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Subject:
From:
Paul Edwards <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Edwards <[log in to unmask]>
Date:
Thu, 1 Sep 2005 10:09:23 -0700
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Phil,

We haven't seen a real difference in solder adhesion with ENIGed PCAs whether they were baked or not....

However we use the 150-160F in-air bake to control moisture in and stabilize the PCAs we get from our customers.  Because we have large prototyping customer base, we have found that the bake seems to minimize the PCB warp during reflow...

I have used "vacuum bake" ovens for both vacuum bake (no inlet gas) and partial- pressure bake (inlet "bleed" gas) and the IR driven vacuum bake process works better and faster than the partial pressure process for reducing the moisture content of the PCB's internal laminate layers.

Paul

Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: Wednesday, August 31, 2005 8:00 PM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking & Solderability

I think I know what this discussion is missing ....someone who isn't working to comment on it!...lol.  Anyways, I think I know what may have gone wrong in Dave's experiment as my experiments yielded the opposite results.  I certainly agree that baking is not going to improve the solderability on ENIG boards, but I have never been able to discern a difference if prebaked using this vacuum oven process.  The vacuum oven was set at 150 F to minimize the oxidation back in the days of HASL boards and still helps on ENIG as the gold does not seal the nickel below 100%.

This process is still being used by my former employer and survived numerous challenges by Quality Engineers who only believed in the  220 F/24 hrs prebake cycle.

Vacuum ovens (at least all of mine) have three ports on them, the largest one being in back and is for the vacuum pump, and then two smaller ports with a hose bibs on them at the front, one for room air which releases the vacuum pressure, and the other one is for bleeding in dry nitrogen gas.

The vacuum and the heat sets the water molecules in motion, but without a nitrogen gas stream flowing across the pwb surface, the water does not want to leave the PWB.  Based on Bev's addition below, my assumption is that most of the moisture is removed by the mechanical flow effects of the gas being bled in and very little of it actually dissolves into the nitrogen.

Important also is that the dry nitrogen gas is being flow limited and therefore trickled (bled) in to the vacuum oven chamber at a slower rate than the pump is pulling the atmosphere out.  The pump is left on continuously and maintains at least 24 inches of vacuum.

I started using this process many years ago for wavesoldering .125" thick PWBs which had stored in wet conditions and exhibited huge outgassing during wavesoldering unless vacuum baked like this.

Recently, a particularly aggresive ENIG stackup design forced me to have to use it in order to keep extremely the high density microvia technology board from delaminating due to the spacing of the drilled holes and the properties of the laminates.

Besides that, it also saves 22 hours when it comes to rebaking boards before reworking them.

Hope this is clear enough for everyone to understand.......

Phil

Bev Christian <[log in to unmask]> wrote:Paul,
There is a big diference in the scale of interest between a printed circuit board and the dimensions on a Si wafer. I am afraid you are going to have to prove to me with data that this could ever be an issue with a PCB.
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Paul Edwards
Sent: August 31, 2005 1:43 PM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability


Bev,

Both N2 and H20 exists as a gas molecules in the air..
In the presence of static charging sources they can be ionized and disassociated to form nitrogen compounds..

This can be easily seen in Si wafer cleaning processes prior to epi...

Paul

Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48

-----Original Message-----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Wednesday, August 31, 2005 10:30 AM
To: TechNet E-Mail Forum; Paul Edwards
Subject: RE: [TN] ENIG/ Immersion Gold Plating and Pre-baking & Solderability

Paul,
Please explain to me how in the world molecular nitrogen (N2) is going to react with water vapor in a benign environment like a near vacuum at less than 200C. I quote from the Webelements website http://www.webelements.com/webelements/elements/text/N/chem.html:
"Reaction of nitrogen with air
Nitrogen gas does not react with air under normal conditions.

Reaction of nitrogen with water
Nitrogen gas does not react with water. It does, however, dissolve to the extent of about x g kg-1 at 20°C (297 K) and 1 atmosphere pressure."


We aren't not talking about lightning bolts of charge here. I would expect the amount of charge caused by static to be relatively small, BUT I can always be educated. :)
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Paul Edwards
Sent: August 31, 2005 1:22 PM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability


Dave,

In the vacuum baking environment, the vacuum is used to lower the
partial pressure to 0 to increase the molecular diffusion of the water
out of the laminate structure..

In your experiments, the gas(N2 or Ar), serves as a thermal transfer
agent so gas filled baking would be more effective than the vacuum
baking, if the thermal energy source were heating coils...

If laminate absorbing IR sources were used instead, a vacuum bakeout
would be more effective than a "dry" gas bakeout.

Again assuming the thermal energy source is a heating coil, the ideal
baking method would be to use "bone-dry" or -100F (1ppm) or better Ar,
assuming that the moisture in the PCAs would not raise the moisture
level within the total Ar load to 0F (800ppm)...

I wouldn't use N2 because of molecular acid formation when the N2 reacts
with H20 and static electric charging sources that could occur in high
air flow environments...

Paul

Paul Edwards
Process Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, August 31, 2005 7:53 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability

Hi Bev! Exactly! And most folks who use a vacuum bake process just kick
on
the vacuum and away they go! They don't realize that the bake/vacuum
recipe
they have chosen might be impeding the removal of moisture.

