The photochemical machining industry takes into account the problems with etchant puddling so they can make square or even acute angles come out like the drawings.  They modify the artwork to increase the "depth" of the etchant cut into square or acute corners.  I have never seen printed circuit shops go out of their way to modifiy artwork for bends or in "corners".  Each photochemical machining shop has its own production rules depending on the etchant, the spray characteristics of the etch machine, the overhang of the photoresist during the etch process, etc.  

The link to their industry association is 

 http://pcmi.org/

You can learn a lot about etching, artwork compensation, and spray processing of flat sheets from these people.  They have been lurking behind the printed circuit board fabrication industry for 40 years - same exposure machines, same etch machines, same problems with thin core transport, etc.  

Denny Fritz
 
-----Original Message-----
From: Brian Ellis <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Thu, Apr 23, 2015 11:38 am
Subject: Re: [TN] Teardrops (Cu trace expansion) on thin traces - needed for OSP?


I don't believe that it is an urban myth. I thought that I had explored 
this
in my book, but I may be mistaken as I cannot find it! I believe 
that the
problem is due to the capillary action retaining etchant in the 
acute angle.
The minute quantity of etchant would quickly become 
exhausted, possibly
leaving poorly soluble precipitates in the "corner" 
as the pH dropped
(alkaline etchant) or increased (acid etchant). 
However, this explanation is
possibly speculative.

On 23/04/2015 18:01, Wayne Thayer wrote:
> Is that
crevice corrosion during board fab, or during service life? For years,
> I've
been wondering what the "acute angle" avoidance issue was and where it
> came
from. Every time I put in an acute angle, it gets under-etched because
> the
photomask makes it hard to get etchant into the tip of the notch, so I've
>
been ignoring that rule as either an anachronism from some etching process
>
used in the 60's or just an urban myth.
>
> -----Original Message-----
>
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
> Sent:
Thursday, April 23, 2015 10:48 AM
> To: [log in to unmask]
> Subject: Re: [TN]
Teardrops (Cu trace expansion) on thin traces - needed for
> OSP?
>
> Hi
folks - the problem isn't so much a galvanic corrosion issue but really a
>
crevice corrosion issue coupled with plating bath parameter control. The NASA
>
DoD Lead-free Solder Consortia experienced this issue on a number of its test
>
vehicles. The problem primarily impacts plated thru hole technology but has
>
been also observed with surface mount technology. Using teardrop shaped pads
>
gives the board fabricator a bit more process robustness. I haven't seen much
>
publication of this issue yet in the public domain but it is a known issue in
>
the industry for folks using immersion silver surface finishes. Details on
>
this issue were documented in the NASA DoD Lead-free Solder Consortia final
>
report.
>
>
>
> Dave Hillman
> Rockwell Collins
>
[log in to unmask]
>
> On Thu, Apr 23, 2015 at 9:29 AM, Brian
Ellis <[log in to unmask]> wrote:
>
>> The likelihood of electrolytic corrosion on
a silver/copper couple is
>> really negligible, because the EMF difference is
only 0.485 V,
>> referred to hydrogen equals 0 V. This is insufficient to
dissociate most
>> ions.
>>
>> On 23/04/2015 15:59, MacFadden, Todd
wrote:
>>
>>> Hello Technet friends,
>>>
>>> We are usually asked by our PCB
suppliers to add teardrops (track
>>> expansion) to thin traces (<=5mil) at
soldermask openings. We
>>> understand the impetus for this on immersion silver
boards, where
>>> there is a risk of galvanic corrosion due to Cu-Ag couple at
the
>>> soldermask/Cu interface of SMT pads.
>>>
>>> But some suppliers also
ask for track expansion on OSP boards. What
>>> would be the motivation in this
case? My understanding is the risk of
>>> corrosion at the soldermask interface
on OSP boards is low, even if
>>> the soldermask undercut is severe. So is
there perhaps some other
>>> reliability advantage to having wider copper at
trace/pad interfaces
>>> on otherwise thin traces? Do others get this question
as well?
>>>
>>> Thanks in advance for any thoughts or insight.
>>>
>>> Todd
MacFadden
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
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