Homebrew PCBs

Ok, thanks for your response. I thought you might have had made a grand 
discovery in getting through holes to place reliably over carbon coated 
holes. As I said earlier, I'm using carbon black to coat the holes and I am 
experiencing the same problems as many did in who tried to make 
carbon/graphite process commercially viable, such as MacDermid. The current 
process by MacDermid is called BLACKHOLE SP, and is well established in 
industry. They claim BLACKHOLE SP to be suitable for even the most high 
tech PCBs, 16+ layers, 0.1 mm hole diameters, et. The early attempts by 
these companies had problems getting the copper to plate on relatively thin 
layer of carbon/graphite. From my experiments of plating carbon coated 
holes, I've found the chemistry of the plating solution to play a huge part 
in obtaining successful copper coverage. For example, I found acid sulfate 
bright tin bath plates the holes much more reliable than acid copper bath. 
I also found small amounts of chloride in the acid copper bath have a 
detrimental effect, with > 300ppm of Cl- will completely inhibit any hole 
wall plating. I am wondering if the alkaline bath you use is the answer to 
getting the holes plated with good reliability. I'm not up to speed with my 
electrochemistry but I believe the reason holes are difficult to plate in 
is that the carbon has a higher overpotential for deposition of copper than 
copper on copper does. In other words you need higher cell voltage to plate 
on carbon than on copper. Overpotentials depend on many things and pH is 
one of them. The carbon surface resistively is relatively high (about 10e+6 
times more resistance of a unit square) compared to the copper foil surface 
resistively. During plating a voltage drop develops at the boundary were 
copper meets the carbon. The result is a forward growth of copper over the 
carbon surface during the plating cycle. The copper growth emanates at the 
hole ends and eventually meets half way inside the hole completing the 
coverage. Its the rate of this copper growth front the determines how good 
the process is running. If the growth rate is too slow (or nonexistent) 
then too much copper will be deposited on copper cladding, and not enough 
inside the holes. Worse still is the voiding inside the holes due to 
patches of extremely thin coatings of carbon. This is the fundamental problem.

There a number of alkaline copper plating chemistries that could be tested. 
The good news is we don't need high speed, smooth bright finishes, since 
the aim is to "flash plate". The acid copper bath will take care of the 
remaining copper. Industrial alkaline copper uses cyanide , which I prefer 
not to use. Another possibility is copper sulfate + ammonia hydroxide + 
ammonium sulfate with around pH 8 so it doesn't smell.

Like you, I am doing this as a hobby so I don't get the time needed to do 
thorough experiments. I did most of my carbon hole work when I had a part 
time job in which I had the time to built a home lab just to explore this 
process. I've had a about 1 year break from it now, but these discussions 
have got me thinking about it again. It might be time to dust out the lab.

Adam




Jeremy Taylor wrote:
> Hard to say - and subjective at best. but IMO yes.
> We can discuss this more tomorrow, I've been trying to take some pics , but
> man is it difficult to even get the holes...







> 
> 
> 
> http://www.soundclick.com/jtsound
> ----- Original Message ----- 
> From: "Adam Seychell" <adam_seychell@...>
> To: <Homebrew_PCBs@yahoogroups.com>
> Sent: Thursday, April 01, 2004 11:09 PM
> Subject: Re: [Homebrew_PCBs] plated tin as the etch resist
> 
> 
> 
>>
>>Jeremy Taylor wrote:
>>
>>>My last post must been sent just prior to yours.
>>>
>>>I actually ended up completely closing up some holes with the acid
> 
> copper.
> 
>>Does that mean the alkaline plate was more capable of completely
>>covering the holes walls with copper than compared to acid copper
>>plating ? Which bath was better ?
>>
>>
>>
>>>It' definitely a more aggressive plate.
>>
>>yes, the acid copper is fast, almost 100% coulombetric
>>efficient, good throwing power and easy to maintain and setup.
>>This is why I suspect you have chosen alkaline plating because it
>>performed better when plating on the ink surface.
>>
>>Regards,
>>
>>Adam
>>
>>
>>
>>Be sure to visit the group home and check for new Bookmarks and files:
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>>
>>
>>
>>
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> Be sure to visit the group home and check for new Bookmarks and files:
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