Dick,
Let me take a real devil's advocates position here as I think you are
making mountains out of mole hills.
First thing I did was to drop a US Penny on the scale to verify the 2.500
mass. Yes, the scale measured 1 penny, then 2, 3 and a few more correctly.
So I consider the scale as "Reasonably" working.
Given that the surface contamination is expected to be WAY down in the mud
at the measured water and scale resolutions:
Why do you want a gauge R&R ???
Even if my measurements were of by 100% it would not make a difference in
the determination that rinse water source (tap, DI, Distilled or RO) is
going to make a measurable difference in final ionic contaminations. At not
through any mechanism that anyone has suggested.
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Richard D. Krug
Sent: Monday, January 30, 2012 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] Measured Water on Board after Rinse
You probably need to perform a gage R&R to know whether the scale is going
to be usable for this experiment. If 3 people could measure the same 5
serialized samples 5 times, in randomized order of samples each time
measured and randomized sequence of people to measure, you could determine
the minimum difference in weight the scale can discern.
The rule of thumb is you need resolution 4x the difference you expect to
measure, and is preferred to be 10x the difference expected.
Dick Krug, CSSBB, CSMTPE
SMT Process Engineer
Sparton Corporation
30167 Power Line Road
Brooksville, FL 34602-8299
p (352) 540-4012 (Internal Ext. 2012)
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, January 30, 2012 1:02 PM
To: [log in to unmask]
Subject: [TN] Measured Water on Board after Rinse
Hi,
I finally received my little scale with .01g resolution. Not a lot but good
enough for government work! J
So I took a small board I make, 2 x 2 inches, weighted, rinsed, shook off
and air gunned. (26cm sq)
See: www.kondner.com/files/smallbrd.jpg
Initial Weight: 17.81 gram
After Rinse and shake: 18.00
After blow off: 17.83
After blow off I had .02g of water. Given my poor scale resolution let say
.03g or .03ml.
After the board dried some I noticed it was down to 17.80. I must have
removed some dust.
Assuming really poor rinse water at 100mg of NaCl equiv per L we can expect
3 * 10**-6 g or 3 ug on the 26 cm sq PCB. Actually the surface area with the
second side and parts is probably several time that but let's use just the
two sides of the PCB or 52 cm sq.
Wow even without the water blow off the tap water is a non-issue. If I
simply rinse in distilled I am fine. Good, I can trash the blow off process
if rinse in distilled.
3ug / 52 = .058 ug / cm sq
Holy Cow, did I get all the conversions and math right?
Actually the final water related remaining contamination is probably 10% of
that as most tap water is cleaner and not all dissolved solids ate as ionic
as NaCl.
The normal NaCl found in epoxy laminates or uncured solder mask and silk
screen ink Ionics are probably more of a problem.
Summary:
Rinse water quality is only a TINY fraction of what goes into total board
cleanliness.
It is not the quality of the rinse water. The quality of the wash, getting
the wash material FLOWING under parts is the killer. And getting the
contaminated wash water removed is VERY important. As for the nature of the
rinse water it is a complete don't care.
Bob K.
OK: Tear into me. I saw the movie "The Grey" with my son over the weekend. I
am ready to be attacked by a pack of wolves. J
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