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
> >
> >
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> --
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> Best Regards,
>
> *Reuven Rokah*
>
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Best Regards,
*Reuven Rokah*
Mobile: 972-52-60-120-18
Tele-fax: 97239360688
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