Homebrew PCBs

i am making a lot of pcbs but after a while they oxidize (corosion). 
i couldnt find any flux on electronics stores (here in israel i 
could hardly find copper boards). is there someplace alse where i 
can get something to prevent corosion????

thanks a lot
mumin

>From: "mumin55555" <MUMIN55555@...>
>Reply-To: Homebrew_PCBs@yahoogroups.com
>To: Homebrew_PCBs@yahoogroups.com
>Subject: [Homebrew_PCBs] how can i prevent corosion?
>Date: Thu, 27 Nov 2003 05:59:42 -0000
>
>i am making a lot of pcbs but after a while they oxidize (corosion).
>i couldnt find any flux on electronics stores (here in israel i
>could hardly find copper boards). is there someplace alse where i
>can get something to prevent corosion????

All the  big suppliers like Farnell stock this sort of stuff. I use 
something called Flux SK10 made by Kontakt Chemie. It's an aerosol flux. It 
takes some time to dry, but works very well. I have also used a 
solder-through lacquer in the past, that can be better as it it isn't sticky 
when handling. Soldering is a bit harder, though.

I don't actually use it much. My home-made PCBs don't seem to oxidise, even 
after several months, probably because there is a small amount of resist 
left on the copper.

Leon
--
Leon Heller, G1HSM Tel: +44 1424 423947
Email: aqzf13 at dsl dot pipex dot com
WWW: http://www.geocities.com/leon_heller

_________________________________________________________________
Sign-up for a FREE BT Broadband connection today! 
http://www.msn.co.uk/specials/btbroadband

--- In Homebrew_PCBs@yahoogroups.com, "mumin55555" <MUMIN55555@H...> 
wrote:
> i am making a lot of pcbs but after a while they oxidize 
(corosion). 
> i couldnt find any flux on electronics stores (here in israel i 
> could hardly find copper boards). is there someplace alse where i 
> can get something to prevent corosion????


I have achieved good results with Tinnit (DATAK) tin "plate".  The 
stuff (shipped in powder form) dissolves in warm to hot water and 
works best when hot (140F).  You immerse the freshly etched/washed 
board into the tinnit mixture and within a few seconds, all the 
copper turns a silver color.  You are supposed to leave the board in 
for 10-30 minutes.  After plating, it is suggested to thouroughly 
clean the board with a mild solution of household ammonia to remove 
all traces of the plating solution. The name of the material would 
suggest tin plating, but there is no electro-plating process.  I'm 
not entirely sure what it is, but I have yet to have one of my boards 
(treated with this stuff) corrode.

You can also use a lacquer spray to paint/protect the board after 
assembly to prevent corrosion.  I used to use a clear coat of MG 
Chemicals Conformal Coating, but I believe it is simply clear acrylic 
Lacquer.

Good luck

I have very good results with colophony solved in laquer thinner.
Can be a bit sticky before totally dry.
colophony is the resin of pine tree i think.



it is a good flux and keeps oxygen away...


If want to protect completed boards, after soldering, you can use any 
laquer.

st

In article <bq43se+5u70@...>,
   mumin55555 <MUMIN55555@...> wrote:
> i am making a lot of pcbs but after a while they oxidize (corosion). 
> i couldnt find any flux on electronics stores (here in israel i 
> could hardly find copper boards). is there someplace alse where i 
> can get something to prevent corosion????


Tin plating can often help. If you can't find the proprietry products
someone recently posted the formula here.

-- 
 __  __  __  __      __ ___   _____________________________________________
|__||__)/ __/  \|\ ||_   |   /
|  ||  \\__/\__/| \||__  |  /...Internet access for all Acorn RISC machines
___________________________/ stuartwinsor@...

101 uses for a Pentium: No2 - A greenhouse heater.

In a message dated 11/27/2003 3:37:15 AM Central Standard Time, 
Johnman@... writes:
The name of the material would suggest tin plating, but there is no 
electro-plating process.  I'm not entirely sure what it is, but I have yet to have one 
of my boards (treated with this stuff) corrode.
It IS tin plating!  There are SOME metals/chemistries that permit 
"non-electroless" plating, i.e., applying the metal WITHOUT the use of DC current.  

Wish there was a way to get that "Tinnit" stuff in larger quantities, AND 
some instructions as to how to increase the "shelf life" (it does seem to "go 
bad", even in a shut bag, in time).  I used only "one bag" one time, as I do 
boards that warrant such so seldom, and it just would NOT work, months later.  
Also, what "good" is the solution after one use?  Will it work months later, if 
left closed?  Didn't seem to "want to"!    Some chemist "in here" please tell 
us!  
Jan Rowland


[Non-text portions of this message have been removed]

I've posted this mixture before:

Mix 3.8 grams of Stannous chloride into 1 liter of
distilled water that is at 80 to 120 degrees
fahrenheit in temperature. Stir well until dissolved.
Add 49.5 grams of thiourea to the dissolved stannous
chloride and distilled water while maintaing the
temperature between 80 and 120 degrees fahrenheit.
Stir well until dissolved. Add 12 ml of sulfuric acid
to this mixture (Be careful! As you add the acid the
mixture is going to get HOT! Also, Always, ALWAYS add
acids to water. Never, NEVER add water to acids!).
Stir well. 

You are now the proud owner of electroless tinning
solution. 


