As I understand it, it does not make a lot of sense to attribute the growth of cracks along the pad to the orientation of the tin unit cell. It is true that in lead free solder primary tin dendrites grow, preferably along the colder interfaces such as the pad or the connector of the components. As these dendrites are monocrystalline after formation they do not allow for grain movement to relax occurring stress with strain. This means that between the copper of the PCB and an adjacent tin dendrite high stress builds up. The rest of the solder joint is to some extend shielded because the dendrite close to the copper or the next one in direction to the solder volume bear all the stress. This can lead to an immediate crack along the Cu- Sn interface or, in thermal cycles, the dendrite is deformed by dislocation creep which is quite damaging thus concentrating the zone where damage occurs to the dendrite. As the number of cycles increases the dendrites close to the copper will recrystallize, allowing the strain to reach the volume of the solder joint. The path the developing crack takes after this point is stochastic because it depends on several influencing factors.



Have a nice Weekend



Günter 







-----Ursprüngliche Nachricht-----

Von: TechNet <[log in to unmask]> Im Auftrag von Bev Christian

Gesendet: Freitag, 10. Januar 2020 14:46

An: [log in to unmask]

Betreff: Re: [TN] [External] [TN] Need interpretation



Dave,



Here below is the sentence before and after. I don’t think the context helps at all.  But let me take a crack at interpretation myself, now that I have had a good night’s sleep.  Let’s assume the person is talking about the solder joint portion near the PCB pad. Laterally the tin-based solder is “seeing” the copper pad, which has a different CTE than the solder.  Vertically the solder is just seeing more solder which has the same CTE, so if the joint is going to crack, it is going to crack parallel to the PCB pad.  Do I, MOI make sense?  How would you rewrite the underlined sentence in question in quotes?



 



“For the reflow oven temperature profile, the thermomechanical response of the Sn content of the SAC305 alloy was considered.  As Sn is anisotropic, the size and orientation of the grain structure contributes to CTE mismatch fractures along the larger axis of a solder joint, laterally rather than vertically.  The grain of the Sn present in the solder joint grows during the reflow and continues to grow during the cooling.”



 



Regards,



Bev



 



From: David Hillman <[log in to unmask]>

Sent: Friday, January 10, 2020 8:27 AM

To: TechNet E-Mail Forum <[log in to unmask]>; [log in to unmask]

Subject: Re: [External] [TN] Need interpretation



 



Hi Bev - the statement standing alone is out of context so it could mean a bunch of things. You would need to know the solder joint configuration and the what the X/Y/Z stress states to have the sentence make sense. I am assuming that the report from which the statement was taken would provide that information.



 



Dave Hillman



Collins Aerospace



[log in to unmask] <mailto:[log in to unmask]> 



 



On Thu, Jan 9, 2020 at 7:50 PM Bev Christian <[log in to unmask] <mailto:[log in to unmask]> > wrote:



TechNetters,







Can someone interpret this sentence for me?  



"As Sn is anisotropic, the size and orientation of the grain structure contributes to CTE mismatch fractures along the larger axis of a solder joint, laterally rather than vertically."







I know the following:



1.      Sn is anisotropic

2.      the size and orientation of the grain structure contributes to CTE

mismatch fractures



Is the latter part (.along the larger axis of a solder joint, laterally rather than vertically.) always true and if so, why?  What am I missing?







Regards,



Bev