In my experience, the most commonly accepted theory is that 100% inspection is only 80% effective AT BEST. That means that if your inspoectors are happy, noy outside concerns, well-rested, etc, they will catch 80% of the defects. This number goes down very quickly when outside factors start to influence them. You can improve this number short of testing by incporporating good, effective process controls (including SPC)and preventive maintenance. Automated Inspection equipment can also catch many of these defects, including solder bridges, polarity, some wrong parts, missing, misaligned, and other solder defects. Incircuit test will cost about between $5K and $10K per fixture and program. Cost depends on complexity. This does not include the tester itself. ICT effectiveness depends on the test node access and board design. ICT can test some IC's functionally, catch many (not all) wrong/missing components or polarity issues, and some solder issues (unsoldered, bridging, etc), and components defects(defective components, out-of-tolerance, etc). I have seen boards with less than 50% test access, meaning that only half the components on the board were actually tested. What functional test will catch will depend on the level of testing. Some components may be set in redundant circuits, and unless each circuit is isolated and checked, one circuit could be bad and the board still pass test. Other defects may not show up unless you are doing ESS testing where the board is subjected to various environmental conditions. Keep in mind that such testing adds labor cost in addition to the NRE and maintenance costs. I do consider these value added though. If you have set up your processes and controls correctly, control your suppliers, and have an aggressive process control procedure inplace, you can eliminate many of your defects via a combination of your supplier controls, process controls, visual, and automated inspection. I say this because I have been part of a team effort that did it. We reduced our SMT and Wave Solder defects to less than 50ppm. ICT can help catch the remainder of component defects, such as defective or out-of tolerance components. Many EMS Customers are starting to drive more of the testing back to the EMS Supplier. They have more confidence and reduce their in-house costs when they received testing electronic assemblies.
Hope this helps. Feel free to e-mail me if you wan t more information.