After finding a batch of Package-on-Package components with a ~20% defect rate (that’s not a typo), I asked the vendor to have the lot of components replaced.
At first replacing the whole order was met with opposition, there were still some components unused that met visual inspection. But how do you use these components from a faulty lot without suspecting possible problems? Just because the components have spheres on them, it does not mean they have been properly attached.
These parts were eventually replaced, but should serve as a lesson. Do not pick out good parts from a suspect lot – it’s just not worth the questionable results you could have during testing.