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April 2006

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Fri, 21 Apr 2006 12:41:25 -0500
Content-Type:
text/plain
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text/plain (136 lines)
Just to let everyone know, we've decided to rebuild the assemblies from
scratch. Not worth the effort to try and rework them...

Thanks again for all the replies! That's why I love this forum!

Have a great weekend, and I'm off to FUJI Flexa school next week in
Chicago.

-Steve Gregory- 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Friday, April 21, 2006 2:54 AM
To: [log in to unmask]
Subject: Re: [TN] Parylene Coating Removal...

When I read this, before all the other replies, I said to myself,
"Whoa!, this is REALLY nasty stuff!" The EU labelling classification is
Symbol: F, Xi, T
R: 11-19-36/37
S: (2-)-16-28-29-33
Highly flammable
Irritant
Toxic
May form explosive peroxides
Irritating to eyes and respiratory system Keep out of the reach of
children Keep away from sources of ignition - No smoking After contact
with skin, wash immediately with plenty of water Do not empty into
drains Take precautionary measures against static discharges

However, that's not all. How does it work? It doesn't chemically react
with the paraxylylene of the coating, but it penetrates into the polymer
matrix, causing it to swell and thus destroy the bonding to the
substrate. As it is volatile, so the vapour will do the same, to a
lesser extent, with surrounding coating and this may well destroy the
integrity of the bond sufficiently for the reliability to be affected,
even if you cannot see the damage.

My advice: don't! The laser method is inherently better and cost should
not be a criterion.

Brian

Steve Gregory wrote:
> Hi All!
>
> I've always thought that when it comes to removing parylene coating, 
> about the only way you can remove it is with some sort of mechanical 
> means, i.e.; microabrasion, or scraping. Burning through it with a 
> soldering iron when trying to do rework it is very, very 
> difficult...I've tried it...it doesn't work well at all.
>
> I was just told that a chemical that I haven't heard of before might 
> work, it's called THF, or Tetrahydrofuran. I was told that it won't 
> dissolve the coating, but will cause it to lift from the PCB where it 
> can be brushed away. Have any of you ever heard of this stuff? I'm not

> a chemist, and I'm not about to use it until I learn more about it.
>
> The issue that I have is with a little tiny assembly that has been 
> built and parylene coated with some bad programmable hall effect 
> switches installed. The board is so small, and there are other 
> components installed really close to where the coating has to be 
> removed. I'm afraid of getting them microblasted because I'm afraid of

> damaging the other components. See the assembly at:
> http://stevezeva.homestead.com/files/8001_assy_backside.JPG ...the 
> small components are 0402's, so that gives you a visual scale of how 
> big the assembly is...
>
> I've got the information in to a company that I found that does laser 
> ablation to remove coatings to give us a quote, so I'm waiting on
that.
> But if that turns out to be really expensive, I was contemplating 
> using the THF as plan "B" (provided the stuff will work and it doesn't

> hurt anything else on the assembly).
>
> Kind regards,
>
> -Steve Gregory-
> Senior Production Engineer
> OAI Electronics
> East 12th Street
> Tulsa, Oklahoma 74112
>
> (918) 836-9077 Ext.-507
> (918) 706-2779 CELL
>
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