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Conformal coating for highimpedence design?

J

Jim Miller

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
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
 
S

Spehro Pefhany

Jan 1, 1970
0
Polyurethane varnish, from the hardware store.
John

Do you think Future (acrylic) would work?

Best regards,
Spehro Pefhany
 
G

Guy Macon

Jan 1, 1970
0
Jim Miller said:
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?

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.
 
K

Ken Smith

Jan 1, 1970
0
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?

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.
 
K

Ken Smith

Jan 1, 1970
0
Polyurethane varnish, from the hardware store.
John

Do you think Future (acrylic) would work?[/QUOTE]

Melted crayons?
 
K

Ken Smith

Jan 1, 1970
0
Guy Macon said:
Also, strongly consider using grounded guard rings.

Better yet, use driven guard rings. This will raise both the DC and AC
impedance.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Ken Smith
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.
 
P

Product developer

Jan 1, 1970
0
Jim Miller said:
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

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.

Conformal coating is only as good as the surface it protects,
especially in a subnanoamp environment. If your circuit can withstand
hipot testing you may consider probing the board to verify that the
flux is completely removed.

I would clean and verify then pre-heat the assembly to around 130 deg
F to flash off any local moisture and coat immediately after or even
during the application of heat. If this is a short run you can use an
enviromental chamber to preheat the board(s) then coat them. Once the
coating is applied verify consistancy with a UV source. If all is well
return the boards to the chamber for curing according to the coating
mfgr's procedures.
 
R

Robert Baer

Jan 1, 1970
0
Jim said:
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

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
 
T

Tony

Jan 1, 1970
0
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.

Guard rings are often a must (grounded if that's appropriate).

FWIW I'd also like to share an experience with a sensitive inverting
amplifier with switched feedback which worked over a 130dB range
starting at around 10fA. The first production batch worked fine, but
the next batch had some subtle drifts. As far as we could determine,
the smt parts in the feedback networks had, more than before, bonded
closely with the solder mask underneath and any flux that started out
underneath couldn't be cleaned out, and caused leakage. The only way
we could guarantee no problems was to route a few 1mm wide x 2mm long
slots under the most sensitive few parts.

Tony (remove the "_" to reply by email)
 
J

John Larkin

Jan 1, 1970
0
Do you think Future (acrylic) would work?

Best regards,
Spehro Pefhany


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
 
J

John Larkin

Jan 1, 1970
0
I read in sci.electronics.design that Ken Smith


'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.

John
 
J

John Larkin

Jan 1, 1970
0
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.

Second that. Ionic contamination from *anything* water-soluble can be
a nightmare.

John
 
R

Roy McCammon

Jan 1, 1970
0
John said:
I read in sci.electronics.design that Ken Smith
'Conformal coating for highimpedence design?', on Thu, 23 Sep 2004:




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

So, John
How is the lead free initiative working out?
 
J

John Woodgate

Jan 1, 1970
0
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:
Now, John, be nice; English is not our native language over here.
I'll believe that when you post in Choctaw. Or, preferably, Lockjaw.
(;-)
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Roy McCammon <[email protected]>
wrote (in said:
So, John
How is the lead free initiative working out?

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.
 
J

John Larkin

Jan 1, 1970
0
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:
I'll believe that when you post in Choctaw. Or, preferably, Lockjaw.
(;-)


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?"
 
J

Jim Thompson

Jan 1, 1970
0
On Thu, 23 Sep 2004 13:23:13 -0700, John Larkin

[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" ?:)

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
 
J

John Larkin

Jan 1, 1970
0
On Thu, 23 Sep 2004 13:23:13 -0700, John Larkin

[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" ?:)

No, you're thinking about the South. New Orleans is its own private
island of weirdness. The native accent resembles, if anything,
Brooklyn.

John
 
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