With dendritic growth, dissolved solids add to the problem, but what I am looking at right now is conductivity mechanisms...
This only requires free ions in a transport media like water..
In the best case I would be trying to measure an ~2x10-4 gram variation on a 25 grams PCA and the typical case using the same component, ~2x10-4 gram on a ~615 gram 6x8 PCA..
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Wednesday, February 02, 2011 10:03 AM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] PCBA Water Wash
Paul,
You want to check my math but a .001 inch diameter drop is 7.2E-12 liter.
This is such a tiny amount of water that I don't see how you could ever get
enough dissolved solids onto a PCB surface that the surface would be
considered "Not Clean".
I was originally looting a water drops 1mm in size which contains 64,000
times the amount of water as your .001 drops. I don't think you have 64,000
little drops under the parts, right? (1mm = 40mils and 40**3 = 64,000)
If the drops are .001inch in diameter you could be rinsing in swamp water
and it would not matter. Right???
Bob
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Wednesday, February 02, 2011 12:19 PM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
The last ones I looked at in an F/O boot last week about 15% of the water
droplets were smaller than 0.001" (0.025 mm)...
This is after I removed the boot so the particulate size could have been
larger in the undisturbed state... The smaller droplets could have been
generated by the shearing action of the liquid on the hydrophobic plastic
surfaces at the unconstrained boundary edge formed by the boot/connector...
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Tuesday, February 01, 2011 6:51 PM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] PCBA Water Wash
Paul,
So how big are these water drops? .5mm or .1mm?
Bob
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Tuesday, February 01, 2011 7:20 PM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
Bob,
When I say "I have seen"... I literally mean I have seen droplets of water
under QFNs, micro-BGAs and in sealed F/O transceivers after air knife
blow-off at 0.7ft per min....
I look for water intrusion in parts and under components at 150X to 300x...
There are few instruments that are capable of measuring the weight of a
single water droplet intrusion capable of providing generating dendritic
growth in a single component on a average size PCA...
With the lab tools that were available to me, the measurement noise far
exceeds the resolution required to make the material measurement.
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Tuesday, February 01, 2011 4:09 PM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] PCBA Water Wash
Paul,
Well, try it. Go ahead, tell me the weights of the PCB when dry and then
after wash and blow off. You can easily calculate the equiv ug / in sq.
And / Or do an ionic contamination test of a DI and that same board you
measured above. I would love to see the numbers. My problem was I did not
have a scale with the required sensitivity.
Instead of anything with an "I have seen ...." do an "I did this experiment
and this is what I measured" type statement.
Enquiring Minds Want to Know! :-)
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Tuesday, February 01, 2011 2:02 PM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
I disagree with Bob...
I have never seen an air knife remove water from all the spaces that can be
on a board...
The work fine if you have plenty of clearance between ALL surfaces...
I have seen air knives drive water into spaces where to water's natural
surface tension would NOT allow it to go...
When the water evaporates, if it evaporates, it leaves the organo-halogen
complexes that supply base materials for dendritic growth...
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Tuesday, February 01, 2011 10:23 AM
To: [log in to unmask]
Subject: Re: [TN] PCBA Water Wash
City water is fine if it is removed (air knife) prior to drying.
Run the number on dissolved solids content and calculate or weigh the amount
of later left on the board. With a simple blow off you will find the
remaining ionic content is very tiny.
Get ionic contamination tests done. City or DI that test results are the
important issue.
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Forrester, Michael (H
USA)
Sent: Tuesday, February 01, 2011 12:08 PM
To: [log in to unmask]
Subject: [TN] PCBA Water Wash
I have three questions I hope someone can help me with:
1) We have a vendor that uses "city water" directly into their SMT PCA
washer. I believe DI water is industry standard.
Does anyone know if it is documented anywhere? Spec?
2) We had an issue where there was Chlorine contamination and gold
dendrites found inside some standard gull-wing 6 pin SMT ICs.
This issue seems to be batch related. The chip manufacturer claims
that chlorine is never used in their process. Should the SMT
process expect to handle the possibility that the wash solution may
get into a part? The PCA vendor uses unfiltered "city water",
with no cleaner, in the wash process and does a ROSE test on one
board of each lot. Since the ROSE test passes, the PCA vendor
is blaming the part manufacturer for having "leaky" parts, since the
thinking is the wash is getting into the part and trapped.
3) One issue I have is the water pressure in the wash is 80-100 psi.
I think this is too strong, and should be half of that? You want to
flood the board with water, not hit the board and bounce off?
Thank you in advance.
Best Regards,
Michael Forrester
Sr. Product Engineer
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