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
Consulenza Affidabilitą e Normative
Via Kennedy Ingresso 2, 20871 Vimercate (MB) - Italy
Desk: +39.039.8908.4547 - Fax: +39.039.8908.5051 - Mobile: +39.335 6833616
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