Steve:
As a vendor who prides himself on being honest and straightforward, and who has yet been roasted more than once...let me respond, at least from the fabricators point of view...
First, perhaps foremost, people use products, aka, chemicals, differently, and the different ways they use them, (and you cannot specify EVERYTHING) affects the outcome...
Secondly, and I understand this is hard for you to believe, sometimes, workers lie about what they have done, and will swear they have checked temperatures, times, etc, when in fact, they have done neither, thus rendering results of testing suspect.
Many times, I have given a product out for test, and it has worked well 3 times, and bombed once.....why? Often because the products being inputed to the chemical process are different....
For example, do you really think everybody plated 63/37 Tin/Lead at every point on the board, back when we were plating Tin/Lead? Well, when it got way out of range, it would not fuse/reflow at a given temperature...and it was always the fault of that $#%^&*@ fuse oil, which didn't do its job...
So, we need to match chemicals and process, and it ain't so standard...
IMHO...:-)
Rudy Sedlak