Dave



Bev Christian
OM>
To
Sent by: TechNet [log in to unmask]

cc


Subject
08/31/2005 09:48 Re: [TN] ENIG/ Immersion Gold
AM Plating and Pre-baking &
Solderability

Please respond to
TechNet E-Mail
Forum

; Please respond
to
Bev Christian
OM>






The trick is to have the evaporation process (driving the concentration
gradient) to be slow enough to not comsume all the heat available so
that
the water stays liquid.
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Vladimir Igoshev
Sent: August 31, 2005 10:15 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability


Hi Dave,

That is a nice example of an adiabatic process, but it's "slightly
different" from "pumping" moisture out of a board. :-)
I'm a bit surprised with the results of your experiments. I'd expect
both
process to complement each other and speed up the process: vacuuming -
to
maintain the concentration gradient (driving force) and heating - to
speed
up diffusion (and prevent moisture from "freezing" :-)).

Regards,

Vladimir

Vladimir Igoshev, Ph. D.
Senior Materials Researcher
Research in Motion
451 Phillip St.
Waterloo, ON, N2L 3X2

Voice: (+1) 519-888-7465, ext. 5283
Fax: (+1) 519-886-0863
E-mail: [log in to unmask]


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of David D. Hillman
Sent: Tuesday, August 30, 2005 6:31 PM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability


Hi Richard! Just a word of caution - I did some experimenting a number
of
years ago to see if either baking or vacuum baking was more effective in
removing moisture from a pwb. I expected that the vacuum baking would be
the winner but the design of experiments results demonstrated that
baking
was more effective. The physical process of pulling a vacuum to remove
water ends up playing with the change of state (liquid to gas, gas to
liquid, etc) behavior which impacts the ability to remove water from the
pwb. I did some research and found a need example of pulling a vacuum on
a
disk of water - the change of state from a liquid to a gas resulted in
the
formation of ice (not exactly the result anyone wants to achieve)! The
research illustrated that with the right controls and a well
characterized
vacuum/heating recipe you can effectively remove moisture faster than
just
baking but in a practical sense, having a nitrogen or argon atmosphere
with
a typical baking process was just as effective and lower cost. Any
moisture baking process is going to degrade the pwb solderability so
avoiding baking if possible is the best option.

Dave Hillman
Rockwell Collins
[log in to unmask]




"Stadem, Richard"
D-AIS.COM>
To
Sent by: TechNet [log in to unmask]

cc


Subject
08/30/2005 08:27 Re: [TN] ENIG/ Immersion Gold
AM Plating and Pre-baking &
Solderability

Please respond to
TechNet E-Mail
Forum

; Please respond
to
"Stadem, Richard"
D-AIS.COM>






I do not know the answer to that, but it sounds like a good idea. I do
know that back in the 70's a major military OEM used to regularly bake
pwb's at 150 F (note F, not C) at a 1/2 gravity vacuum, and they claimed
the moisture was not removed if they did not have the 1/2 gravity
vacuum.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Creswick, Steven
Sent: Tuesday, August 30, 2005 8:24 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability

Richard,

Is it an approved/acceptable process to place the PWB's into a vacuum
chamber at room temp?

I would not hazard a guess as to how great of a vacuum for what
duration, however. Just curious if it is done in the surface mount
industry.

Nothing gets any more simple than a dessicantor box though!

Steve Creswick - Gentex Corp


-----Original Message-----
From: Stadem, Richard [mailto:[log in to unmask]]
Sent: Tuesday, August 30, 2005 9:05 AM
To: [log in to unmask]
Subject: Re: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability


Baking of ENIG plated boards may be a requirement in order to drive out
moisture even though it can exacerbate a nickel oxide formation that is
already started. But there are other options for moisture removal,
including lower bake temperatures at longer times. You could even lease
a precision scale and see at what length of time is required for simply
putting the PWB into a drybox (a dessicator box such as McDry, not a
nitrogen box) and allowing it to dry at room temperature for a period
long enough to reduce the moisture content by 90% or more. J-STD-030
does state that the bake times and temperatures they list for components
and pwbs are guidelines. If you can prove your bake process or
dessication process is removing the moisture, then you are qualifying a
lower temp and longer time. You will need to use a number of pwbs in
order to do this, as there is some variation in weights caused by resin
content, etc. But you should be able to show a reduction in the average
of weights (mean weight).

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Schaefer
Sent: Friday, August 26, 2005 10:10 AM
To: [log in to unmask]
Subject: [TN] ENIG/ Immersion Gold Plating and Pre-baking &
Solderability

Does baking these types of plating affect solderability? If so what
factors play a part (i.e. baking temperature, plating thickness, baking
time, etc...). I know that if baked it can cause the Nickel Oxide layer
to become less solderable if the plating process is Poor (thanks Richard
S.). Is there is a factual study that proves that baking any type of
gold plated boards in any way shape or form reduces the solderability
and/ or reliablity of the joints/ product? I am trying to gather data to
prove to management that baking is a bad decision with this type of
plating and should never be performed. Also if a Gold Plated board of
any type is suspect for high level moisture content - how do you remove
the moisture "IF" baking is not acceptable?


Chris Schaefer
Suntron Corporation

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