Best regards

Marvin Dickens
Alpharetta, Georgia USA



--- JanRwl@... wrote:
   Some
> chemist "in here" please tell 
> us!  
> Jan Rowland


=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

On Sun, 30 Nov 2003 23:54:21 -0800 (PST), mpdickens <md30022@...> 
wrote:

> I've posted this mixture before:
>
> Mix 3.8 grams of Stannous chloride into 1 liter of
> distilled water that is at 80 to 120 degrees
> fahrenheit in temperature. Stir well until dissolved.
> Add 49.5 grams of thiourea to the dissolved stannous
> chloride and distilled water while maintaing the
> temperature between 80 and 120 degrees fahrenheit.
> Stir well until dissolved. Add 12 ml of sulfuric acid
> to this mixture (Be careful! As you add the acid the
> mixture is going to get HOT! Also, Always, ALWAYS add
> acids to water. Never, NEVER add water to acids!).
> Stir well.
>
> You are now the proud owner of electroless tinning
> solution.
>
>
> Best regards
>
> Marvin Dickens
> Alpharetta, Georgia USA
>
>

Well, that sounds easy..
BUT thiourea ia pretty nasty stuff!
It is known to produce cancer (no idea which dose etc.).
and it is very toxic to the environment (DON'T put it down the drain!).

If you really need to use it look at the msds and the safety precautions
and follow to them.

I myself rather use other methods.

st


Why never add water to acid?
I mean i know that but i don't know why.
I thought in the past maybe because any splashes would be more
likely from the water (if you add acid to water) but that can't be the 
reason?

Stefan Trethan wrote:
> On Sun, 30 Nov 2003 23:54:21 -0800 (PST), mpdickens <md30022@...> 
> wrote:
> 
> Why never add water to acid?
> I mean i know that but i don't know why.
> I thought in the past maybe because any splashes would be more
> likely from the water (if you add acid to water) but that can't be the 
> reason?

The dilution action can release heat, so a small drop of water
in acid can make a very high temperature and pressure explosion
or splash when the water turns to steam (like water in hot oil).

> The dilution action can release heat, so a small drop of water
> in acid can make a very high temperature and pressure explosion
> or splash when the water turns to steam (like water in hot oil).
>
>

i see

--- Stefan Trethan <stefan_trethan@...> wrote:

> Well, that sounds easy..
> BUT thiourea ia pretty nasty stuff!
> It is known to produce cancer (no idea which dose
> etc.).

Not as nasty as:

1.) Gasoline which is a known carcinogen
2.) Kerosine which is a know cacinogen
3.) Methlyine Chloride which is a known carcinogin and
also cause neurological damage.
4.) Tolulene (Main ingredient in a lot of glues) which
is a known carcinogin and causes neurological damage

I could go on and on..................


> and it is very toxic to the environment (DON'T put
> it down the drain!).

I see. Like fecl or copper chloride baths are not...?
What about the tinnit (TM) solution? The tinnit (TM)
crystals ARE composed of the compounds used in this
same solution. Add water and you have what I posted.
FWIW, this solution is the only currently known route
to electroless tinning that is cyanide free. 

If you would prefer a cyanide based method (That does
not involve thiourea, I'll be happy to email you
one...

> I myself rather use other methods.

What method do you use?

> Why never add water to acid?

You will end up wearing the acid. Further, when
pouring /measuring/ dispensing any chemical (Acid or
not...), do it at eye level and always wear eye
protection and protective clothing.




Best regards


Marvin Dickens
Alpharetta, Georgia  USA


=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

I bought a clear spray acrylic/epoxy, but I am thinking now that clear
nail polish might be useful too for PCBs with components that can not be
covered with the acrylic.

Ted


mumin55555 wrote:

>  i am making a lot of pcbs but after a while they oxidize (corosion).
> i couldnt find any flux on electronics stores (here in israel i
> could hardly find copper boards). is there someplace alse where i
> can get something to prevent corosion????
>
> thanks a lot
> mumin
>
>
>                    Yahoo! Groups Sponsor
                        ADVERTISEMENT
                         [click here]

>
> Be sure to visit the group home and check for new Bookmarks and files:
>
> http://groups.yahoo.com/group/Homebrew_PCBs
>
> To unsubscribe from this group, send an email to:
> Homebrew_PCBs-unsubscribe@yahoogroups.com
>
>
>
> Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.

--
Ted Huntington
Programmer Analyst I
Main Library
University of California, Irvine
PO Box 19557
Irvine, CA 92623-9557
emesgs:  thunting@...
web page:  http://business.lib.uci.edu/webpages/ted.htm
8:00a-12:00p Business Office (949) 824-8926
1:00p-5:00p  Multimedia Resource Center (949) 824-1674
"Stop violence, teach science."



[Non-text portions of this message have been removed]

>
> Not as nasty as:
>

> 1.) Gasoline which is a known carcinogen
> 2.) Kerosine which is a know cacinogen
> 3.) Methlyine Chloride which is a known carcinogin and
> also cause neurological damage.
> 4.) Tolulene (Main ingredient in a lot of glues) which
> is a known carcinogin and causes neurological damage
>
> I could go on and on..................
>
>
>> and it is very toxic to the environment (DON'T put
>> it down the drain!).
>
> I see. Like fecl or copper chloride baths are not...?
> What about the tinnit (TM) solution? The tinnit (TM)
> crystals ARE composed of the compounds used in this
> same solution. Add water and you have what I posted.
> FWIW, this solution is the only currently known route
> to electroless tinning that is cyanide free.
>
> If you would prefer a cyanide based method (That does
> not involve thiourea, I'll be happy to email you
> one...
>
>> I myself rather use other methods.
>
> What method do you use?
>

I just want to say no need for any dangerous chemicals if a simple
acrylic paint will do after soldering or a paint with colophony if you need 
to protect
it before soldering.

