Find the nearest time from another table and add column
This answer might contain some typing erros, and is untested, because i cannot test from a picture... 

SELECT
   AssemblyNo,
   SerialNo,
   AssemblyType,
   DefectType,
   BuildDate,
   BuildTime,
   x.Machine
FROM Table1 t1
CROSS APPLY (
    SELECT TOP 1 t2.Machine
    FROM Table2 t2
    WHERE t1.BuildDate=t2.WaveDate AND t2.Wavetime &lt;=t1.BuildTime) x