Yep. Here's what happens. 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:
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Now, normally to prevent the balls from coming loose because of circuit board flex, thermal expansion, etc... a polymer called "underfill" is injected underneath.
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This stabilizes the connection points.
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In addition, in previous iPhones, there was a full metal shield that helped make the board more rigid:
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But in the iPhone 6, BOTH the full shield AND the underfill were designed out
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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.
Hmm. 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.