I was searching for answers over days to this because I have had to re-glue my screen on an Xperia Z3 twice now, and tore the flex in the process, so I needed a whole new screen. There are very few numbers to go around and I wanted to put this here so anyone googling for answers will get an idea with numbers.
0) Screen and digitizer flexes seem to get thinner and thinner and over time get brittle, mine was weaker than paper when it tore.
1) Remove all traces of the old glue and make sure to prep surfaces with isopropyl alcohol (I use 99.9%). With a microfibre (lens) cloth. A pencil eraser cut to size helps with this but I use a pin moving in one direction like a chisel.
2) Pre-cut black adhesive seems to be 0.2mm thick.
3) Red tape is 0.2mm according to the place I bought it from and is clear adhesive with a red backing you peel away. I measured it with my calibrated vernier calipers, and it is 0.2mm thick with a 0.1mm backing thickness (that is peeled away).
4) The tape I bought did not claim to be 3M, it was from ebay. I measured the strength of 1mm wide tape at room temperature. I put a 16cm (~6”) length on a stainless steel ruler, and pressed it down to an aluminium plate of a heat plate (turned off). It took about 2kg (4.4lbs) of force to lift off the plate plus or minus 0.5kg (1.1 lb). I had pressed the two together for 30 seconds to ensure a bond and left a 0.4kg (0.9lb) weight on it for 3 minutes.
5) I repeated the above with 55 degree C heat (131F) for 3 minutes. I tried lifting it off and it took about 3.5kg of weight plus or minus 0.5kg. My force measurement was not very accurate, but I was not doing a lab test here.
6) So at this temperature and amount of time the strength of the glue is better by 3.5/2 = 1.75 times. 75% increase in strength is worth it to me. It translates to 31psi of strength.
7) You can try other temperatures and timing to see what works for you. I was happy with this.
8) When I used pre-cut adhesive I heated the phone up to 85 degrees C four times (for each side of the adhesive for the front and back of the phone) and it lifted within days. I was worried about damaging the screen but this did not happen.
9) Another way of looking at it is the total force the perimeter of the phone would take to lift and it works out to be 9kg (19.8lbs).
10) These repairs are not waterproof, but if your edges between glue strips is very close, it’s probably rain resistant. You can try a bead of b7000 glue in the crevices to make it more waterproof, but I haven’t tried this yet.
P.S. Please be careful when ordering screens online that cannot prove they are OEM or compatible in all dimensions. I had to return a screen that was 0.2mm too thick. This means a lot of wasted time for something that has a screen sticking out that will get dirty and chipped and if you do this for a living, it will attract refunds. I’d say measure the screen thickness first with calipers as well as your glue thickness. Compare this to the recess in the frame. They should match!