Hello Enrico,

I agree with Ed's comments about void area.  Joint failures due to voiding typically manifests as mechanical lifting or separation in the joint.   The area of the void can be related to overall joint strength.  

The folks who make X-ray systems talk a lot about the importance of void location.  If the voids are located at either the board or component interfaces, then joint strength might be compromised.   If the voids are in the center of the joint, then the joint strength might be acceptable.  

Best regards,

Tony Lentz

FCT Assembly
Field Application
[log in to unmask]
970-566-0360 Mobile

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ed Hare
Sent: Friday, May 8, 2015 7:56 AM
To: [log in to unmask]
Subject: Re: [TN] Voiding evaluation

I would suggest that the area criterion is appropriate since stress = load/area.  It is not a missing mas issue in my opinion, it is a reduction in load bearing area that is of concern.

Ed Hare
VP SEM Lab, Inc.
www.semlab.com

On Fri, May 8, 2015 at 5:15 AM, Enrico Galbiati <[log in to unmask]
> wrote:

> I would like to ask anyone if there are any reliability data regarding 
> the evaluation of voiding in a solder ball (BGA).
>
> In both the standards IPC-7095C and IPC-A-610F, the voiding in the 
> solder balls is evaluated measuring the area of the voids. However, 
> the weakening of balls caused by voiding should depend on the amount 
> of the missing material caused by the presence of voids. If this is 
> true, the amount of the missing material should be measured by the 
> total *volume* of voids, not by the area. Consequently, the limit 
> should be set on the volume, instead of the area.
>
> For example, with the present rule based on the percentage of area of 
> the voids, a solder ball of 0,85 mm diameter, with a single void of 
> 0,45 mm diameter, is acceptable, since the percentage of voiding is 
> 28%, thus less than the maximum limit of 30% (ref. IPC-A-610F). In 
> this case, the missing volume of the material is 15%.
>
> Considering another example, if a solder ball has 6 voids of a 0,20 mm 
> diameter each, giving 33% of the area of voiding, would be rejected.
> However, in this last case the percentage of the missing volume is 
> only 8%, i.e. less than the previous case (about 53% of the previous case!).
>
> So, the ball of the second case is rejected, even if it stronger that 
> the one of the first case. On the contrary, it is the solder ball of 
> the first case that should be rejected.
>
> Enrico
>
> --
> Enrico Galbiati
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Ed Hare
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