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dirty boards

K

Ken Smith

Jan 1, 1970
0
What kind of trouble?

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.
 
R

ratman

Jan 1, 1970
0
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

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.
 
B

Brian

Jan 1, 1970
0
John Larkin said:
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

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!
 
Ken said:
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.

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.
 
M

Mike Monett

Jan 1, 1970
0
Ken Smith wrote:
In article <[email protected]>, John
Larkin <[email protected]> wrote:
[...]
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.
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
 
J

Jeff L

Jan 1, 1970
0
John Larkin said:
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.

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.
 
K

Ken Smith

Jan 1, 1970
0
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.
 
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