dirty boards
John Larkin
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
Ken Smith
John Larkin <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
[....]
>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.
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.
>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?
The only flux I've really had trouble with was the "no clean" type. I
don't think it should ever be used.
>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.
Using dirty solvent is trouble, whether or not it is 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.
... 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.
>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?
I believe that there is a detergent of some sort added to the first wash.
After that it is washed with pure water.
>Are there any lead-free implications as regards leakage? This board
>uses regular pb/sn solder, but it may become a concerm some day.
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.
--
--
kens...@rahul.net forging knowledge
John Popelish
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.
Jim Thompson
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>
>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
[snip]
Due to allowing ESD devices to be used as clamps ?:-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
John Larkin
wrote:
>
>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.
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
Pooh Bear
Jim Thompson wrote:
> On Sat, 27 May 2006 10:08:14 -0700, John Larkin
> <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>
> >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
>
> [snip]
>
> Due to allowing ESD devices to be used as clamps ?:-)
>
> ...Jim Thompson
Snort !!!
Graham
qrk
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
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?
---
Mark
John Popelish
> 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.
Yes, you would want to soak them in the highest specified humidity
before testing, so the thorough dry out is probably pointless.
> We've never had any leakage problems with rosin flux. It's not
> conductive whether you clean it or not.
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.
Phil Hobbs
> In article <8l0h72lgnv3goedb1...@4ax.com>,
> John Larkin <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
> [....]
>
>>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.
>
>
> 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.
>
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
John Larkin
(Ken Smith) wrote:
>
>The only flux I've really had trouble with was the "no clean" type. I
>don't think it should ever be used.
>
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.
>
>>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.
>
>Using dirty solvent is trouble, whether or not it is water.
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.
>>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.
>
>
>... 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.
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
mc
> don't think it should ever be used.
What kind of trouble?
Rich Grise
>
> 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.
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
Spehro Pefhany
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>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.
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
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
John Larkin
wrote:
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
John Popelish
> 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.
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.
Mike Monett
> 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
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
In article
<D8D251B0224E7494.0844C212...@lp.airnews.net>,
jste...@jkmicro.com says...
> Dave Rosenbloom wrote:
>> Disc Brake cleaner works well and evaporates very quickly. It
>> seems to come in cans with higher pressures than many other
>> aerosol products and is usually equipped with a blaster straw. I
>> currently have a can of CRC brand Brakleen on my bench and it
>> works great. I often apply it with cotton swabs to clean small
>> areas.
> Have you had any problems with it attacking plastic parts?
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
RST Engineering (jw)
solvent. I'd probably wash in a mild soap and water solution followed by a
rinse in distilled.
Tequila would probably work also {;-)
Jim
Leon
Try scrubbing them with cellulose thinners (xylene) followed by IPA. I
find that works very well on all types of flux.
Leon
Martin Riddle
message news:8l0h72lgnv3goedb1...@4ax.com...
>
> 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?
>
under it from the water cleaning process.
> 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.
>
They all seem to be aware of the problem
> 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?
>
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.
> Are there any lead-free implications as regards leakage? This board
> uses regular pb/sn solder, but it may become a concerm some day.
>
Wiskers, selective conformal coating time.
> John
>
Cheers
Ken Smith
John Larkin <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>On Sat, 27 May 2006 17:33:14 +0000 (UTC), kens...@green.rahul.net
>(Ken Smith) wrote:
>
>>
>>The only flux I've really had trouble with was the "no clean" type. I
>>don't think it should ever be used.
>>
>
>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.
I've seen it conduct and ruin PCBs. The environment was full of decane
though so that may be a special case.
>>... 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.
>
>We'd just need a distilled-water fire hydrant.
Actually, you could use ordinary tap water in most places and then follow
with a distilled-water bath.
>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.
In almost every place in North America, tap water is the way to go for
both washing and drinking.
