For anyone wondering what these problems are about, here's an old post I made on the topic:
Many of today's chips have so many connectors, that they're placed underneath in what's known as a BGA (ball grid array). Here is the touch controller desoldered from its pad on the board:
Now, normally to prevent the balls from coming loose because of circuit board flex, thermal expansion, etc... a polymer called "underfill" is injected underneath.
This stabilizes the connection points.
In addition, in previous iPhones, there was a full metal shield that helped make the board more rigid:
But in the iPhone 6, BOTH the full shield AND the underfill were designed out
Those board design decisions, along with a comparatively weak case with a mechanical brace design mistake (Bendgate), allow the board to flex and the BGA solder joints to separate over time.
It's probably not even a question of IF it will happen, but more of WHEN it will happen. Thermal expansion alone could cause it sooner or later, much less any natural flexing during normal use.
I think what might have happened, was that Apple was planning to begin using spray top coatings that can double as a shield and a kind of overlayment brace, but had not quite gotten to that point for some reason. IIRC, some avionics makers have gone to this method.