly
    toluene). This mixture will remove ALL parts of the flux. Pure 99%
    Isopropanol (aka "rubbing alcohol") WILL work as well, since it DOES
    dissolve both polar and non polar residues, BUT it is extremely slow
    to dissolve the non-polar stuff (resin). The above mixture acts much
    faster,and still does not harm most components (polystyrene caps are
    the exception, but they dissolve in nearly anything anyway).

    The other alternative is to remove the resin based flux with a
    proprietry Saponifier in a water solution (Kester makes one). This
    requires very strong agitation, and hot water/saponifier solution.

    Finally, one can just use water-soluable-flux solder. Some brands so
    not have very good "fluxing" action, although I have had good luck
    with the stuff made by Alpha Metals. Warm, HIGH PRESSURE water spray
    (to get under ICs and so on) is a must to remove this. One good way
    is to stick the PCBs in a dishwasher. Personally, I use water
    soluable flux wherever possible. It works well and DOES remove
    completely. One final point: you must ALWAYS clean water soluable
    flux off the PCB. NEVER leave it on there. It is extremely
    hygrosopic and will result is a malfunctioning PCB after it has
    absorbed atmospheric water (1 to 4 weeks later).

    Bpb.

    You need a solvent blend that contains both ionic and non-ionic
    solvents. A very good blend is 70% trichloroethane and 30%
    isopropanol (isopropyl, or "rubbing" alcohol). The trichlorethane is
    an aggrressive non-ionic solvent and the isopropanol attacks the
    ionic salts.

    Although isopropanol is a drugstore item, trichloroethane is not. If
    you have a problem with this, xylene or toluene (or even ordinary
    lacquer) can be used instead, although these are more aggressive
    solvents and some parts may be affected (test first). Increasing the
    proportions to 50-50 will reduce the aggressiveness of the blend.

    Even straight isopropanol can be used, since it can dissolve bothe
    ionic and non-ionic residues. However it is a very feeble non-ionic
    solvent, so it will take a long time to attach the resin and other
    non-ionic residues.

    Be sure to immerse the PCB in a reasonable volume of solvent, and
    rinse in a fresh bath. **Use lots!!** DO NOT JUST SWAB IT AROUND
    WITH A COTTON SWAB!!! All this will do is to spread the flux all
    over the place, and partially dissolve the rosin, exposing the
    activator salts that were previously trapped harmlessly. EITHER
    CLEAN THE PCB PROPERLY OR NOT AT ALL!

    BOb.

    Naptha would work well if the crud were purely non-ionic, however I
    suggested the Full Monte cleaning because the true makup of the crud
    was not known, and also because there will CERTAINLY be some small
    amount of flux residue if the motherboard were made in the orient
    (which most are).

    You are correct that there is no rule that a solvent mix must be
    used. Sequential cleaning (as long as the rosin is removed first to
    expose the activators) is perfectly acceptable.

    I agree with Bob. Most of the flux is no clean today. Why use an
    extra processing step (washing) if you don't need to. Especially
    when you have some non-washable parts on the board.

    Now, I use a turpine based solvent that:

    1) Smells great (it is orange peeling oil really)
    2) Is predominantly non-toxic and non polluting.
    3) Most importantly, does a great job in cleaning the boards.

    The only drawback is that it takes a long time (when compared to
    freon) to evaporate. If you use compressed air to get the majority
    of it off, then it will dry in a few minutes. It does a great job
    and if used in liberal quantities and some scrubbing, leaves no
    residue. That cannot usually be said about acohols and freon based
    solvents.

    Chuck

    I agree, that clean water wash is good if all the components are
    sealed. Another method that does not require careful drying is to
    use a turpine (orange/lemon peeling oil) based solvent. It smells
    good, cleans good, takes a while to evaporate (I suggest using
    compressed air) and leaves very little or no residue. It will eat
    styrene but that is the only plastic that I have found that it
    doesn't like. BTW never use any type of alcohol on poly carbonate
    components. Like the plugs on the ends of phone cords.

    Chuck

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Regards,

    Mike Monett
     
  18. Leon

    Leon Guest

    Try scrubbing them with cellulose thinners (xylene) followed by IPA. I
    find that works very well on all types of flux.

    Leon
     
  19. I would guess not, We had a simlar problem with a crystal and residue
    under it from the water cleaning process.
    They all seem to be aware of the problem
    We tried a alcohol wash after the fact, and the crap was still under the
    crystal. It seem that once it dries its very tuff to wash out. John P's
    wash and rinse might work.
    I would recomend a Milled slot under the part in the next design,
    similar to HV stuff.
    Wiskers, selective conformal coating time.

    Cheers
     
  20. Ken Smith

    Ken Smith Guest

    I've seen it conduct and ruin PCBs. The environment was full of decane
    though so that may be a special case.
    Actually, you could use ordinary tap water in most places and then follow
    with a distilled-water bath.
    In almost every place in North America, tap water is the way to go for
    both washing and drinking.
     
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