Ken Smith
The two biggies are:
Electrical leakage of a very noisy sort was one of the worst problems.
I had some boards that were intended to be potted. The flux messed with
the potting process so they had to clean the PCBs with very nasty stuff to
make the potting work right.
There is also an inspection problem. The goo hides things that I'd like
to have inspected.
ratman
Scrub the worst of the flux off under running tap water with a
soft brush.
Soak for several hours in hot TAP ***not deionized*** water.
Rince with a little deionized water.
Soak for several hours in hot, fresh deionized water. Dry in
a warm oven overnight.
This procedure was written by a chemist and it has worked
brilliantly on very sensitive boards where every pA counts. However,
it assumes that the board has no parts that will be bothered by either
the heat or the water.
Brian
"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
news:8l0h72lgnv3goedb1...@4ax.com...
>
Let me say that most replies to this post were full of spit....
I have seen MORE failures do to difficult cleaning of water-soluable flux in
SMT than in No-Clean processes. Some of you sound like you gave up on
no-clean 15 years ago. Well, your knowledge is outdated. How do I know? I
have seen contamination tests on modern formulations and they are
consistently BETTER than the water wash.
The trouble with no-clean comes when people muss with it and don't
understand it. DON'T clean it improperly; it will fail. Don't conformal coat
it with just anything, use something water-based. Used properly, it works
WELL.
John, for your issue, get some isopropyl alcohol as stated in one post. Use
it in an ultrasonic cleaner with your boards in it. Some toluene in it, to,
will help if needed. This will clean the boards off and get them working
(assuming the assembly fits in the available ultrasonic cleaner).
Keep us posted!
bill....@ieee.org
I spent a couple of years working on a conductivity meter in Venlo. The
tap water in Venlo and Nijmegen has a conductivity of about 300uS/cm. I
don't have any reason to believe that North American tap water is much
different.
For the record, our de-ionised water measured less than 1uS/cm when
first dispensed, but crept up to a couple of uS/cm when left open to
the air for any length of time, which I believe to be caused by CO2
from the air dissovling in the water and forming carbonic acid (H2CO3).
Tap water os conductive by virtue of dissolved carbonates and
sulphates, mostly calcium and magnesium - the stuff that water
softeners convert into chlorides.
--
Bill Sloman, Nijmegen
Mike Monett
> Ken Smith wrote:
>> In article <6h5h72dbfb81a0k79...@4ax.com>, John
>> Larkin <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
[...]
>> Actually, you could use ordinary tap water in most places and
>> then follow with a distilled-water bath.
>>> 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.
>> In almost every place in North America, tap water is the way to
>> go for both washing and drinking.
> I spent a couple of years working on a conductivity meter in
> Venlo.
> The tap water in Venlo and Nijmegen has a conductivity of about
> 300uS/cm. I don't have any reason to believe that North American
> tap water is much different.
> For the record, our de-ionised water measured less than 1uS/cm
> when first dispensed, but crept up to a couple of uS/cm when left
> open to the air for any length of time, which I believe to be
> caused by CO2 from the air dissovling in the water and forming
> carbonic acid (H2CO3).
> Tap water os conductive by virtue of dissolved carbonates and
> sulphates, mostly calcium and magnesium - the stuff that water
> softeners convert into chlorides.
I use a Hanna 98308 PWT daily to monitor the quality of distilled
water and can confirm your observations on CO2. Most dw I get
measures 0.6uS to 0.8uS when fresh, and it increases slightly as it
absorbs CO2. It may go as high as 1.2uS.
I recently switched to another brand of dw that measures 1.2uS and
seems to stay at that value regardless of how long it has been
opened. That may be the limit of CO2 absorption in dw.
The tap water here in Midland is off scale on the Hanna, so it is
over 199uS/cm. It is very hard water and leaves calcium deposits on
containers. It would be unusable for cleaning high-impedance pcb's.
Regards,
Mike Monett
Jeff L
"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
news:8l0h72lgnv3goedb1...@4ax.com...
