the acceptable and reject should not be based on calculation.  It is  
based on the correlation between your reliability data to the  
observed X-ray image of the voiding - material/interface specific.   
majority of the voids are sphere, including the via in pad type.  the  
approximation with good reliability test data plus common sense  
safety margin would get you in a good engineering acceptable/reject  
area.  you always want to be conservative if you do not have  
redundancy - again, your design should know what is achievable of  
your MFG.  stay within the design margin would be the key.  if your  
MFG control  is not there - spc cpk is not good, you can forget void  
calculation.  Evaluation criterion for voids is not a practical  
problem.  the design/MFG fully understand what they are deal with in  
the margin is a problem... my 1.6 cents.
             jk
On May 13, 2015, at 3:52 PM, Enrico Galbiati wrote:

> I just have some clarifications.
>
> If voids are not spheres, both the volume criterion and area  
> criterion may fail. In fact, if the void is not a sphere, the area  
> can not be uniquely determined, because it depends on the direction  
> of viewing. So, if one wants to do any numeric calculation  
> considering areas or volumes, the assumption that the voids can be  
> considered spheres, with an acceptable error, should be done in any  
> case.
>
> In fact, the standard IPC-7095C (see figure 7-45 and table 7-7)  
> indicates the diameters of the voids, and a void can be  
> characterized by a diameter only if a spherical shape is considered  
> the actual shape or, at least, a good approximation of the actual  
> shape.
>
> People can use Xrays for the evaluation of volumes or areas exactly  
> in the same way. Assuming spherical voids (see above), in whatever  
> way we determine area or diameter, we can calculate the volume  
> univocally by a simply algebraic formula (no need of CT scan).
>
> The problem of comparative densities or contrast ratios affects the  
> Xrays measurements independently from the fact that we use the area  
> or the volume for the calculation (if we determine the area, the  
> volume is determined accordingly, and vice versa).
>
> The evaluation of the criterion for voids is a practical problem,  
> because based on this criterion we accept or reject, or simply  
> evaluate, the solder joints. Basing on a specific criterion, we  
> could be too conservative or too permissive.
>
> Obviously, the volumes, as well as the areas, are not the only  
> parameters to be considered in the evaluation of a solder joint,  
> but a good rule about it surely helps.
>
> Enrico
>
> Il 12/05/2015 16.50, Wayne Thayer ha scritto:
>>
>> Enrico-
>>
>> Ah, but the voids are often not spheres and the XRAY is not black  
>> & white! So your equation would need to also include comparative  
>> densities (contrast ratios) between the void and adjacent non-void  
>> regimes. Add to this that there is stuff internal to the board and  
>> on the opposite side of the board, and estimating area is about  
>> all you can do, unless you get a highly detailed CT scan!
>>
>> Typically, people bring up practical problems in this forum. Is  
>> this a practical problem? Can you share some snapshots of how the  
>> IPC guidelines are either too permissive or too conservative in  
>> certain situations? Basically, the IPC guidelines are in place to  
>> establish relatively easy-to-measure characteristics to serve as a  
>> basis for civilized discussion amongst the various stakeholders  
>> for an electronic assembly. Presently, the area of voiding is the  
>> standard we’re using. Lots of software packages do an automated  
>> calculation of this, and I’m not aware of practical problems with  
>> the results. Zero voids as measured with an XRAY does not mean  
>> there’s actual contact being made, so it’s not a perfect  
>> measurement. You can still have “head-in-pillow”, “champagne  
>> voids” too small to be picked up on the XRAY, brittle  
>> intermetallics, etc., which will either prevent connection  
>> entirely or substantially reduce useful life. But your suggested  
>> changes won’t help with any of these.
>>
>> Wayne
>>
>> *From:*Enrico Galbiati [mailto:[log in to unmask]]
>> *Sent:* Tuesday, May 12, 2015 10:12 AM
>> *To:* TechNet E-Mail Forum; Wayne Thayer
>> *Subject:* Re: [TN] Voiding evaluation
>>
>> If you are considering the missing mass (and I agree on it), the  
>> volume of the voids is more appropriate.
>>
>> Also the evaluation of the volume of the voids can be done easily  
>> with the X-rays: you just sum the void diameters raised to power 3  
>> (instead of power 2 as in case of the evaluation of the area) and  
>> divide the result by the solder joint diameter raised to power 3 too.
>>
>> So the evaluation of the volume is not more complicated than the  
>> evaluation of the area.
>>
>
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> Enrico Galbiati
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