If i can avoid a class 3 carcinogen i will do it, no matter how much
other dangerous substances there are commonly used.
And if one has to use it, because there is no substitute, i would
stress following the suggested safety precautions rather than saying
"other stuff nasty too".

I agree it is maybe the best electroless tinning, but still nothing
to handle without care.


no point in discussing this further (in my opinion).

st

and thanks to all for the many acid/water, water/acid descriptions.
i think i got the picture now ;-)

Does anyone here have experience and suggestions for low-leakage
coatings? I've got some circuits in mind that in the past I've done
dead-bug style to avoid surface leakage on a PCB. Is there a good not
to expensive low leakage coating?

I'm thinking of an input with an impedance in the rage of 100M to 10G.
Yes, I even wash the parts themselves, I've been either leaving them
uncoated or coating them with paraffin. Something I got from a book on
static electricity machines, mix a bit of lighter fluid and wax and
paint it on. The lighter fluid (not butane, I mean the stuff that goes
in those old metal lighters) evaporates leaving a very thin even
coating of wax.

Steve

----- Original Message ----- 

From: "Steve" <alienrelics@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, December 01, 2003 6:28 PM
Subject: [Homebrew_PCBs] Low leakage coatings? Re: how can i prevent
corosion?


> Does anyone here have experience and suggestions for low-leakage
> coatings? I've got some circuits in mind that in the past I've done
> dead-bug style to avoid surface leakage on a PCB. Is there a good not
> to expensive low leakage coating?
>
> I'm thinking of an input with an impedance in the rage of 100M to 10G.
> Yes, I even wash the parts themselves, I've been either leaving them
> uncoated or coating them with paraffin. Something I got from a book on
> static electricity machines, mix a bit of lighter fluid and wax and
> paint it on. The lighter fluid (not butane, I mean the stuff that goes
> in those old metal lighters) evaporates leaving a very thin even
> coating of wax.

Have you used 'guard ring' techniques and Teflon standoffs?

Leon

Its very difficult to even get that level with solder mask, so say nothing of flux. I've tried different coatings from Dupont, however our assembly techniques nullified the coatings, thus we returned to dead-bug. 

A friend in Singapore made thousands of production sensor conditioners a month, and even then had to dead bug the input circuits. At least his labor costs were low enough to make this practical.

I will keep an eye on this topic, as I'm very interested if someone has a workable solution.

Thanks
Ron

  ----- Original Message ----- 
  From: Steve 
  To: Homebrew_PCBs@yahoogroups.com 
  Sent: Monday, December 01, 2003 1:28 PM
  Subject: [Homebrew_PCBs] Low leakage coatings? Re: how can i prevent corosion?


  Does anyone here have experience and suggestions for low-leakage
  coatings? I've got some circuits in mind that in the past I've done
  dead-bug style to avoid surface leakage on a PCB. Is there a good not
  to expensive low leakage coating?

  I'm thinking of an input with an impedance in the rage of 100M to 10G.
  Yes, I even wash the parts themselves, I've been either leaving them
  uncoated or coating them with paraffin. Something I got from a book on
  static electricity machines, mix a bit of lighter fluid and wax and
  paint it on. The lighter fluid (not butane, I mean the stuff that goes
  in those old metal lighters) evaporates leaving a very thin even
  coating of wax.

  Steve


        Yahoo! Groups Sponsor 
              ADVERTISEMENT
             
       
       

  Be sure to visit the group home and check for new Bookmarks and files:
  http://groups.yahoo.com/group/Homebrew_PCBs

  To unsubscribe from this group, send an email to:
  Homebrew_PCBs-unsubscribe@yahoogroups.com



  Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. 


[Non-text portions of this message have been removed]

--- In Homebrew_PCBs@yahoogroups.com, "Leon Heller" <leon_heller@h...>
wrote:
> 
> ----- Original Message ----- 
> From: "Steve" <alienrelics@y...>
> To: <Homebrew_PCBs@yahoogroups.com>
> Sent: Monday, December 01, 2003 6:28 PM
> Subject: [Homebrew_PCBs] Low leakage coatings? Re: how can i prevent
> corosion?
> 
> 
> > Does anyone here have experience and suggestions for low-leakage
> > coatings? I've got some circuits in mind that in the past I've done
> > dead-bug style to avoid surface leakage on a PCB. Is there a good not
> > to expensive low leakage coating?
> >
> > I'm thinking of an input with an impedance in the rage of 100M to 10G.

> 
> Have you used 'guard ring' techniques and Teflon standoffs?
> 

That blocks interference but it can actually make leakage worse as the
path to ground is shorter.

No, haven't tried Teflon. Is it that good an insulator? What am I
using the stand-offs for? If you mean the supports for the PCB, that
still leaves me with surface leakage across the PCB.

Steve

Here is yet another approach. Volatile corrosion inhibitors. We've tried
them in nasty outdoor environments and they are just amazing. The only
disadvantage is life span. However in extremely nasty environments such as
Minnesota winters, they do really well and allow for extended service life.