>
> 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.
Water soluble is not nice stuff - it is conductive, will tarnish metal, make
solder joints brown if left on for one to several days, and it can be hard
to clean. On the other hand, it is easy to thermo profile as it has a wide
process window, is very active, which means it will wet oxidized parts
better then most other fluxes. It is also easy and cheap to clean, if you
can get to the flux under the parts.
To prevent any problems like you describe, we only consider using water
soluble for thru - hole and mixed boards in the wave, since the parts are
big and chunky (mixed boards would have RMA (to be solvent washed later) or
No Clean on the SMT preventing the entrapped areas from getting water
soluble in them), and wash off easily, and some though hole parts have
lacquer coatings (mainly resistors!) which will wash off with all but the
weakest solvents.
To get the water soluble off, we do a pre wash by soaking the boards with
tap water for several minutes, change the water (the tap water soaking is
only to help make our expensive carbon and DI water filters last much
longer), soak in a water soluble neutralizer for a few minutes, and then
wash in a *closed loop* DI water washing machine, which is basically a
modified stainless steel dish washer. Putting the boards in the washer until
the wash sump resistivity reaches over 2.0 meg ohm (compensated for standard
temp, and is the standard 1 cm cubed measurement), which is mil spec for
ionic contaminants generally ensures the boards are clean. For problem or
high impedance boards, we may set the washer to run until 2.5 meg ohm to
make sure all the hard to clean areas are clean. The wash temp is around 140
deg F and the water entering the sump is generally 16.x meg ohm. Perfectly
clean DI water at standard temp (~20 deg C) is 18.4 meg ohm. DI water very
quickly wants to remineralize it's self and will grab CO2, trace gases,
parts of the container, etc and quickly become under a few meg ohms within
minutes depending on surface area and temp. Over-washing boards with 2+ meg
ohm DI for an hour or two will dissolve some of the metals out of the solder
and leave an interesting spider web like pattern when viewed under a
microscope, so over washing is likely not a good thing, and shows how
aggressive DI water is. Putting 1 clean board in the wash, with a single
touched up lead with a small dab of water soluble flux will drop the sump
resistivity to 0.0xx to 0.1xx meg ohms!
Washing with something that has decent agitation, can monitor the water
resistivity, and keeping it cleaning until you at least meet the mil spec of
2.00 meg ohm should get the boards clean, as DI really wants it's ions back.
Using hot water should reduce the surface tension and increase the
aggressiveness enough to clean under tight SMT parts. You may even be able
to get away with washing the boards in an ordinary kitchen dishwasher
(keeping in mind ESD, as most kitchen dishwashers are not in a ESD protected
area)!
Also don't leave the boards wet for a long time - the solder will corrode,
and any steel / iron parts, leads and wire may rust. Bake at ~ 90 deg C for
at least 1/2 an hour to dry and remove traces of moisture. Note that some
solder masks which may not be fully cured may absorb water and turn murky
looking - heat generally drives off this moisture returning the mask to it's
original appearance.
> 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.
If this was working when your boards went through, it should have a DI water
final rinse, however, some companies just use tap water as DI is much more
expensive. You may want to ask them about the resistivity of the water in
the final rinse zone.
>
> 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?
Large, tight to the board, dense leaded QFP's seem to be the hardest parts
to clean, but they do come clean.
>
> 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.
As long as it's the wash or pre wash water, that could be OK. If it's the
rinse or especially final rinse water, then that's a BIG problem.
>
> 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.
Try a hot water wash in a normal dishwasher as mentioned above.
Ken Smith
<bill....@ieee.org> wrote:
[...]
>Tap water os conductive by virtue of dissolved carbonates and
>sulphates, mostly calcium and magnesium - the stuff that water
>softeners convert into chlorides.
It is what stays behind on the PCB after washing that really matters. A
little CO2 in the water won't matter much.