Hi all,
I think many industries make their decisions based on empirical data and own
experience.
My own opinion is, and few will agree, that the whole idea of a standardized
humidity level is disputable, because the humdity is not constant from x to
y, nor from Z 1 to Z2. Furthermore, the humidity is varying with time and
temperature, sometimes within seconds. Moreover we have the board's surface
condition, percentage of copper, access to mobile ions and a lot
more. Which was pointed at by Dave
Hillman.
It all starts with the ability of water condensation on the surface followed
by the rate of penetration of gas and water vapor into the board structure.
The water content on the very surface is not included in the discussion,
because it's not relevant for the soldering process. It's the inside
accumulation of water that is the input for the standard.
To get knowledge of the water content, you 'simply' perform a weight control
of the board, starting with boards that have been in a humid atmosphere,
e.g. from stock or from a humidity test. Then you bake out the water and do
a weight control again. The diff is the water content.
Here starts the questions and the aquestionings. The bakeout is the first
problem, because you don't know how effective this process is. The actual
standard says, that we hope that at least 90 % of the water will find its
way out and disappear. But who knows. It's a question of the leakage of
water molecules through millions of micron sized pores in the epoxy
structure. And the one who has learned about the percolation of molecules
know that this is very complicated a thing. furthermore we have the
diffusion of water vapor through the same micron sized 'orifices', also a
far from easy to understand process. Depending on the charachter of these
labyrints, we have a variation of water escaping rate. Compare with the
water on the surface. Water molecules can stick to the outermost crystals as
billions of droplets, or cover the entire surface as one and a single one
water layer. The evaporation of water can vary a lot. Imagine what happens
when you clean your glasses. You breathe on them, water condensates in less
than seconds, and you have some more seconds to rapidly wipe the glass with
a tissue. If you are late, you'll see how the water leaves rapidly. If you
do the same on a sheet of plastic, the water film can stay for minutes.
Likewise, the transportation of water from the inner regions of a PWB can
vary a lot, depending on what base material is used, number of copper
layers, reinforcement materials etc. Some water can be so bound to the
structure by the capillary force, that it's nearly impossible to get rid of
it. There is a lot more to say, but I stop here (to the relief of many).
Now, what about the small activity with small resources for analysis and
advanced incoming inspection? They have no idea about the water content
Perhaps a passus in the purchase handling. Yes, it's one way. Another way
is to look back and see how their products behave in a long term. A further
way is to avoid long storage before soldering. Perhaps they have someone who
knows a little about nature's behaviour, i.e. the ambition to get a state of
equilibrium. If you have 50% RH in a room and put a PWB on the desk,
humidity will increase in the board until there is an equality on inside vs.
outside. How fast? It depends on how many PTHs you have, o how many vias
and how the edges of the board are humidity inlets. One way to get an idea
is to create buried capacitors in the PWB, expose the board to humidity and
measure the capacitance change vs. time. You'd be surprised how fast
humitity gets on inside of a board. Halfways to quilibrium can occur from 10
hs to 100 hs. So, a board that has been lying for weeks in a humid store,
may hold such a lot of water, that you are far from 0.1 wt% after bakeout.
You may have 0.5 wt%, 1 wt% or more. Without knowing. Tests on a variety of
boards gives that equilibrium is obtain after as short a time as a few hours
to as long long a time as one year!
So, that's why I wonder with what confidence we follow the 0.2 % rule. With
all respect to those who work with the IPC norms. Am not against, nor pro.
End of old man's gagging
Inge
PS, If you don't have Chris Hunt's splendid report (he is a TN member), I
got it. Title: Moisture Measurements in PCBs and Impact of Design on
Desorption Behaviour. Not a paper with pages and pages with maths that
you don't understand, but a language and with illustrations that explains
complicated things so even a gaga like me can melt.
Inge
On 23 October 2011 00:03, David D. Hillman <[log in to unmask]>wrote:
> Hi Rigo - yes, Doug would give you the standard " it depends" response
> because as you detailed,there are too many inputs to your question. The
> reason that the IPC-1601 committee only put suggested values in the
> standard is that clear, data determined, industry wide applicable values
> for requirements could not be established. The committee is continuing to
> investigate, research and gather published data on that task. Establishing
> industry wide applicable values is a difficult task as the committee does
> not want to impose any requirements that are not based on data and are not
> value added for the industry.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> "Garcia, Rigo (GSFC-300.0)[MANTECH SRS TECHNOLOGIES]"
> <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 10/21/2011 01:31 PM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> "Garcia, Rigo (GSFC-300.0)[MANTECH SRS TECHNOLOGIES]"
> <[log in to unmask]>
>
>
> To
> <[log in to unmask]>
> cc
>
> Subject
> [TN] How do we determine the Maximum Acceptable Moisture Content (MAMC)
> for a board?
>
>
>
>
>
>
> Hi everybody,
>
> Does anybody know how do we determine the Maximum Acceptable Moisture
> Content (MAMC) for a board?
>
> I understand that IPC-1601 states that for most designs a MAMC between
> 0.1% and 0.5% of moisture weight to resin weight is adequate. However, it
> does not describe the basis to establish the range. I can think of many
> variables that can affect this number like minimum pitch, voltage across,
> component clearance, whether the board gets conformal coating or not, etc.
> I would like to know if anybody knows of a safe method to determine the
> MAMC for a given board.
>
> Paul... I can already see your "it depends" coming... :)!
>
> Thanks for the help!
>
> Rigo Garcia
>
> Sr. Quality Assurance Engineer
> Workmanship Standards, Code 300
> NASA, Goddard Space Flight Center
> Greenbelt, MD 20771
> Phone. (301) 286-6129
> Fax. (301) 286-6576
>
>
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