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From:
"Goodyear, Patrick" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Goodyear, Patrick
Date:
Tue, 25 Oct 2011 15:57:48 +0000
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All,

While we don't make circuit boards we do do a lot of repair on vintage circuit boards.  We are located on the Pacific coast RH runs around 80% or higher.  We have a huge issue with measling on boards when we rework.   If we have time we put them in an oven at 150 deg for a day or so to dry them out, when we can't we use a heat gun to raise a localized area to a higher temperature to drive off the moisture, this works fairly well, but issues still prevail.  

One could put the boards in a vacuum chamber at a higher temp say 100 deg F. something similar to an autoclave to dry the boards, I suppose they could then be sealed with a moisture barrier, vacuum vapor impregnated, hmmm it might work.

Pat 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, October 25, 2011 6:08 AM
To: [log in to unmask]
Subject: Re: [TN] How do we determine the Maximum Acceptable Moisture Content (MAMC) for a board?

Hi, Inge.
Delamination and measling still occur, but much more infrequently. I believe this is simply due to a much more robust pre-preg quality in the market. However, delamination does occur more frequently with polyimide slash types. You just gotta pre-bake or you will eventually have issues.

And don't try to save money by buying PWBs from China. You will get burned- big time. I have been getting calls quite frequently in the past year to look at various lots of PWBs manufactured in China with all kinds of quality issues. Problem is, some of the more reliable PWB companies have attempted to form offshore partnerships in an effort to reduce costs and become more competitive, but did not do their homework when selecting an offshore supplier, and did not place a company rep at the site. PWB suppliers never voluntarily reveal to their customer that they are buying offshore. Invariably, poor workmanship and/or poor materials slip through and are not detected until during manufacturing or after.

I am not saying all Chinese or all offshore PWB fabricators are of poor quality, I am simply stating that there are a lot of companies out there that make crap, and you have to be very careful of your PWB's pedigree.

Waterproof PWBs? I would need a lot of data to believe that claim.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
Sent: Monday, October 24, 2011 4:43 PM
To: [log in to unmask]
Subject: Re: [TN] How do we determine the Maximum Acceptable Moisture Content (MAMC) for a board?

I forgot a question I had to you all:  how frequent is delamination (caused by water vapour) nowadays. It's a long time since I heard of serious problems. Perhaps one or two cases with a single board or two, but in a mass production that would not be count as an issue.

IF delamination is much lower today, what is then the reason, and is the improvement global or significant for a limited number of suppliers?  And also this (perhaps stupid) question: Has anyone seen boards marked 'waterproof' . I read somewhere about such boards and my guess is that the boards were coated. But waterproof? Hmmm..

Inge



                              -------------------------

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


- Show quoted text -
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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On 23 October 2011 07:31, Reuven Rokah <[log in to unmask]> wrote:

> Hi Bev,
> You can check the graph for vapor pressure (psi) vs temp. of Werner's 
> white paper for PCBs.
> I believe this graph is based on experimental measurements. I assume 
> that there are more factors (than the PV formula) influencing the 
> vapor pressure inside the PCB layers during reflow.
> Reuven
>
> On Sun, Oct 23, 2011 at 3:48 PM, Bev Christian
> <[log in to unmask]>wrote:
>
> > Reuven,
> > I respectfully disagree.  Even if we assume a peak reflow 
> > temperature for eutectic tin/lead solder of only 205C and a high 
> > 260C for SAC305 the
> vapor
> > pressure due to trapped air and/or moisture is still not going to double.
> >
> > P1V1=n1RT1
> > P2V2=n2RT2
> >
> > R = a constant
> > n1 = n2
> > V1 = V2
> >
> > Leaving
> > P1/P2 = T1/T2
> >
> > 300/P2 = (205+273)/260+273)
> >
> > P2 = 300*533/478
> >
> > P2 = 334  That is only an 11% increase.
> >
> > Not sure where the 300 came from in the first place, but no matter 
> > what number you start with, it does not double.
> >
> > Bev
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
> > Sent: Sunday, October 23, 2011 5:03 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] How do we determine the Maximum Acceptable 
> > Moisture Content (MAMC) for a board?
> >
> > Hi Garcia,
> > You didn't mention the most important variable: the process 
> > technology,
> the
> > vapor pressure (300 to 650 psi) of Lead free is more than double 
> > than Leaded process at the reflow peak temp.
> > the MAMC for Leaded process should be 0.20% and for Lead Free 0.17%.
> (ref.
> > Werner Engelmaier PCB FAB Notes).
> > Reuven
> >
> > On Fri, Oct 21, 2011 at 8:31 PM, Garcia, Rigo (GSFC-300.0)[MANTECH 
> > SRS TECHNOLOGIES] <[log in to unmask]> wrote:
> >
> > > 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
> > >
> > >
> > > __________________________________________________________________
> > > ____ This email has been scanned by the MessageLabs Email Security 
> > > System.
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> > [log in to unmask]
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> > > ____
> > >
> > > ---------------------------------------------------
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> > > -----------------------------------------------------
> > >
> >
> >
> >
> > --
> >
> > Best Regards,
> >
> > *Reuven Rokah*
> >
> > Mobile: 972-52-60-120-18
> > Tele-fax: 97239360688
> > <http://www.rokah-technologies.com/>[log in to unmask]
> > [log in to unmask]
> > www.rokah-technologies.com
> >
> > **
> > This e-mail message is intended for the recipient only and contains 
> > information which is CONFIDENTIAL and which may be proprietary of 
> > Rokah Technologies. If you have received this transmission in error, 
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> > of the original files and all other copies exist.
> >
> >
> > ______________________________________________________________________
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> >
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> > For additional information, or contact Keach Sasamori at [log in to unmask]
> > 847-615-7100 ext.2815
> > -----------------------------------------------------
> >
> >
>
>
> --
>
> Best Regards,
>
> *Reuven Rokah*
>
> Mobile: 972-52-60-120-18
> Tele-fax: 97239360688
> <http://www.rokah-technologies.com/>[log in to unmask]
> [log in to unmask]
> www.rokah-technologies.com
>
> **
> This e-mail message is intended for the recipient only and contains
> information which is CONFIDENTIAL and which may be proprietary of Rokah
> Technologies. If you have received this transmission in error, please
> inform
> me by e-mail, phone or fax, and then please delete all of the original
> files
> and all other copies exist.
>
>
> ______________________________________________________________________
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