Hi Werner,
You do need "liquid water" for vapor pressure. Vapor pressure is the
pressure imposed by evaporation. I just want all to understand that the
pressure exerted by vapor pressure is limited to the amount of available
water, or other volatiles, in the system. I mean molecular water, not
water bound as -OH and +H ions in the dielectric. The increase in the
rate of pressure due to temperature is reduced when all available
molecules of the volatiles are in a gaseous state. You can't drive a
steam engine with one drop of water.
This is not my idea. It was pointed out to me by Wayne Rothschild
formerly from IBM. The amount of water counts when calculating the
pressure imposed by heating.
I vaguely remember this from P Chem. I was calculating the number of
impacts per second on the surface of a vessel based on the heat (read
speed), size and weight of the molecules, and number of molecules in the
vessel in order to calculate pressure. More molecules present in the
vessel, at an isotherm, means more hits on the inner surface of the
vessel which in turn equals greater pressure. Over a liquid the gas is
in equilibrium with liquid, something akin to an unlimited water source,
and pressure rises. Once all the liquid is evaporated the rate of
increase in pressure is reduced.
I believe the gas laws include this concept.
Baking does not reduce the vapor pressure (read evaporation rate) of the
volatile, baking reduces the amount of water. The internal pressure is
reduced because the amount of water is reduced.
From Wikipedia (only used this source because I can cut and paste,
reference books say the same thing)
"Vapor pressure or equilibrium vapor pressure is the pressure of a vapor
in thermodynamic equilibrium with its condensed phases in a closed
container. All liquids and solids have a tendency to evaporate into a
gaseous form, and all gases have a tendency to condense back to their
liquid or solid form.
The equilibrium vapor pressure is an indication of a liquid's
evaporation rate. It relates to the tendency of particles to escape from
the liquid (or a solid). A substance with a high vapor pressure at
normal temperatures is often referred to as volatile."
Note the definition states "in thermodynamic equilibrium". When you run
out of water the pressure in the board is effectively limited.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Werner
Engelmaier
Sent: February 1, 2010 7:38 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Bundle Crack in Microsection
Hi Vicka-and please forward to Paul,
Unfortunately, what Paul says is incorrect. You do NOT need liquid water
to have an increase in pressure-without superheated steam, no steam
turbines would be possible.
I would doubt that any liquid H2O exists in any PCB, but for sure the
vapor pressure increases with temperature.
Werner
-----Original Message-----
From: White, Vicka (EHCOE) <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, Feb 1, 2010 2:49 pm
Subject: Re: [IPC-600-6012] Bundle Crack in Microsection
Sent on behalf of Paul Reid:
Hi Everyone,
As a result of high temperature assembly cycles, forced on us by RoHS,
our company developed a capacitance method to find material damage in
representative coupons. We developed the method about 5 years ago and
have been reviewing and categorizing delamination ever since. The first
materials tested, where we tested 21 lead free materials produced 19
materials with delamination (all had been baked). Different types of
delamination present different effects in reliability testing, by
changing failure modes or accelerating or preventing failures.
Based on our experience what is exhibited in the photo is an atypical
delamination. We would call this adhesive delamination not crazing,
measeling, cohesive failure nor material decomposition. What makes it an
atypical crack is that it is limited to the center of the glass bundle.
Most material cracks I see do not stop at epoxy or glass bundle
interfaces. They usually progress right through boundaries but
frequently stay within the b-stage or c-stage layer. On occasion
cohesive failure will traverse the b-stage C-stage boundary.
Given that the crack is limited to the glass bundle and it is
directionally propagating down what appears to be the wrap of the glass
fabric, in the c-stage layer, my vote would be a glass wetting problem.
I do not believe baking would mediate this failure mode nor do I believe
stress from vapor pressure would produce this type of delamination.
