Chosen Solution

One day, I tried to upgrade the SSD by removing the LCD display, but unfortunately the display connector was broken and falling off, so I bought a new connector, and try to soldering it back to logic board (disassemble the logic board and do the soldering job), after it is done, assemble the logic board with all wires connected and turn on the iMac, but no image shows on screen, I then tried to connect my iMac with an external monitor, it works fine, which means the connector may not be soldered correctly or properly, so, I performed the soldering process again (but this time, I did have the logic board stayed with iMac with all wires connected, and of course the power cable is disconnected), but after that, the iMac won’t power up at all, nothing happened by pressing the power button, I tried all possible solutions such as disconnect the power cable for 10 seconds, then put it back, or remove the RAM, etc. but no luck. Does it indicate the logic board is gone completely? Or is there some trick I should try? Any thoughts? Thanks, Update (02/28/2020)

The issue you describe sounds like a P.O.S.T. failure. When you first start a computer whether it’s a smartphone or a super computer and everything in between runs a Power On Self Test. This is the first program that every computer runs before the operating system is loaded. If there is a failure the computer will not boot. The P.O.S.T. is controlled by the BIOS. Chance are your BIOS cant’ see the drive. The easy way to resolve this issue is to boot into your BIOS set-up utility and change your boot order manually.

@hru1207 - It does sound like you have a bridged solder joint on the connector. Let’s take it off again and check the solder pads. Take a few pics what you have now and again after removal (cleaned) Adding images to an existing question Also, locate the onboard diagnostic LED’s hidden under the tape

@hru1207 remove the connector once more. You have too many solder bridges and looks like poor connections on some contacts. Clean the pads with flux and solder-braid. After that try to solder the connector with a technique called drag soldering to attach a new connector. Right now every attempt to start your computer runs a chance of shortening power-rails and creating more damage