 They also have a conformal coating material as well

http://www.cortecvci.com/Products/products.php?showonly=Electric

Ron

Ron Amundson wrote:
> Its very difficult to even get that level with solder mask, so say nothing of flux. I've tried different coatings from Dupont, however our assembly techniques nullified the coatings, thus we returned to dead-bug. 
> 
> A friend in Singapore made thousands of production sensor conditioners a month, and even then had to dead bug the input circuits. At least his labor costs were low enough to make this practical.
> 
> I will keep an eye on this topic, as I'm very interested if someone has a workable solution.

Why do you need *any* coating on the pcb?

Russell Shaw wrote:
> 
> Why do you need *any* coating on the pcb?

To prevent any possibility of the PCB being coated in something 
you don't want it coated with. e.g. microscopic water 
condensation or traces of dried salts.

--- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...> wrote:

> Why do you need *any* coating on the pcb?

Exposed conductors mean they are exposed to humid or dusty air, and
the surface of the fiberglass substrate will build up contaminants and
humidity.

Steve

Steve wrote:
> --- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...> wrote:
> 
>>Why do you need *any* coating on the pcb?
> 
> Exposed conductors mean they are exposed to humid or dusty air, and
> the surface of the fiberglass substrate will build up contaminants and
> humidity.

You could put the pcb in a sealed container with dry nitrogen
or normal air with moisture-absorbing gel.

--- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...> wrote:
> Steve wrote:
> > --- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...>
wrote:
> > 
> >>Why do you need *any* coating on the pcb?
> > 
> > Exposed conductors mean they are exposed to humid or dusty air, and
> > the surface of the fiberglass substrate will build up contaminants and
> > humidity.
> 
> You could put the pcb in a sealed container with dry nitrogen
> or normal air with moisture-absorbing gel.

I was hoping for something that would allow repair and that would not
require hermetically sealed pressure tested enclosures. Otherwise
those are not bad suggestions.

If you are wondering why I say pressure tested, the first time it
ships by air it's got to stand more than a few PSI. That means a lid
2x4 inches at, say, 10psi air pressure in an unpressurized hold has to
stand up to about 36 pounds forcing it open. I'm just shy of sea level.

Steve

--- Ron AmAmundsonroronmamundsonohotmailom> wrote:
> Here is yet another approach. Volatile corrosion
> inhibitors. We'veveried
> them in nasty outdoor environments and they are just
> amazing. The only
> disadvantage is life span. However in extremely
> nasty environments such as
> Minnesota winters, they do really well and allow for
> extended service life.

Using a disposable brush, coat the board in 1 hour
epoxy. The coat does not have to be that thick: You
just cover the board (And the parts on the board if
you want to) in a thin coat. After it dries, it's
water proof, impervious to most chemicals including
most hydrocarbon based solvents (Including most acids,
most strong bases and gasoline) and it has a service
life somewhere between 7 to 10 years in reridiculously
harsh environments and climates (Extreme cold or
hot...). Plus epoxy is an insulator. The service life
can be extended 4 or 5 additional years if the board
is not exposed to UV light sources (Natural sun
light).

During the life span of a expoxy coated board, if the
need arises to remove part or all of the epoxy
coating, there are chemicals that you can use to do it
without a lot of fuss. 

FWIW, I've been involved in projects where we potted
*entire* electronic assemblies in epoxy and then
deployed those assemblies on the floor of the atlantic
ocean for up to a year. 

It's been my experience that epoxy is your friend
against harsh environments and climates...


Best regards


Marvin Dickens
Alpharetta, Georgia  USA


=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

That blocks interference but it can actually make leakage worse as the
path to ground is shorter.

No, haven't tried Teflon. Is it that good an insulator? What am I
using the stand-offs for? If you mean the supports for the PCB, that
still leaves me with surface leakage across the PCB.

Actually a driven guard ring will reduce the problems of leakage. If you are refering a grounded guard ring, you are correct. However most guard rings are driven to the same signal levels as the signal of interest in order to reduce problems with leakage.

You can get teflon for your pcb material instead of FR4, but it is a lot more expensive. Teflon also will hold a electrostatic charge and can create miuch havoc.

See
http://www.national.com/rap/Story/0,1562,4,00.html

for an example of how teflon can create a problem as it ages.

Best regards
Ron

[Non-text portions of this message have been removed]

Just one note: I found methylene chloride paint stripper does 
excellent job removing epoxy, unfortunately that includes the FR4 
epoxy. After several hours it turns to mush.

mpdickens wrote:
> It's been my experience that epoxy is your friend
> against harsh environments and climates...

>It's been my experience that epoxy is your friend
>against harsh environments and climates...

I've used epoxy too, and have filled potting boxes with it. However it can
raise havoc with heat producing parts, as well as creating additional
thermal and conductive paths. However if the economic conditions allow it,
its quite workable except when it comes to service.

On key thing with some epoxies is that they can cause stress related
failures, eg op amp Vos drift is exagerated with temp, and ferrite beads
really hate the stuff unless you balance their coefficient of expansion with
the epoxy.

The VCI stuff we used was in addition to conformal coatings on the pcb, and
as such protected the wire harnesses  as well.

The worst corrosion on a pcb I've ever seen was in the washdown area of a
cheese plant. While the box used liquid tight fittings and was NEMA 4X,
water vapor made inroads, and then would condense. After 6 months, the unit
died due to corroded traces.  Conformal coating was mandatory after this
problem, despite the extra cost.

The worst case of corrosion I've ever seen was on a stainless steel sensor
housing in a very nasty and proprietary chemical process. After 3 months,
the atmospheric vapors had eaten away 50% of a 0.1" stainless steel shell.
They had to go to a series of air knives to keep the nasty atmosphere away.

