At the solder ball mounting process on the CSP (chip size package), must have inspection of solder ball residue on the vacuum nozzle (inhaling tool). This study is how to detect solder ball residue during vacuum nozzle moving when using laser through-beam sensor. First, create a slit on the laser receiver that increasing sensitivity of laser through-beam sensor and confirmed the improvement. The slit is created to increase sensor output value that compares result of solder ball residue on the vacuum nozzle with or without solder ball. Next, we supposition production line and study the vacuum nozzle movement error. Therefore when vacuum nozzle movement error is occur then measure sensor output value. Then we attempt to use first differential value. With this result, we discover following. (1) The solder ball recognition on the CSP can be detect by laser through-beam sensor. (2) We confirmed effectiveness of the slit on the laser receiver unit. (3) If vacuum nozzle has movement error, we can use first differential value to avoid influence of vacuum nozzle movement error to recognize the CSP solder ball.