Hi Bev - Guenter's explanation is much better than I was going to put
together and is really good way to think of the topic.

Dave

On Fri, Jan 10, 2020 at 9:20 AM Bev Christian <[log in to unmask]>
wrote:

> Gunter, thank you, as always.
> Bev
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of Grossmann, Günter
> Sent: Friday, January 10, 2020 9:29 AM
> To: [log in to unmask]
> Subject: [TN] AW: [TN] [External] [TN] Need interpretation
>
> 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
>