Why do you find this difficult? I suppose you find it difficult to dose the laser due to the different heat output of different pads?
You have no clue about lasers (I have $3k CO
2 cutter/engraver) and they are not accurate at all. It's a one thing when you need to burn through, and just use more power than actually needed for some margin. And normally used on homogenous materials. Engraving as well, there is not much difference if you deliver 20% more or less power. Once you need to deliver exact amount of power, it becomes a big problem. I've played enough with removing material to certain depth, even made countersunk holes. It's a black magic and pain in the ass. Each pad will be different. Even if pads are the same by themselves, traces connected to them are not. Imagine as if you are using a soldering iron with 2000
oC tip temperature, and need to find exact time for each solder joint so it does not burn. Let's assume you spent a zillion of man-hours and hundred of destroyed PCBs to calibrate each pad. When you receive the next batch of PCBs and especially if from different manufacturer, likely it will not be the same. Nor laser will keep it's power exactly as it was before, unless it's some weak semiconductor type operated in stable conditions.
Even a simple toy laser is well dosed and makes a fairly accurate burning on wood or leather.
If your intent is burning PCBs, it can be done easily. BTW there is not much use from a few W lasers for soldering, too weak.