The focus information of each solder ball is used to determine {HEAD ~ the distance} to {TAIL ~ the tips} of each solder ball 
to
The focus information of each solder ball is used to determine { HEAD ~ the distance } to { TAIL ~ the tips } of each solder ball
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH