Inspection & Test of electronic boards - concepts & strategies
Today typical elements of a quality system for electronic manufacturing are: manual or automatically optical inspection, in-circuit test , functional test and post-sale services - related to warranty and customer claims.
Manual optical inspection of populated electronic board (PCB) - done using operators and microscopes is a flexible& cheap method used especially in East Europe, Pacific and Latin America. It has a disadvantage that only 50%-80% of errors are detected and the small components (eg: 0201,01005) are very difficult to detect. Then the errors escape in the next production phase and create unnecessary rework.
The alternative is 3D automatically optical inspection (AOI). It uses high resolution cameras and multiple light sources to acquire components and solder images and to detect: missing components, wrong placement, solder quality problems (volume, shape, shorts, opens). Also, the machine is doing OCR (optical character recognition) very useful for reading the components codes and barcodes. The most important thing is that using this machine you can control the process of paste printing, components placement and soldering process.
Using the feedback of 3D AOI you can have a process traceability and you can improve your technological process. ALFA TEST is the distributor in Romania and Serbia for 3D Orbotech (www.orbotech.com) machines.
In circuit test is an electrical test on component level done using a direct access from internal stimulus and measurement system to electrical nodes of electronic boards. The advantage is that the electrical diagnosis is done on component level ad we can detect shorts, opens, missing components, out of tolerance analog & digital components. Additionally we can measure frequencies/times, we can do/verify device programming and power up tests.
1st type is an in-circuit test with standard matrix interface. The contact between the tester interface and PCB is done using a test adapter (fixture) specific for every PCB . The advantage of this ICT is the fastest test time (eg. 40s time for a board with 800 electric nodes) and the wide range of testing capabilities, especially when we are talking about digital test using multiple logic levels.
Alfa Test is doing in East Europe the hardware support for Teradyne ICT (www.teradyne.com), the worldwide leader in in-circuit test. In East Europe we have more then 160 Teradyne ICTs installed base.
2nd type - Flying probe ICT is using for measuring and stimulus 4-6 mobiles probes that are moved by precise motor drivers on X, Y, Z axes very fast with 30 um combined precision. Then you can access very narrow spaces on electrical node, SMD pins, vias paths, things that are not possible sometimes using an ICT with standard fixture. As the design of PCB has sometimes testability access issues or if on the density of the components on the PCB is very high the flying probe ICT is the only solution to test electrically the components. In the case of low volumes production or if the design of the board is changed often, the flying probe tester is the most efficient test solution. ALFA TEST SRL is the distributor in Balkans for the market leader TAKAYA flying probe tester.
ICT solutions can be combined with Boundary Scan (JTAG) test. Boundary scan it is an innovative stand-alone solution that allow to test a group of ICs using a serial shift register interconnected internally with the IC pins. This test can be done on the ICs that are designed according with IEEE 1149.1 standard. ALFA TEST is the distributor in Romania, Serbia and Hungary of XJTAG (www.xjtag.com) boundary scan tools.
Functional test is a stage where we verify the input/output parameters of the boards in standard working conditions or/and under stress conditions. In that phase the testing time is longer and the functional test platform is custom designed, expensive and seldom it can be re-used for other products.
Having in the mind the main methods of testing the populated PCB boards there are some conclusions. Even if the inspection methods have some crossing points , if the error are detected in the earlier process phase the costs are lower. If we consider an error that is detected during paste printing like 1x cost, if the error related to the same component is detected after optical inspection the cost is 10x, after in-circuit test is 100x and if the complete product came from customer site like claim the cost can top 1000x initial repair cost.
CAD conversion, testability reports, fixture design and files for test and inspection machines can be generated using Assembly & Test Expert software, a powerful software tool. Alfa Test is distributing in Balkans Tecnomatix electronic tools for test & inspection.
Then, knowing what type of technology can be used in specific conditions a very good strategy for test & inspection will minimize the production costs and will increase the quality of the products in a very efficient manner.