thick layer not "think layer" ... my bad.
On Sep 6, 2018, at 12:41 PM, Yuan-chia Joyce Koo wrote:

> 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.