Dear Liqi,
in case you have a noteworthy difference between any two bond connections that involve identic materials, there is something wrong with your equipment.
Maybe there is a settling effect in the bond tool or the bond wire remaining for the first bond was damaged during setting up the machinery.
Generally, bond connections done with the same parameter set using the same partners to interconnect should have the same mechanical robustness.
"Strength and Time" may also be misleading parameters to measure as they do not hint out exactly, if the same energy was spent to create the interconnection.
In case the copper on the substrate was less hard in one position, the energy absorbed may have been different.
Hello Liqi,
In my opinion, there are 2 things we should think of.
1. There could be difference in the local temperature of the bonds. May be the local temperature and current density are high at the stitch bond area compared to other bond.
2. It is also possible that more per-damage could happen at the stitch bond since after bonding the first bond, the pulling force by the bonder at the stitch/first bond is more than the force that is applied at the last bond.
Regards,
Raju
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