What is the name of the small circuit board above the trackpad that the flex cable connects to on the MacBook pro retina 2015 13"?
There are two places where the track pad connects - the logic board (red) arrow and to the track pad PCB (red square)


Image Source:  Ifixit.com

If what you're referring to is what's identified by the red square at the bottom of the image above, then it's the PCB. It's  not sold as a separate assembly (AFAIK) and you have to replace the entire track pad assembly (PN# 923-00518). If it's the top portion (red arrow), you have to get the logic board repaired/replaced.

It's possible you could buy a salvaged track pad from eBay just to get the PCB, but it's an "unknown."  You could end up getting a track pad with a bad PCB again, but if the price is good, it may be worth the gamble. 

Finally, if you got a spark when connecting the flex cable, replace the flex cable as well.  When you see a spark, that means enough voltage passed through the cable to arc which could have permanently damaged the cable - it's cheap enough that you should just do it anyway and not try to salvage a cable of questionable function.