Aileen - take a big step backwards and think about what you are controlling here. Your ball shear test assures your solder ball attach process.
Go back and look at the amount of failures, you've had and I'll bet you never had any or if it was, it was special cause such as units that have a fine solder ball pitch. Even if your spec limit is 10gF, I bet your min readings are 2x your spec limit.
Your ball attach process is going to be influenced by your reflow process, your substrate lot and solder ball lot. Your substrate lot should come with a CoC (Certificate of Conformance) that assures pad finish, pad plating thickness, warpage etc, Your solder ball attach profile should be under SPC control and your solder ball supplier should also assure the ball diameter and alloy %'s.
Solder ball shear per assembly lot has redundancy especially as 10 - 30 assembly lots will all use the same supplier substrate lot and go through the the same reflow process + solder ball type. Thus, already you can start to reduce the amount of sampling by doing it on a per substrate lot basis instead of a per assembly lot run.
Carry on going with your research and I would be sure that if the package technology is mature and has a good track record of ball off, you can group this by BGA family / substrate supplier and you can start to get coverage by smarter sampling on a family basis. - device x,y and z that use same substrat supplier and design rule can be grouped together. It is quite acceptable to control SBS (Solder ball shear test) on a package family basis where SBS is sampled per day, per shift or whatever.
Be aware in too much generality as there will be some devices more susceptable than others - For example, larger BGAs could be susceptable to warpage and cause a ball off issue.
You need to pay attention to solder ball - SnPb solder balls have lower solder temperatures than SAC solder ball types. Even SAC balls can have different amounts of silver in them that could require slightly different solder profiles. Pitch of the solder ball is also important - smaller ones could be more susceptable to ball off.
Plan into your sampling special cases such as low volume runner devices where your substrates may be old. If not controlled, the finish could be oxidized that could result in sub-standard ball adhesion. You may get around this if you have plasma clean but if not, old substrates should be deemed as risky if not stored appropriately.
Plan a SBS test if you shut your reflow oven down for maintenance or if you are changing it, a repeat of SBS on a larger basis.
Finally, if you are reducing blind SBS testing to a family control basis, I strongly recommened you consider SBS test as a reaction plan to your final Visual inspection. If you are seeing unacceptable amount of ball off at VM or other logpoints such as test, then it could be that your have damaged them at a back end of the lines operation that use connectors to the solder balls. Even doing a SBS in your line may not catch this as your SBS test could be done while your BGAs are still in strip format.
In summary, you have excellent opportunity to move to a predictive sampling instead of random sampling especially as in most cases, the solder ball strength will be assured earlier steps.
