Conformal coating for highimpedence design?

J

Jim Miller

Guest
I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?

tnx
jtm
 
On Wed, 22 Sep 2004 22:02:11 -0400, "Jim Miller"
<jim@removethisjtmiller.com> wrote:

I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?

tnx
jtm
Click to expand...

Polyurethane varnish, from the hardware store.

John
 
On Wed, 22 Sep 2004 19:47:47 -0700, the renowned John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Wed, 22 Sep 2004 22:02:11 -0400, "Jim Miller"
jim@removethisjtmiller.com> wrote:

I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?

tnx
jtm



Polyurethane varnish, from the hardware store.
John
Click to expand...
Do you think Future (acrylic) would work?

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Jim Miller <jim@removethisjtmiller.com> says...

I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?
Click to expand...
Look here:
Acrylic Lacquer Conformal Coating 419B
http://www.mgchemicals.com/products/419b.html

Avoid any hardware store products. You don't know how well they adhere
and whether they attack anything on the board, and they don't have a
PWB-safe stripper.

Also, strongly consider using grounded guard rings.

--
Guy Macon <http://www.guymacon.com>
 
In article <zOadndrQV8Yis8_cRVn-pw@comcast.com>,
Jim Miller <jim@removethisjtmiller.com> wrote:
I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?
Click to expand...
The simple answer is "no; put it in a sealed box".

If this is a production unit, you need to study the conformal coating
issue before you layout the PCB. In my experience, a sealed box is less
costly in the long run.

Also:

Don't use "no-clean flux" and be careful in selecting water washed fluxes.

It looks like some lead free solder actually works so if CE will be an
issue you can try them. I think they replace the lead with an mixture of
plutonium and unobtainium.

Dry the board before coating or placing in the box. This is usually done
by placing them in the hot box over night or for a few days. In the long
run the coated board will have the same vapor pressure inside as out but
at least you get a clean start on it.

Never apply power to the board until the coating has aged to the point
where the strong smell is gone. Some coatings contain ions that you can
move around when they are wet. If it still smells wet, its still "wet".

Don't believe anything the maker of the coatings tells you. If they say
the sky is blue, check. This is from sad experience. The people that
apply the stuff are much more likely to know the right answers.

Mil-spec coatings are just ones that have made it past the approval
process. They worked well on tube based radios. If you aren't having to
meet some mil-spec. do your own picking based on performance.




--
--
kensmith@rahul.net forging knowledge
 
In article <n5f4l0t4q81p8tcjjfn5amcm0f0ub5p67g@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:
[...]
Polyurethane varnish, from the hardware store.
John

Do you think Future (acrylic) would work?
Click to expand...
Melted crayons?

--
--
kensmith@rahul.net forging knowledge
 
In article <10l4f7ahok31d6b@news.supernews.com>,
Guy Macon <http://www.guymacon.com> wrote:
[...]
Also, strongly consider using grounded guard rings.
Click to expand...
Better yet, use driven guard rings. This will raise both the DC and AC
impedance.

--
--
kensmith@rahul.net forging knowledge
 
I read in sci.electronics.design that Ken Smith
<kensmith@green.rahul.net> wrote (in <citfd2$h5$5@blue.rahul.net>) about
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:

I think they replace the lead with an mixture
of plutonium and unobtainium.
Click to expand...
'An' mixture? What a dreadful grammatical error! (;-) It's not
plutonium, it's goofium.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Jim Miller wrote:
I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?

tnx
jtm
Click to expand...
You also might consider replacing FR-4 with a better material, such as
Megtron 5 made by Matsushita Electronics Materials,
http://www.mem-or.com also available thru Oil4LessLLC.com
 
On Thu, 23 Sep 2004 02:52:58 GMT, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Wed, 22 Sep 2004 19:47:47 -0700, the renowned John Larkin
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Wed, 22 Sep 2004 22:02:11 -0400, "Jim Miller"
jim@removethisjtmiller.com> wrote:

I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?

tnx
jtm



Polyurethane varnish, from the hardware store.
John

Do you think Future (acrylic) would work?

Best regards,
Spehro Pefhany
Click to expand...

Don't know. You've got to be careful that the stuff doesn't attack any
parts, and that it doesn't absorb moisture, as some varnishes do. The
polyurethane varnish I used to use (when I was poor, and did this all
myself) was nice except on polystyrene caps, and you could solder
right through it for rework. My production people do this now, and I
think they buy proper conformal coating material, probably the same as
my old varnish but 5x the price.

