I need to encase an assembled PIC device in a polymer block. For example,
place the tested PCB+components in a mould and add a polymer so it sets
around the board and components to make a waterproof case. I also need to
encase a sensor which needs only be used up to 100 degrees Celsius.
I am concerned that the polymer, whether it be epoxy, neutral cure silicone
or some other compound may change in resistance with time (the device uses
15 bits for each measurement, so small changes count). This change could be
due to water absorption or structural reorganisation.
Please share your successes and failures. One problem with epoxy is the
exothermic polymerisation, however, the setting temperature should not
exceed 100 degrees Celsius.
Dean Biddle wrote:
>
> Hi,
>
> I need to encase an assembled PIC device in a polymer block. For example,
> place the tested PCB+components in a mould and add a polymer so it sets
> around the board and components to make a waterproof case. I also need to
> encase a sensor which needs only be used up to 100 degrees Celsius.
>
> I am concerned that the polymer, whether it be epoxy, neutral cure silicone
> or some other compound may change in resistance with time (the device uses
> 15 bits for each measurement, so small changes count). This change could be
> due to water absorption or structural reorganisation.
>
> Please share your successes and failures. One problem with epoxy is the
> exothermic polymerisation, however, the setting temperature should not
> exceed 100 degrees Celsius.
Be careful of the epoxy type you use. If it is a hard setting type and
it shrinks even slightly, it can put great pressure on the components
and lead to failure.
Some neutral cure silicone is nuetral after curing and eats away at
leads etc while curing.
> Hi,
>
> I need to encase an assembled PIC device in a polymer block. For example,
> place the tested PCB+components in a mould and add a polymer so it sets
> around the board and components to make a waterproof case. I also need to
> encase a sensor which needs only be used up to 100 degrees Celsius.
>
> I am concerned that the polymer, whether it be epoxy, neutral cure silicone
> or some other compound may change in resistance with time (the device uses
> 15 bits for each measurement, so small changes count). This change could be
> due to water absorption or structural reorganisation.
>
> Please share your successes and failures. One problem with epoxy is the
> exothermic polymerisation, however, the setting temperature should not
> exceed 100 degrees Celsius.
There are a large number of possible products that can
be selected depending on what you are looking for... e.g.
minimum shrinkage, moisture absorption, dielectric
strength, optical clarity, heat ductabilty, flexibility etc etc.
Checkout Electrolube, I dont know who the agents are in
Aus but they are Electroplus here in NZ.
For most potting I use a hard polyurethane UR 5528
(electrolube), I get the two-part bag kit... no de-gassing
needed if you are careful. (n.b. UR 5528 is close to the
compressbility of water).
For most non physically harsh environments (marine is
the worst I have ever come across) and low voltage
circuitry I am happy to use cheap old wax. Easy to get at
the components while maintaining a modicum of
protection from the elements.
_____________________________
Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
<x-flowed>At 09:46 AM 3/13/2000 +1000, you wrote:
>I am concerned that the polymer, whether it be epoxy, neutral cure silicone
>or some other compound may change in resistance with time (the device uses
>15 bits for each measurement, so small changes count). This change could be
>due to water absorption or structural reorganisation.
I've had reasonably good luck in the past with two-part silicone elastomer
- it's like epoxy in that it's a two part system that doesn't harden until
mixed, and it has a long working time... If fact, as I remember, it takes
about a week to completely cure - but it's hard in 12 hours or
so. However, it starts out as a very thin liquid, so it creeps around and
doesn't leave gaps or unsealed areas. I did usually wind up putting it in
a vacuum chamber to cure, as otherwise there were usually a few small
bubbles trapped in the material, but not nearly large enough to decrease
the protection it provided. This was used on boards with precision trimmed
resistive dividers, and I've yet to have a problem with them.
ND Holmes
--------------------------------------------------------------------------
Nathan D. Holmes .....maverickKILLspam@spam@drgw.net,ndholmesKILLspamiastate.edu
122 Shepard #3 Box 328 Gilbert, IA 50105 Iowa State University - EE http://www.drgw.net/~maverick PH: 515-663-9368
"Unless a man has creativity and self-motivation, freedom is an irksome
burden."
-------------------------------------------------------------------------
For non-repairable, I use a urethane two part mix. It is fairly rubbery,
and can be stiffened and the thermal transfer improved by adding alumina
powder. Can be cut with a hot knife, but is general not easily
repairable--you can get it off, but do more damage in the process.
For repairable, but much more fragile, I use a two part silicone, water
clear, very high surface resistance, cuts easily with a razor blade, and
peels off of PCBs--which makes repair very easy. Both cure in about 4
hours at 50 degC.
At the present time I use the GE (?) RTV series with great success in our
small data loggers. Have pretty much abandoned the use of the urethanes in
favor of the silicones.
Kelly
>Hi,
>
>I need to encase an assembled PIC device in a polymer block. For example,
>place the tested PCB+components in a mould and add a polymer so it sets
>around the board and components to make a waterproof case. I also need to
>encase a sensor which needs only be used up to 100 degrees Celsius.
>
>I am concerned that the polymer, whether it be epoxy, neutral cure silicone
>or some other compound may change in resistance with time (the device uses
>15 bits for each measurement, so small changes count). This change could be
>due to water absorption or structural reorganisation.
>
>Please share your successes and failures. One problem with epoxy is the
>exothermic polymerisation, however, the setting temperature should not
>exceed 100 degrees Celsius.
>
>
>> Hi,
>>
>> I need to encase an assembled PIC device in a polymer block. For
example,
>> place the tested PCB+components in a mould and add a polymer so it sets
..