The underwater stuff sounds really cool. What did you have to do for cases,
as I imagine the pressure loads were really high.

Thanks
Ron

----- Original Message ----- 

From: "Steve" <alienrelics@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, December 01, 2003 11:29 PM
Subject: [Homebrew_PCBs] Low leakage coatings? Re: how can i prevent
corosion?


> --- In Homebrew_PCBs@yahoogroups.com, "Leon Heller" <leon_heller@h...>
> wrote:
> >
> > ----- Original Message ----- 
> > From: "Steve" <alienrelics@y...>
> > To: <Homebrew_PCBs@yahoogroups.com>
> > Sent: Monday, December 01, 2003 6:28 PM
> > Subject: [Homebrew_PCBs] Low leakage coatings? Re: how can i prevent
> > corosion?
> >
> >
> > > Does anyone here have experience and suggestions for low-leakage
> > > coatings? I've got some circuits in mind that in the past I've done
> > > dead-bug style to avoid surface leakage on a PCB. Is there a good not
> > > to expensive low leakage coating?
> > >
> > > I'm thinking of an input with an impedance in the rage of 100M to 10G.
>
> >
> > Have you used 'guard ring' techniques and Teflon standoffs?
> >
>
> That blocks interference but it can actually make leakage worse as the
> path to ground is shorter.
>
> No, haven't tried Teflon. Is it that good an insulator? What am I
> using the stand-offs for? If you mean the supports for the PCB, that
> still leaves me with surface leakage across the PCB.

The technique I was thinking of was to bend up the input pin and solder it
to a Teflon standoff. Teflon is a very good insulator. This is sometimes
done if a very high input impedance is required.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@...
http://www.geocities.com/leon_heller

Steve wrote:
> --- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...> wrote:
> 
>>Steve wrote:
>>
>>>--- In Homebrew_PCBs@yahoogroups.com, Russell Shaw <rjshaw@i...> wrote:
> 
>>>>Why do you need *any* coating on the pcb?
>>>
>>>Exposed conductors mean they are exposed to humid or dusty air, and
>>>the surface of the fiberglass substrate will build up contaminants and
>>>humidity.
>>
>>You could put the pcb in a sealed container with dry nitrogen
>>or normal air with moisture-absorbing gel.
> 
> I was hoping for something that would allow repair and that would not
> require hermetically sealed pressure tested enclosures. Otherwise
> those are not bad suggestions.
> 
> If you are wondering why I say pressure tested, the first time it
> ships by air it's got to stand more than a few PSI. That means a lid
> 2x4 inches at, say, 10psi air pressure in an unpressurized hold has to
> stand up to about 36 pounds forcing it open. I'm just shy of sea level.

With a couple of extra screws thru the center, the pressure would be
less problem.

On the next design, the high impedance circuit could be confined to
a small circuit board in a box, with a lead or pins plugging in to
the main pcb.

--- Ron Amundson <ron_amundson@...> wrote:
> 
> >It's been my experience that epoxy is your friend
> >against harsh environments and climates...
> 
> I've used epoxy too, and have filled potting boxes
> with it. However it can
> raise havoc with heat producing parts, as well as
> creating additional
> thermal and conductive paths. However if the
> economic conditions allow it,
> its quite workable except when it comes to service.

Heat was never an issue when the box is in the water.
The entire box acted as a heat sink. Hoever, we also
did it like this when devices had heat issues and the
unit was not submersed in water:

     --------------------------------
     |epoxy potting                 |
     |  ------------           ------- 
     |  |           ----------| heat |
     |  | PCB       |componet | sink |
     |  |           ----------|      |
     |  ------------           -------
     |                              |
     --------------------------------

The heat sink protrudes from the expoxy and dissapates
the heat.

> On key thing with some epoxies is that they can
> cause stress related
> failures, eg op amp Vos drift is exagerated with
> temp, and ferrite beads
> really hate the stuff unless you balance their
> coefficient of expansion with
> the epoxy.

Extruding the heat sink as diagramed above will fix
the op amp problem. Regarding ferrite beads, This is
strange to me. I've hever had an application where
they conducted heat and I cannot think of one. 

> The underwater stuff sounds really cool. What did
> you have to do for cases,
> as I imagine the pressure loads were really high.

The expoxy potting *was* the case. In fact, it was
perfect: Cheap, reliable and never leaked and good to
1000 Feet. Woods Hole Oceanographic Institute still
does it this way. Pressure was never an issue because
epoxy contracts very little under high pressure.  

Best regards

Marvin Dickens
Alpharetta, Georgia  USA

=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

> > No, haven't tried Teflon. Is it that good an insulator? What am I
> > using the stand-offs for? If you mean the supports for the PCB, that
> > still leaves me with surface leakage across the PCB.
> 
> The technique I was thinking of was to bend up the input pin and
solder it
> to a Teflon standoff. Teflon is a very good insulator. This is sometimes
> done if a very high input impedance is required.

That is essentially what I end up doing now, and was hoping to find
something more elegant and professional looking.

Steve

Re: filled with dry nitrogen and sealed

> > If you are wondering why I say pressure tested, the first time it
> > ships by air it's got to stand more than a few PSI. That means a lid
> > 2x4 inches at, say, 10psi air pressure in an unpressurized hold has to
> > stand up to about 36 pounds forcing it open. I'm just shy of sea
level.
> 
> With a couple of extra screws thru the center, the pressure would be
> less problem.
> 
> On the next design, the high impedance circuit could be confined to
> a small circuit board in a box, with a lead or pins plugging in to
> the main pcb.

