Homebrew PCBs

Stefan Trethan wrote:


> Ok, here what i see:
> Some liquid is created, which i think also sizzles.

   Melt something.  It sits there as a liquid.  Anything sizzling has a gas 
escaping.  Can't see anything as moving and making noise if it is all sitting 
there just melted and there isn't something expanding at a different rate. 
Liquids won't just make noise sitting there on their own, a cup of water or oil 
never sizzles by itself, and when it's heated and does it's the gas escaping 
that does it.  Water sizzles on a hot iron from the steam escaping, not from the 
liquid part.  You'd have to have a liquid with an unreal coefficient of 
expansion to have it displace air enough to make a sound no matter how much you 
heat it.

> When i remove the board quickly the liquid is still there, and it can be  
> used to wipe partially toner off the board like a oil or solvent. i dunno  
> what it is, it is not there with high-temp silicone, no difference how  
> long you let cure, so i do assume it comes from non-high temp silicone.
> 

   Exactly, it is the solvent remaining in the silicone.  The high temp simply 
dries out more thoroughly in curing, the low temp just retains more in the cured 
mix.  Actually the high temp must be staying rubberized by something else, hard 
to see it locking in solvent or atmospheric water enough to hold it at 600 deg plus.

   Low temp is known to retain quite a bit of acid for a long time, sealing it 
up will cause the acid vapor to eat up electronics.  If you haven't let it cure 
for a year you're not as done with the acid as you may think..

   And after refreshing my memory a sec on the net, it does pull in atmospheric 
water as part of the curing process, and that is also what eventually dries out. 
  So you may be driving off both remaining acid and water, there's just little 
way it's the solids part melting, I've seen it survive much higher temps for a 
very long time with little apparent change besides getting hard, it's just not a 
surviving as a gasket at that point thus you need a higher temp version.  Can't 
see an elastomeric compound like this that's solid at room temp melting at such 
a low temp.

   Looking at the hardware store there are a couple that are based on the oily 
liquid silicon.  But even those should cure into an elastomer, maybe you're 
reverting a partial cure but doubt it's really melting a fully cured rubberized 
compound.  Also this was a completely different product, even the high temp RTV 
seems to be a much more normal type.


   And recall RTV stands for Room Temperature Vulcanizing.  Vulcanizing normally 
takes higher temps, RTV is a special process for it to happen at room temp.  And 
it's slow, the full curing of these is also largely advanced by heat.  Which 
means if you haven't cooked it out there's still extra stuff in there unless 
it's had a very long time to cure, it'd probably take 50 or 100 fuser passes to 
cook it well enough.  Used a bunch of different types of this stuff in many 
applications over 20 years, and I've never had instance to think of it as 
something that I was going to melt.  Other things may happen but melting isn't 
one I've ever seen at 200 deg C fusing temps or even 400 deg F engine temps. 
May have to put some in the oven just to see what it does, I'd expect simply get 
hard from anything I've ever seen before.




   And while checking the hardware store and getting some high temp RTV (only 
black so it'll be a pita probably to see), there was some really clear acrylic 
sealant listed as super elastic so I got some to try.  Flammable solvents so 
would take some good drying and flame testing to make sure it won't result in 
printer immolation, but it feels very good in the tube, worth a try could be 
better than silicone.  If it's pliant and adheres to copper, aluminum, crome, 
glass, and many others as they say it might be very good.  Same pricing level as 
the high temp, little bit nastier solvents but worth a look.  Ah toluene, good 
old model airplane glue smell.  Excellent feel and about perfect tackiness to a 
small smear, time for some testing.

   With 7 HP 6L's laying around now, it's also time to start working to mod one 
to print direct to copper.  I still think this can be accomplished by proper 
techniques, my aluminum foil prints worked very well.

Alan