April 2002  SMT Magazine
Any money spent on inspection, test and repair is wasted money. It adds only to cost and, hence, adversely impacts the bottom line. It does not add any value. So why do we need to perform these non-value added functions? The reasons are simple. As long as humans are involved, and to err is only human, we are stuck with inspection, test and rework. My modest goal in this column is to focus on ways to reduce the total cost of inspection (namely automated optical inspection [AOI]), test and repair.

Of these three non-value added functions, AOI is the only function that can be performed at any point in the line. Test and repair functions generally are performed at the end of the line. Also, AOI and test are somewhat interchangeable. Both these functions accomplish the same goal ¾ finding faults, although their capabilities may vary and they may be performed expensively or cheaply.

AOI After Reflow. The reflow oven itself can cause defects. However, if you have taken the time to develop a unique profile for each product or product group, you can be sure that the reflow oven is not going to be your major concern. Also, AOI after reflow may have excessive false accepts and rejects. For example, it is difficult even for humans to decipher the subtle differences between cold solder, dewetting or non-wetting. How can an AOI system be expected to be accurate? Even if it is accurate, keep in mind that solder joint inspection after reflow is an after-the-fact step. It is too late to rework the defects you find at this stage. It not only adds to the total cost but adversely impacts solder joint reliability.