I know this thread is a little bit old but I'll try to give in my 2 cents maybe someone here may find it helpful
So I come from background where I do around 20 bga reballs per week, so I do know a thing or two I guess about this although my knowledge on Samsung platform is relatively low compared to an iphone logic.
So to begin with replacing the emmc chip alone is not enough as you'll need a programmer box which connects to a jtag interface which is able to rewrite the initial files like bootrom to the emmc. You can find these boxes at any prominent gsm repair shops; boxes named such as RiffBox or Z3X Samsung box are the best I found recently.
Having said that before any repair is attempted by mainly removing the flash chip it is imperative to try to resurrect the phone using these said boxes, to try to find whether or not the NAND chip is actually detected. As one may have simply installed a ROM which is not compatible with the phone and all that is required is to rewrite the bootrom files. If the NAND (basically the same name as a flash chip) fails to be detected then obviously something went wrong and it either could be the NAND is burnt inside, or the NAND has some cracks under its critical ball pins or even may be a problem that the main power management chip inside the phone is failing from supplying usually around 3V to power up the NAND.
The emmc chip at least found in a samsung is a 14 by 14 pins which only about 1/3 of it's pins are critical, the rest are dummy and do not worry if they eventually get removed, while removing the chip or cleaning the board after desoldering prior installing the new chip.
Some tips on reworking:
- Always cover critical glued components like CPU + POP (package on package) RAM, baseband processor usually XGOLD found in Samsung.
- Clean surrounding chip glue before attempting to remove by giving around 250C of heat and with a needle scratching the glue around
- Do not exceed more than 350C to remove the actual chip to prevent more damage to the built in tracks inside the motherboard.
Last and not least a schematic for your phone would always be a lot of help to help you detect what voltages are missing on bootup to make sure that the boot up sequence is starting fine and also the relative points of each pin under a chip while knowing which pins are critical and which are dummies, or NC (not connected)
If you need any help you can always message me and I'll try my best to answer your questions.