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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