<snippo>
..
>For most non physically harsh environments (marine is
>the worst I have ever come across) and low voltage
>circuitry I am happy to use cheap old wax. Easy to get at
>the components while maintaining a modicum of
>protection from the elements.
Could you elaborate a bit on the `wax' ?
I've seen wax used to hold parts in place in el-cheapo
radio sets & the like, but it just looks like candle-wax,
same stuff ? or something more esoteric ?
best regards, John
e-mail from the desk of John Sanderson, JS Controls.
Snailmail: PO Box 1887, Boksburg 1460, Rep. of South Africa.
Tel/fax: Johannesburg 893 4154
Cellphone no: 082 469 0446
email: jsandspam_OUTpixie.co.za
Manufacturer & purveyor of laboratory force testing apparatus, and related
products and services.
Adding my 2 cents worth in as well. I've personally had good luck with a
product from Hysol (P/N: ES 4412). Have used it for YEARS and have not noticed
any problems (yet). Sets up hard enough that you don't need a enclosure. We
pour into a form and pop it out afterwards. Used in the automotive industry.
There is a certain amount of infant death associated with it (probabally stress
due to the curing process) so we always burn in the units but few die after the
first week.
> Hi,
>
> I need to encase an assembled PIC device in a polymer block. For example,
> place the tested PCB+components in a mould and add a polymer so it sets
> around the board and components to make a waterproof case. I also need to
> encase a sensor which needs only be used up to 100 degrees Celsius.
>
> I am concerned that the polymer, whether it be epoxy, neutral cure silicone
> or some other compound may change in resistance with time (the device uses
> 15 bits for each measurement, so small changes count). This change could be
> due to water absorption or structural reorganisation.
>
> Please share your successes and failures. One problem with epoxy is the
> exothermic polymerisation, however, the setting temperature should not
> exceed 100 degrees Celsius.
> Could you elaborate a bit on the `wax' ?
> I've seen wax used to hold parts in place in el-cheapo
> radio sets & the like, but it just looks like candle-wax,
> same stuff ? or something more esoteric ?
Candle wax seems to be too corrosive for electronic use.
I have tried it.
> Hello Lance & PIC.ers,
>
>
> Could you elaborate a bit on the `wax' ?
> I've seen wax used to hold parts in place in el-cheapo
> radio sets & the like, but it just looks like candle-wax,
> same stuff ? or something more esoteric ?
>
>
> best regards, John
>
Ummmm...
I don't know.
I inherited a huge block of brown-ish wax when I started
here 10 years ago and haven't finished it yet. We don't do
a lot of "outside" work these days. The heady days of
whale research saw us use this and urethane a lot. There
were no reported problems due to encasing but I will add
that most of our gear survived max 3-4 months due to
outrageous harsh conditions although this was
acceptable since there was no substitute, the other gear
available lasted a few hours.
I will find out what this wax is though, it is not candle
wax.. that much I know. I think is a petroleum based
product but ,as I said, I will find out what it is.
I have posted what the urethane was.
_____________________________
Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
Brian Gregory wrote:
>
> In-Reply-To: <000d01bf91d2$08ffba40$4f7c1ec4@server>
>
> > Could you elaborate a bit on the `wax' ?
> > I've seen wax used to hold parts in place in el-cheapo
> > radio sets & the like, but it just looks like candle-wax,
> > same stuff ? or something more esoteric ?
>
> Candle wax seems to be too corrosive for electronic use.
> I have tried it.
>
> Brian Gregory.
> KILLspambriangKILLspamcix.co.uk
=============================
Seem to me that beeswax was used. Non-corrosive, easy to work.
Some years ago I needed to pot one or two circuits but didn't want to pay
the price of commercial potting compound.
Instead I purchased a quantity of epoxy resin and associated hardener from a
local automotive repairs/spares store.
This worked out extremely cheaply - it set/hardened a darkish/murky grey. If
this is just for weather proofing then that should be ok, but black pigment
could be added for 'circuitry privacy'.
For real good 'circuitry privacy' add very dry sand to mix instead of
pigment it sets like rock, also you could add a few used odd conponents for
anybody to find sould they try to open it.
all the best Arthur
| Some years ago I needed to pot one or two circuits but didn't want to pay
| the price of commercial potting compound.
|
| Instead I purchased a quantity of epoxy resin and associated hardener from
a
| local automotive repairs/spares store.
|
| This worked out extremely cheaply - it set/hardened a darkish/murky grey.
If
| this is just for weather proofing then that should be ok, but black
pigment
| could be added for 'circuitry privacy'.
|
| Regards,
| Neil
|
| {Original Message removed}
> Hello Lance & PIC.ers,
>
>
> >Date: Mon, 13 Mar 2000 14:00:58 +1200
> >From: Lance Allen <EraseMEl.allenAUCKLAND.AC.NZ>
> >Subject: Re: Encase PCB with epoxy, silicone or ??? (OT)
>
> >> Hi,
> >>
> >> I need to encase an assembled PIC device in a polymer block. For
> example,
> >> place the tested PCB+components in a mould and add a polymer so it sets
> ..
> <snippo>
> ..
Howdy...I have 2 cents on this matter also. there are a large number of
potting compounds that you can choose from. One that I use is a 3M product
(DP270). It comes in a convienent 50 by 50 two part dispensing tube and
cures at about 70 degrees celsius. It has a good dielectric constant and is
relative impermiable once it sets up. There are MANY other products that do
this from Applitech and others.
picnpoke.com