Yes, but my case costs will go up drastically.

The 2nd solution would cause less problems as far as serviceability as
only that high impedance portion would cause problems when serviced,
but then I still need nitrogen tanks and a way to fill all this,
astromomically increasing the start-up costs.

I know I'm nay-saying a lot of this, but I do think they are good and
valid ideas. Just not for my particular purpose and situation.

Steve

--- In Homebrew_PCBs@yahoogroups.com, mpdickens <md30022@y...> wrote:
> 
> --- Ron Amundson <ron_amundson@h...> wrote:
> > 
> > >It's been my experience that epoxy is your friend
> > >against harsh environments and climates...
> > 
> > I've used epoxy too, and have filled potting boxes
> > with it. However it can
> > raise havoc with heat producing parts, as well as
> > creating additional
> > thermal and conductive paths. However if the
> > economic conditions allow it,
> > its quite workable except when it comes to service.
> 
> Heat was never an issue when the box is in the water.
> The entire box acted as a heat sink. Hoever, we also
> did it like this when devices had heat issues and the
> unit was not submersed in water:
> 
>      --------------------------------
>      |epoxy potting                 |
>      |  ------------           ------- 
>      |  |           ----------| heat |
>      |  | PCB       |componet | sink |
>      |  |           ----------|      |
>      |  ------------           -------
>      |                              |
>      --------------------------------
> 
> The heat sink protrudes from the expoxy and dissapates
> the heat.

The epoxy itself can dissipate the heat as long as you design enough
surface area between the heat sink and the epoxy, and from the epoxy
to the water. Water is a great conductor of heat and seawater is
pretty cold.

Long ago I fixed those overheating C64 power supplies by sawing off
the plastic top and epoxying an aluminum heat sink right on top of the
epoxy encapsulating everything. I knew it worked well because the base
of the heat sink got warm within 20 seconds of turning it on but never
got any warmer. In that case things like conductivity made no difference.

> > On key thing with some epoxies is that they can
> > cause stress related
> > failures, eg op amp Vos drift is exagerated with
> > temp, and ferrite beads
> > really hate the stuff unless you balance their
> > coefficient of expansion with
> > the epoxy.
> 
> Extruding the heat sink as diagramed above will fix
> the op amp problem. Regarding ferrite beads, This is
> strange to me. I've hever had an application where
> they conducted heat and I cannot think of one. 

He meant the different rates of expansion as the entire device changes
temp. This will not be much of a problem with something in seawater,
should be at a pretty constant temp as long as the power producing
components are designed to give up their heat pretty efficiently to
the seawater.

> > The underwater stuff sounds really cool. What did
> > you have to do for cases,
> > as I imagine the pressure loads were really high.
> 
> The expoxy potting *was* the case. In fact, it was
> perfect: Cheap, reliable and never leaked and good to
> 1000 Feet. Woods Hole Oceanographic Institute still
> does it this way. Pressure was never an issue because
> epoxy contracts very little under high pressure.  

My favorite kind of project box.

For my side branch of the thread on low leakage, I may just pot the
high impedance part of the circuit if I can find an epoxy encapsulant
that has very little conductance.

Steve

--- Steve <alienrelics@...> wrote:
 
> I  may just pot the high impedance part of the
circuit > if I can find an epoxy encapsulant that has
very little 
> conductance.

Polyester resin (Epoxy is the trade name for polyester
resin) has an extremely low conductance. In fact, the
conductance of expoxy is close to that of glass.

FWIW, here is a something that everybody may want to
take a look at:

http://www.alchemetal.com/Characteristics.htm


I'm ordering the a sample kit for a project that I'm
working on for a customer. The kit consists of:

    * 1 pound of Alchemetal AC-78
    * 1 pound of Alchemetal special solvent
    * 1 ounce of Kester flux 819

The sample kit will cover approximately 30 square feet
of substrate at a 2 mil wet thickness. If anyone wants
some, email me off list and I'll sell you some at my
cost (Naturally, in new, appropriate packaging). There
is no way I can use it all...

Best regards

Marvin Dickens
Alpharetta, Georgia  USA 


=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

--- In Homebrew_PCBs@yahoogroups.com, mpdickens <md30022@y...> wrote:
> 
> --- Steve <alienrelics@y...> wrote:
>  
> > I  may just pot the high impedance part of the
> circuit > if I can find an epoxy encapsulant that has
> very little 
> > conductance.
> 
> Polyester resin (Epoxy is the trade name for polyester
> resin) has an extremely low conductance. In fact, the
> conductance of expoxy is close to that of glass.

I think you've said this before, but I've never heard that. And epoxy
resin is always listed separately from polyester resin.

Like this site:
http://www.shopmaninc.com/resins.html

I'll readily admit that I don't know what kind of plastic is in epoxy.

> FWIW, here is a something that everybody may want to
> take a look at:
> 
> http://www.alchemetal.com/Characteristics.htm

Fantastic! A conductive castable polymer, can be soldered and
electroplated. I added it to the links under Cases for Projects.

