U2 ic replacement issue

The U2 replacement probably needs to be done again.

Here a couple of things to look out for when doing micro BGA replacements. You may have already applied all or most of these best practices but just in case...:

• Set your hot air to ~220C to soften up any underfill (black epoxy)
• Carefully pick away at it to remove it. Be careful not to remove any of the adjacent passive components (capacitors etc)
• Apply flux and heat ~380C to remove the IC. Don't pull or push it with any force until you can see it floating on the melted solder (a slight nudge to see if it moves)
• As much as possible, lift the IC as opposed to pulling it sideways
• Clean the pads of any lead-free solder and remove any leftover underfill
• Clean with IPA and inspect under magnification to see if any traces, pads or components were damaged or removed
• Apply fresh flux and set the new IC...apply 380C heat. When the solder has melted, the IC should float and properly position itself.
• Clean and test

Obviously, too much heat can cause damage not only to the IC you are replacing but also the surrounding area and the flip side of the logic board. Sometimes passives get disconnected electrically even though they look okay. Those that are encapsulated in underfill are more prone to that.

Also, when testing for "float", we can sometimes nudge the IC too far and cause a solder bridge underneath the IC, shorting out pins that can cause all kinds of problems.

So in essence, I would remove Tristar and start again with a new one.