I see, so assuming the SoC is good (that the heat wasn’t temporarily resolving a die issue or something) and it was indeed joint related issues, then I suppose we have to assume it’s a board level defect either as a result of board warp which is causing the balls to detach or not making proper contatct with the board following reflow or that there’s pad/via issues on the board… think the best course of action would be to transfer the SoC and EMMC to another board just to take it out of the equation, unless of course you pull the SoC and can positively identify something along the lines of a detached or soft pad/s etc and conduct the repairs.
Best advice I can give on this is to practice reballing on your other dead SoC, repeat reballing it over and over until it’s perfect every single time, practice makes perfect
Don’t use solder paste, the chip is too big for it and it never reliably works, even with those wanky magnetic holders Instead use a dedicated 80x80 or 90x90 stencil with the proper jig (the bronze coloured ones are perfect for this)
When removing the SoC from the board, preheat the opposing side of the board first with your hot air (everywhere) at 150C then step it up to 200/250C for a min or two, then flip the board and add flux around the edge of the SoC while continuing to heat the SoC at 200/250C, when the flux is applied crank your temp up to approx 420C medium air, run your air around the wafer and don’t concentrate on the die itself, nudge the edge of the wafer to ensure the chips is fully molten below and when it is use one of those suction pens to lift the chip, while the board is still hot, tin then wick the pads on the board, then do the same on the SoC, if your iron is struggling on the board have your hot air assist you, try not to remove any solder mask while wicking otherwise this could hinder you when it comes time to put the SoC back on (if you decide to re-use this board)
After chip removal take a close look at the resin/epoxy around the die, if it’s a greyish/bluish colour your good, if it’s a nasty looking brown then the chip is almost guaranteed to be dead.
On the SoC itself, after the pads have been wicked and then cleaned, add a thin film of flux, I usually just use a cotton swab like a paintbrush, it has to be just the right amount, which you’ll only know with practice, too much flux and you’ll cake your stencil up and the balls will not roll into the holes easily or get stuck in the stencil when you remove the upper portion of the jig, too little and the balls will piss off on you when it comes time to reflow them.
when the balls have all been swirled into place with the jig, I tilt the whole jig at a 45 and gently brush over the stencil with a dry cotton swab, this knocks out any double balls etc, then you want to closely look at the stencil to make sure none are missing, after remove the top part of the jig with the stencil, lift out the chip carefully, take another close look and make sure it’s not missing any balls, if it is then repopulate them manually, reflow the balls at a lower temp and minimum aireflow, I use 320C
After it’s similar to the removal, I preheat the opposing side upto 200/250C, add a thin layer of flux to the SoC pads, align the chip , crank the temp up to about 420C and reflow it into place
Hope that helps