Steve

--- Steve <alienrelics@...> wrote:
> --- In Homebrew_PCBs@yahoogroups.com, mpdickens
> <md30022@y...> wrote:
> > 
> > --- Steve <alienrelics@y...> wrote:
> >  
> > > I  may just pot the high impedance part of the
> > circuit > if I can find an epoxy encapsulant that
> has
> > very little 
> > > conductance.
> > 
> > Polyester resin (Epoxy is the trade name for
> polyester
> > resin) has an extremely low conductance. In fact,
> the
> > conductance of expoxy is close to that of glass.
> I think you've said this before, but I've never
> heard that. And epoxy
> resin is always listed separately from polyester
> resin.
> 
> Like this site:
> http://www.shopmaninc.com/resins.html

Your right. You nailed me ;) Most of the stuff they
sell in stores that is labeled "Epoxy" is really
polyester resin. In fact, true epoxies have to be
heated in order to cure (In an autoclave). For our
purposes (At least mine...), polyester resin is just
fine. I happen to know a little chemistry. Here is the
difference between the two:

Polyester resin is a simplier molecule than an epoxy
molecule. Polyester is made by reacting dibasic acids
such as maleic anhydride or phthalic anhydride with
dihydric alcohols such as ethylene glycol (antifreeze)
in equal amounts.  These resins are short chain
polymers  and are set into amorphous solids by cross
linking the polyester chains to each other (This
results in an exothermic reaction which is why it get
hot when it cures).

OTOH, Epoxy polymers are made by reacting
epichlorohydrin with bisphenol-A in an alkaline
solution. Each polymer chain has one epoxide grouping
at each end of the chain (but none within the polymer
chain). The hardners that are used are short chain
diamines such as ethylene diamine. Once they are
mixed, the reaction to produce an epoxy molecule has a
high heat of reaction and requires the introduction of
energy (Heat) into the system. Also, hcl is produced
as  a by product (along with water) during this
synthesis. Both the hcl and the water must be burned
off (By heat) as it is produced in order to completely
cure.

Epoxy is the strongest and most flexible of the two
(And also real expensive because you need an
autoclave).

Best regards

Marvin Dickens
Alpharetta, Georgia 

=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

--- Steve <alienrelics@...> wrote:
> --- In Homebrew_PCBs@yahoogroups.com, mpdickens
> <md30022@y...> wrote:
> > 
> > --- Steve <alienrelics@y...> wrote:
> >  
> > > I  may just pot the high impedance part of the
> > circuit > if I can find an epoxy encapsulant that
> has
> > very little 
> > > conductance.
> > 
> > Polyester resin (Epoxy is the trade name for
> polyester
> > resin) has an extremely low conductance. In fact,
> the
> > conductance of expoxy is close to that of glass.
> I think you've said this before, but I've never
> heard that. And epoxy
> resin is always listed separately from polyester
> resin.
> 
> Like this site:
> http://www.shopmaninc.com/resins.html

Your right. You nailed me ;) Most of the stuff they
sell in stores that is labeled "Epoxy" is really
polyester resin. In fact, true epoxies have to be
heated in order to cure (In an autoclave). For our
purposes (At least mine...), polyester resin is just
fine. I happen to know a little chemistry. Here is the
difference between the two:

Polyester resin is a simplier molecule than an epoxy
molecule. Polyester is made by reacting dibasic acids
such as maleic anhydride or phthalic anhydride with
dihydric alcohols such as ethylene glycol (antifreeze)
in equal amounts.  These resins are short chain
polymers  and are set into amorphous solids by cross
linking the polyester chains to each other (This
results in an exothermic reaction which is why it get
hot when it cures).

OTOH, Epoxy polymers are made by reacting
epichlorohydrin with bisphenol-A in an alkaline
solution. Each polymer chain has one epoxide grouping
at each end of the chain (but none within the polymer
chain). The hardners that are used are short chain
diamines such as ethylene diamine. Once they are
mixed, the reaction to produce an epoxy molecule has a
high heat of reaction and requires the introduction of
energy (Heat) into the system. Also, hcl is produced
as  a by product (along with water) during this
synthesis. Both the hcl and the water must be burned
off (By heat) as it is produced in order to completely
cure.

Epoxy is the strongest and most flexible of the two
(And also real expensive because you need an
autoclave).

Best regards

Marvin Dickens
Alpharetta, Georgia 

=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

mpdickens wrote:
> 
> Your right. You nailed me ;) Most of the stuff they
> sell in stores that is labeled "Epoxy" is really
> polyester resin. In fact, true epoxies have to be
> heated in order to cure (In an autoclave). 


Then in that case most of the retailers have got the 
labeling wrong. If they call polyester resin "Epoxy" then 
what do they call polyester resin ?
A shop I went to made distinct differences between the epoxy 
resins and polyester resins they sold. If I buy a two part 
resin which part A is labeled "contains %100 epoxy" and part 
B is labeled "contains: > %70 aliphatic amines", then is 
that  polyester or epoxy ? , it doesn't need autoclaving

> Then in that case most of the retailers have got the labeling wrong. If 
> they call polyester resin "Epoxy" then what do they call polyester resin 
> ?
> A shop I went to made distinct differences between the epoxy resins and 
> polyester resins they sold. If I buy a two part resin which part A is 
> labeled "contains %100 epoxy" and part B is labeled "contains: > %70 
> aliphatic amines", then is that  polyester or epoxy ? , it doesn't need 
> autoclaving
>

all the "epoxys" here for model building etc. don't need an autoclave.
They are calles epoxy, and there is also stuff called polyester resin, 
which i thought is sort of older eqivalent. No idea if they are all labeled 
wrong.

