First, any results of pull or shear tests are unscientific at best. [We pop our BGA from boards with, appropriately enough, a beverage can opener.]
Second, we have no have problems with your ENIG specification.
Third, as with your customer, we'd expect foil tearing and board-based distruction when popping BGA [or any other part] from the board. Breaking at the nickel tin intermetallic is not necessarily a bad thing. While we have no ENIG boards, in that case, we: * Don't expect to see a flat nickel plane for nothing. * Expect a rough solder surface attached to the pad.
So, let's assume that your customer is seeing nickel pads with no [or little] solder. Failure of this type does not have to be hypercorroded nickel [black pad]. From a board fab stand-point, oxidized nickel could result in similar problems. * Could the gold be pourous or unevenly plated for any reason? * Could the nickel have sat around for a while prior to immersion gold? * What are the results of the customer's in-bound solderability test of the this lot of bare boards?
Finaly, OK. OK. Let's give the poor fab a break. [Everyone blames the fab. They're just such easy targets.] You only mentioned the BGA popping from the board. * If this is a fab problem, other components should be just as poorly attached. * What if your customer [or their BGA suppler] changed their process? Maybe the alloy of solder between the paste and the BGA solder balls has changed and it's not as strong as it used to be.
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