Teardown update! It turns out this logic board has a secret weapon: tiny seals surround each of the cable connectors. (They look like black foam hedges surrounding each of the gold connectors.) What can it mean?

Answer: we think those are waterproof silicone seals. They appear to match a patent Apple filed back in March for waterproofing board-to-board connectors.

When it comes to liquid damage, those cable connectors are among the most vulnerable parts of the phone.

This would seem to explain recent tests showing the 6s and 6s Plus to be dramatically more resistant to liquid damage.