The PAF model, which is one of techniques about the cost of quality (CoQ), has been introduced to the head stack assembly process in the hard disk drive industry. Purpose to measure the total CoQ in the current process and use it as a tool to identify problems related to quality costs in order to find improvement plans. The total CoQ has been calculated in the percentage of the cost of quality per the cost of finished goods. Cost elements that impacted to the total CoQ have been prioritized and analyzed by using the Fish Bone diagram and the FMEA tool to find the potential causes of problems. The improvement plans have been implemented in order to reduce the chance of occurrences and improve detection ability. The result on two head stack models showed that the total CoQ could be reduced from 1.45% to 1.36% or 6.21% of improvement on the model A and reduced from 1.11% to 1.08% or 2.70% of improvement on the model B. The optimum points between the total CoQ and the quality level are 1.32% and 99.32% on the model A and 1.07% and 98.95% on the model B.