I have seen "hot coating tanks" for coating pcbs with acrylic.
maybe that can be used for high resistance.
the tanks somehow melt acrylic plastic but i have no futher information on 
that.

st

--- Adam Seychell <adam_seychell@...> wrote:

> Then in that case most of the retailers have got the
> 
> labeling wrong. If they call polyester resin "Epoxy"
> then 
> what do they call polyester resin ?
> A shop I went to made distinct differences between
> the epoxy 
> resins and polyester resins they sold. If I buy a
> two part 
> resin which part A is labeled "contains %100 epoxy"
> and part 
> B is labeled "contains: > %70 aliphatic amines",
> then is 
> that  polyester or epoxy ? , it doesn't need
> autoclaving
> 
> 
Any resins that are mixed with an amine are epoxies.
Bisphenol-A is an amine. If your not having to
autoclave it, then the 70% aliphatic amines are mixed
with 30% (Or maybe less...) of something that causes
an exothermic reaction. Epoxy has got to have heat to
cure. The strength of the non autoclaved, but cured
expoxy would, in my opinion, not have the strength
(But it would still be stronger than polyester resin)
that epoxy that was cured in an autoclave would have.
The stuff sold in places like home depot say epoxy,
but are polyester resins according to the ingredients
listed on the packaging. You are obviously purchasing
your resins from a company that sells a lot of these
types of composite materials (A specialty store of
some type?)


 All of the epoxy resins I have had experience with
(Back when Lockheed-Grumman was Grumman Aerospace...)
require autoclaving. Basically, we had a mold, put the
electronics in the mold and poured the resin in with
the electronics. Then, we hooked up a vacum to it
(Around 60 psi) to remove any air bubbles in the resin
and sent it to the autoclave. When we would get the
device back from the autoclave we would pull it out of
the mold. It would have a nice, smooth conformal
finish on all sides. 


Best regards


Marvin Dickens
Alpharetta, Georgia  USA

=====
Registered Linux User No. 80253
If you use linux, get counted at: 
http://www.linuxcounter.org

__________________________________
Do you Yahoo!?
Free Pop-Up Blocker - Get it now
http://companion.yahoo.com/

In article <20031203080535.74060.qmail@...>,
   mpdickens <md30022@...> wrote:


> Epoxy is the strongest and most flexible of the two
> (And also real expensive because you need an
> autoclave).


OK, now, the Epoxy I normally use, trade name Araldite, is two part and
the warning on the side of one of the tubes, which I am holding at the
moment says (next to the "irritant warning cross) "contains reaction
product Bisphenol A-Epichlorohydrin Epoxy Resin (number average molecular
weight <700). 

It sets fully in around 24hrs at normal room temperature though setting
can be speeded up by heating. No autoclave.

Polyester resins, often used as fillers in car body repairs have a very
distinct smell and Araldite smells nothing like them.

Please explain.

Stuart.

-- 
 __  __  __  __      __ ___   _____________________________________________
|__||__)/ __/  \|\ ||_   |   /
|  ||  \\__/\__/| \||__  |  /...Internet access for all Acorn RISC machines
___________________________/ stuartwinsor@...

101 uses for a Pentium: No2 - A greenhouse heater.

> Any resins that are mixed with an amine are epoxies.
> Bisphenol-A is an amine. If your not having to
> autoclave it, then the 70% aliphatic amines are mixed
> with 30% (Or maybe less...) of something that causes
> an exothermic reaction. Epoxy has got to have heat to
> cure. The strength of the non autoclaved, but cured
> expoxy would, in my opinion, not have the strength
> (But it would still be stronger than polyester resin)
> that epoxy that was cured in an autoclave would have.
>

> Marvin Dickens
> Alpharetta, Georgia  USA
>

I was told that curing the epoxy in an oven at 80 degree or so
improves the strength, even if it is hardening only by mixing the 2 
components too.
The "epoxy" i have (e.g 5 minute 1:1) gets warm when it cures, but not hot. 
I had never problems with the strength, and never tried baking it...

st

mpdickens wrote:
> The stuff sold in places like home depot say epoxy,
> but are polyester resins according to the ingredients
> listed on the packaging. You are obviously purchasing
> 

http://www.wordreference.com/english/definition.asp?en=epoxy


   Seems the best single definition.  Problem is you won't find a single 
solid answer, because the term epoxy has been broadened by use to also 
include any thermosetting plastic adhesive, you'll see that definition 
in many other sources.  So the polyester resin isn't technically an 
epoxy but it's perfectly fine to call it an epoxy.  A bit ridiculous but 
hell that's English.  :)  It's either a technically accurate chemical 
term or a loose term, depending on use.

Alan

mpdickens wrote:
> Epoxy has got to have heat to
> cure.

Then how do you explain when mixing a  < 1 gram of epoxy on the 
surface of a heavy steel plate that is at room temperature (15 ~ 
30\ufffdC) and it still cures relatively ok ? Any exothermic reaction 
will have nil effect in this situation.

There is a lot of info on epoxy here.
http://www.dow.com/epoxy/tech/index.htm

According to the document titled "DOW Liquid Epoxy Resins" it is 
the type of curing agent which determines the temperature and 
rate of cure. The aliphatic amines are the relatively fast room 
temperature type. There are others which require curing at 
elevated temperatures, which sounds like the type you once worked 
with. From what I gather from the document, the curing agents 
that need more heat and time give you a better cured epoxy resin, 
i.e. higher service temperature and more resistant to chemicals 
and solvents.

Another suggestion, use the green paint at this website 
http://www.dynaart.com/index.html  or www.pulsar.gs
or go to your local hobbyist shop and look for a similar product.

David