John
 
On Thu, 23 Sep 2004 07:59:07 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <citfd2$h5$5@blue.rahul.net>) about
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:

I think they replace the lead with an mixture
of plutonium and unobtainium.

'An' mixture? What a dreadful grammatical error! (;-) It's not
plutonium, it's goofium.
Click to expand...

Now, John, be nice; English is not our native language over here.

John
 
On 23 Sep 2004 01:28:34 -0700, jdurban@vorel.com (Product developer)
wrote:

Most any of the commercially available UV viewable coatings will work
but your biggest concern will be just how clean is clean. If you use a
typical water soluable flux used in wave soldering you must be very,
very certain that the boards are absolutely clean. If you coat over
just the slightest amount of flux residue in an application like that
you have identified you will have nothing but trouble when that flux
becomes conductive in the presense of just the slightest humidity.
Click to expand...
Second that. Ionic contamination from *anything* water-soluble can be
a nightmare.

John
 
John Woodgate wrote:
I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <citfd2$h5$5@blue.rahul.net>) about
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:


I think they replace the lead with an mixture
of plutonium and unobtainium.


'An' mixture? What a dreadful grammatical error! (;-) It's not
plutonium, it's goofium.
Click to expand...
So, John
How is the lead free initiative working out?

--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com
 
I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <mtp5l0dtul490les07vso30mfqums3a4ln@
4ax.com>) about 'Conformal coating for highimpedence design?', on Thu,
23 Sep 2004:
On Thu, 23 Sep 2004 07:59:07 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <citfd2$h5$5@blue.rahul.net>) about
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:

I think they replace the lead with an mixture
of plutonium and unobtainium.

'An' mixture? What a dreadful grammatical error! (;-) It's not
plutonium, it's goofium.


Now, John, be nice; English is not our native language over here.

I'll believe that when you post in Choctaw. Or, preferably, Lockjaw.
Click to expand...
(;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Thu, 23 Sep 2004 13:23:13 -0700, John Larkin
<jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

[snip]
My native language is Yat [1]. We conjugate...

I We
You Y'all
He, She, It They

John


[1] The spoken language of New Orleans, named for the Aloha-like
universal greeting, "Where y'at?"
Click to expand...
Aren't the correct forms "We'uns" and "Them" ?:)

I called back to WV the other day to make hotel reservations for my
wife's 45th high school reunion. I almost died from the slow talking
;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On Thu, 23 Sep 2004 13:29:53 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Thu, 23 Sep 2004 13:23:13 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

[snip]

My native language is Yat [1]. We conjugate...

I We
You Y'all
He, She, It They

John


[1] The spoken language of New Orleans, named for the Aloha-like
universal greeting, "Where y'at?"


Aren't the correct forms "We'uns" and "Them" ?:)
Click to expand...
No, you're thinking about the South. New Orleans is its own private
island of weirdness. The native accent resembles, if anything,
Brooklyn.

John
 
On Wednesday 22 September 2004 11:59 pm, John Woodgate did deign to grace us
with the following:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <citfd2$h5$5@blue.rahul.net>) about
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:

I think they replace the lead with an mixture
of plutonium and unobtainium.

'An' mixture? What a dreadful grammatical error! (;-) It's not
plutonium, it's goofium.
--
I worked at an outfit once that did analysis by electron spectroscopy.
Click to expand...
They had a lot of samples composed mainly of mysterium. :)

Cheers!
Rich
 
I read in sci.electronics.design that Roy McCammon <rbmccammon@mmm.com>
wrote (in <41530757.2000204@mmm.com>) about 'Conformal coating for
highimpedence design?', on Thu, 23 Sep 2004:
So, John
How is the lead free initiative working out?
Click to expand...
I'm not following it in detail, because no-one is paying me to do so,
and people ARE paying me to do many other things.

I believe that many of the consumer products manufacturer have switched
to lead-free, and there has lately been some sort of agreement on which
alloys to use. Initially, there were compatibility problems between the
alloys used by manufacturers and those available to service technicians.

I still have a large reel of 60/40, and I'm not proposing to throw it
away, even if I could do that legally. (;-)

I've been brought up on lead; lead water pipes, heavy lead paint, 40/60
solder initially, then 60/40. Any damage has already been done. That's
probably why I'm not as clever as you guys.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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