excessive IMC usually related to repeated reflow, such as excessive  
rework (both overall time above TL and multiple time rework).  If  
your design not allow (good design  dictate no of reflow allowed) X  
times reflow, you would not see the failure caused by IMC.  In  
addition, if the Ni layer is thin  - usually due to price of Ni, you  
have limited IMC - you will  see non-wetting oxide formation for  
repeated rework  before you can form proper joints - usually is a  
reject for further rework -therefore, you have limited chances see  
failure in the field due to excessive think layer of IMC... my 2  
cents... believe me, if condition is correct, thick  IMC do have  
problem in field failure, although is rare...
jk
On Sep 6, 2018, at 9:37 AM, Wayne Showers wrote:

> IMC has been the bane of my existence more than once.  So here is  
> my take from experience backed up with some lab evidence.
> I have had failures attributable to insufficient IMC.  Almost all  
> of these were corrected by either ramp-to-peak or increased time  
> above liquidus / intimate contact time / temperature.
> For SAC, the intimate contact temperature I have been able to dial  
> in on is time above 208C after liquidus is achieved.
> For SnPb, the intimate contact temperature I have been able to dial  
> in on is time above 177C after liquidus is achieved.
> I have found that if I can back end a little more time above these  
> temperatures on the profile, I have more consistent joints.  I have  
> not devoted a whole lot of 'science' on these numbers just 20+  
> years of trial and error coupled with white papers, IPC BOK, and  
> other research.
>
> I have yet to have a problem or a return tied to excessive IMC.  I  
> am sure that at some point excessive IMC may cause or have caused a  
> failure mode, I just have not seen it.  The few times where it may  
> have been a contributing cause, I also cooked the parts making  
> cross-sectioning of the IMC moot.