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.
>
--
Enrico Galbiati
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