This is the age old question on AOI. Fortunately, the answer hasn't changed much over the years. It really depends on your business...period. A high-mix low-to-mid volume manufacturer, will need to concentrate on equipment that may be progammed very easily, with very quick debug time from assembly to assembly. These attributes are typical of comparison based systems.
Higher volume manufacturers, we are talking tens of thousands of assemblies per production run, may opt for algorithm based systems. These systems are inherently more accurate than comparison based systems, but they are difficult to program and require more technical expertise at the helm.
We are a low volume high mix manufacturer. When we did our equipment evaluation, we looked at some of the higher end algorithm based machines, but determined that we simply did not have the time or resources to program these systems for our typical batch sizes. Our consultant steered us toward comparison based systems. In doing our research, we found that there were really only two vendors that were suitable for hig-mix low-to-mid volulme application: Yestech and Mirtec. We brought both systems in for evaluation.
We found that the Yestech programmed very easily, but the debug process took longer than expected, and there were several defects that were not caught right up front. This was alarming. We also ran the same boards through the machine several times and kept getting inconsistent results. The biggest advantage with Yestech was the price. They are by far the most inexpensive machine out there, but the performance was not the best.
The Mirtec machine also programmed very easily. My team and I thought that the Mirtec machine, however, was more simple to operate especially when reviewing defects. The debug time was also much faster and the machine caught every one of our defects on the first pass, as well as a cracked component which my team was not aware of. Lastly, when we ran the same boards through the machine, we got consistent inspection results as advertised. Mirtec is more expensive, but we felt that it was well worth the money based on the performance of the equipment.
Needles to say, we selected Mirtec. We now have six machines and our production quality has improved dramatically. The standard SPC software package has been a great tool in helping us resolve many of our process issues.
Hope this helps,