Interesting discussion on the definition of solder-NiAu failures. One I am often 
confronted with. For what it's worth, my idea of looking at Black Pad and 
Brittle Fracture at the SnNi IMC-Ni interface.
To my opinion the distinction between both is clear when looking at the root 
cause. It is much less clear if one looks at it from the end result: failing SnNi 
IMC-Ni interface.

Brittle fracture:
It seems that the SnNi IMC-Ni interface is intrinsically weak (whatever the 
concentration of P even for P=0%), at least compared to the SnCu IMC-Cu 
interface. (Does anybody know why? - Never seen a BF between the SnCuIMC-
Cu interface.)
The higher the concentration of P in the Ni, the weaker the interface. P 
detoriates the already weak interface further. (always compared to Cu).
Brittle fracture is not limited to ENIG NiAu

Black pad:
Black pad is related to an immersion Au process problem in which the Au 
immersion bath attacks the electroless Ni layer. The higher the P bulk 
concentration in the electroless Ni layer the lower the probability of Black Pad.
There are specific signs for Black Pad (according to this definition) which 
makes it possible to distinguish it from pure Brittle Fracture:
- rough Ni surface, "mud-crack"-like look, reduction of Ni thickness along grain 
boundaries; (BF will show a smooth surface)
- Strong P accumulation at the surface but a lower than typical P 
concentration (7-9%)in the "bulk" of the Ni layer.
- Unsoldered PCB: A thicker than typical Au layer. Au is more rapidly deposited 
under Black Pad conditions which is linked to the high P accumulation seen at 
the interface.
- Soldered PCB: very little or no IMC. Solder does not solder to the corroded Ni.
Therefore, with Black Pad you may just pop-off the components if it is wide 
spread. For BF you will need a bit more force although hitting a heat sink on 
top of a BGA can be enough.

Off course, a low degree of Black Pad can hardly be distinguished from BF. In 
that case it is just another interface detoriating phenomenon aggravating BF.

Interested to read your comments.

Geert Willems
IMEC

 

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