Done.
Thank you.
-----Original Message-----
From: Green, Mike [mailto:[log in to unmask]]
Sent: Monday, December 17, 2007 7:14 PM
To: Stadem, Richard D.
Subject: RE: [TN] PCA washing
This is an excellent discussion of the topic. Perhaps you might want to
share it with TechNet.
Mike Green
Electronic Packaging Design
LMCO-Sunnyvale
408-743-1635
One Corporation, One Team
-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Monday, December 17, 2007 3:16 PM
To: Green, Mike
Subject: RE: [TN] PCA washing
Yes, of course. If you want to do it right, start with all clean boards
(unsoldered bare pwbs), and then load them into a condensation chamber
and saturate them. Measure their saturated weight, then bake them dry
but weigh them every hour or every two hours. You establish their
average dry weight and rate of moisture dissipation by baking them and
weighing them once every one or two hours while baking. The weights
typically decrease rapidly in the first 2-8 hours depending on pwb
material type, number of layers, amount of copper, etc. Once you reach
the point on the the curve where very little change is seen, and it
appears that more than 80% of the saturated weight is gone, then remove
them from the oven, let them come to room temperature, and run them
through the wash process, blow off all of the surface moisture (make
sure to get it all out of the vias), and weigh the boards immediately
after the wash and blow dry all within 20 minutes or so from removal
from the oven. You will then know how much moisture is put into the PWBs
from short-term immersion in water. Usually, little change is seen in
rates of moisture adsorption due to short-term immersion in liquids for
both populated boards and non-populated. You determine the rates of
adsorption and dissipation using the bare PWBs, as the assemblies have
too many variables introduced with the components and the amount of
solder varying all over the place.
To determine the rates of re-adsorption of moisture when exposed to room
environment (relative humidity), simply remove the dry boards from the
oven and weigh them every hour or two hours. This curve is usually
shorter, ie, most boards re-absorb moisture to saturation (limited by
the RH of the environment) faster than the same amount of moisture can
be baked out, at least when using 105 C for a bake temperature. The RH
at the time also influences the rate of adsorption from air, but not
directly. What you are looking for is the average RATE of moisture
adsorption and dessication for each board for an average RH, say
40%-60%.
Of course, the boards will all vary a small amount in weight, but the
rate of removal and re-adsorption curves should be very identical for a
set of PWBs that are all the same part number.
I forgot to mention; the number of hours you need to bake to get to less
than 20% of the "normal" saturated weight is what I use for establishing
standard bake times prior to assembly, never mind how well the vendor
packages them, etc. To be safe, bake the boards for that many hours. I
have never seen a board delaminate or otherwise display any adverse
affects indicative of high moisture content when 80% of the moisture
content was removed. The time past removal of the first 80% of moisture
removal usually does nothing except degrade the solderability. The last
20% moisture content is not usually enough to cause serious reliability
issues, BUT SOMETIMES IT CAN!
But dry-packaging of PWBs IS still important, as it helps prevent
oxidation (reduction in solderability) in IAg, ISn, and HASL and FTL
PWBs that is then brought "over the top" by the bare board bake process
just prior to assembly. ENIG boards stand up a little better in terms of
little or no reduction in solderability due to oxidation. But they also
need to be kept dry until reflow or wave solder, and again at rework.
It is not only delamination that you are trying to prevent, but Z-axis
expansion on the vias, and CAF formation, both which are also
contributed to by high internal moisture levels.
-----Original Message-----
From: Green, Mike [mailto:[log in to unmask]]
Sent: Monday, December 17, 2007 4:14 PM
To: Stadem, Richard D.
Subject: RE: [TN] PCA washing
Don't you need clean boards for this experiment? Dirty boards would
wash off the dirt and thus weigh less when clean.
Mike Green
Electronic Packaging Design
LMCO-Sunnyvale
408-743-1635
One Corporation, One Team
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, November 26, 2007 8:30 AM
To: [log in to unmask]
Subject: Re: [TN] PCA washing
Short term immersion in water typically does not contribute to any
significant moisture adsorption in most PWBs. It is the longer term
exposure to humidity that causes PWBs to eventually reach saturation.
However, there are always exceptions. Simply get a precision scale and
weigh a number of pwbs prior to and after wash and record the delta. The
scales can be leased for very reasonable cost and many scale providers.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Monday, November 26, 2007 9:45 AM
To: [log in to unmask]
Subject: [TN] PCA washing
Hi Technos,
I need to set the process of aqueous washing on our in-line machine and
have a BIG preliminary question:
How to make sure that my boards are properly dried at the end of the
machine? Is the absence of water on the surface of the board, including
under the parts, a thorough indicator? Is it possible that residual
humidity be found inside the laminate?
Thanks for your help,
Ioan
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