@Cedric Yes, the voltage regulator is reused, soldered to the battery tabs, and retaped between the batteries. That is where the cable is coming from.
Oops, I stand corrected. There isn’t necessarily any electrical contact through the legs of their snap domes. It’s the center four bumps that are the electrical contacts, the upper two that have minimal sliding force.
When the snap dome switches my company was using started to fail well before our 3 million cycle test goal, it turned out to be a couple of problems. We weren’t using a hard gold (alloy) plating on our PCB and the snap domes, instead of being the usual round shape, had legs much wider than the Apple snap domes.
The sharp corners of the legs on our snap domes wore through the gold plating, and the stainless/copper interface corroded so that we lost electrical contact.
Maybe the Apple snap dome legs scrape along the gold surfaces and either kick up contaminants that prevent electrical contact in the middle, or eventually wear through the gold and cause corrosion at the leg contact points. The corrosion only has to occur at the very end of the button travel (legs splayed as much as possible) to prevent electrical contact.
If we assume they’re pretty competent people at Apple, it would be easy enough for them to diagnose that and fix those corrosion and wear issues. So maybe it’s not that.
I’m not sure you have to remove the black screws, I think they hold the camera lens holder to the motherboard.
The only way I've been able to remove the screen is to work from the other side.
Work some isopropyl alcohol through the battery compartment, LCD cable slot, camera mount, and headphone jack area. Flex the glass back and forth to suck the IPA in there, then heat up the aluminum frame to loosen the adhesive holding the OLED panel to the frame. Usually I start from the edge closest to the battery and buttons. As soon as there's a slight gap, work in a pry tool to break the adhesive, add a bit more IPA then heat, and work your way around to the top and bottom, then the other edge just levers off.
My rationale for starting from that side is that you're heating the adhesive holding the OLED to the frame more than you're heating the adhesive holding the glass to the OLED. It still shouldn't get too hot to handle with rubber/nitrile gloves on.
I've done this with an 80+% success rate on more than a dozen phones with only two destroyed screens, maybe three, but I wasn't trying to save that last one.
Step 20 first bullet: I assume you mean that the frame was heatstaked to the front bezel.