On Baking - It should be noted that a heated PWB can have internal
volatiles that exert a vapor pressure which builds stress very quickly
when the temperature is above the volatile's boiling point. Pressure
builds quickly with an increase in temperature but only until all the
liquid has become gaseous. The vapor pressure we studied in chemistry
and physics was usually at equilibrium over a liquid. There was an ample
supply of liquid available to produce vapor. In a circuit board there is
limited water and not much at that. Vapor pressure reaches a limit when
available water is vaporized. The amount of water trapped in the board
by weight is very small. Pressure increases slowdown when all available
liquid has been volatilized. That is why we find baking so rarely makes
a difference in reducing delamination in test coupons. Coupons that
delaminate when tested "as received", will still delaminate after an
appropriate bake most of the time.
Asked why he smoked cigars, knowing that smoking cigars was bad for the
health, Mark Twain answered (I paraphrase) "I smoke cigar so I can have
hope." "If I get an incurable disease I can always give up cigars and
hope I get better." "If I didn't have a vice to give up, and I
contracted an incurable disease, then I would have no hope."
I think baking most FR4 materials in an effort to prevent delamination
instills hope more than prevents delamination. I believe that polyimide
materials should be baked prior to thermal excursion.
The product should be rejected.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Francis
Byle
Sent: Monday, February 01, 2010 7:42 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Bundle Crack in Microsection
I would at very least view it as a process indicator. Given that the
sample is thermally stressed, the design and/or materials of the PWB may
well be contributing factors. The fabricator may, therefore, not be the
only place I need to look for root cause.
If I found this in an area with closely spaced through-vias, I would
certainly regard it as a CAF risk, and I would not accept it.
Fritz Byle
Astronautics Corp. of America
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Jose A
Rios
Sent: Sunday, January 31, 2010 6:43 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Bundle Crack in Microsection
i'd be curious to know from other oem's whether they reject for this
condition.
i got the impression in phoenix that not all oem's reject for this.
Joey Rios
PWB & Process Quality Eng'r
Endicott Interconnect Technologies
1093 Clark St.
Endicott, NY 13760
Office: 607-755-5896
Denise Chevalier <[log in to unmask]>
Sent by: IPC-600-6012 <[log in to unmask]>
01/29/2010 09:15 AM
Please respond to
"(Combined Forum of D-33a and 7-31a Subcommittees)"
<[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [IPC-600-6012] Bundle Crack in Microsection
Historically this has been termed as measling or crazing and has been
evaluated from the surface not from micro sections, see section 2.3 of
IPC-600 for photos and criteria. Lately we have had some customers that
are rejecting for this condition when seen in the sections (terming as
bundle cracks). Think that an open discussion would be of value - would
also like to see if there is any data out there that would contradict
the current requirement.
Denise
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Jose A
Rios
Sent: Thursday, January 28, 2010 9:09 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Bundle Crack in Microsection
isnt this crazing (or a cousin of...) ??
i'd be interested to see the area of interest at higher mag, to see if
the
crack goes thru an actual glass fiber or not.
i've seen cracks induced by sectioning (aggressive grinding/polishing)
where individual glass bundles are cut in half.
as to possible caf paths, couldnt measles and/or crazes act as such, yet
neither one is rejectable.
Joey Rios
PWB & Process Quality Eng'r
Endicott Interconnect Technologies
1093 Clark St.
Endicott, NY 13760
Office: 607-755-5896
John Perry <[log in to unmask]>
Sent by: IPC-600-6012 <[log in to unmask]>
01/28/2010 07:35 PM
Please respond to
"(Combined Forum of D-33a and 7-31a Subcommittees)"
<[log in to unmask]>
To
[log in to unmask]
cc
Subject
[IPC-600-6012] Bundle Crack in Microsection
Hi Everyone,
We've got a request for feedback on the anomaly in the attached .ppt
file
which was detected in one bundle of glass in a coupon. This is in the
center of the board stack, which is a 24 layer construction. Since it
is
in the core material it may be related to glass non-wetting of the resin
or poor glass treatment.
I'd also like to know if this is something we can consider developing
criteria for in the next generation of IPC-6012/A-600 when we meet in
Las
Vegas in a couple months.
Regards,
John Perry
Technical Project Manager
IPC - Association Connecting Electronics Industries(r)
3000 Lakeside Drive # 309S
Bannockburn, IL 60015-1249 USA
+1 847-597-2818 (tel)
+1 847-615-7105 (fax)
+1 847-615-7100 (Main)
[log in to unmask]
www.ipc.org
|
