Building a Manufacturing Test for the system which ensures quality and performance of the system is very critical and the pivotal task for every product manufacturer. In case of bulk manufacturing, the test strategies and component coverages are very important to ensure adherence to industry standards as well as delivery of good quality products to the customers.
Today’s manufacturing tests involve very complex strategies managing multiple devices in parallel. Design of an effective test equipment is cardinal in terms of ensuring the system to be uniform in quality and robust, rugged and highly reliable with respect to frequent use in lifespan or serviceability of the product. Adjunct requirement is to provide uninterrupted service, which ensures any undesired interruption in manufacturing process. While in operation in field, the manufacturing test equipment requires timely re-checking and re-validation to ensure quality, reliability and performance meet specified standards and if not, the equipment are needed be repaired or replaced with a qualified one.
Before shipping a product, it undergoes numerous testing process to verify its intended functionalities. Each printed circuit board assembled with components undergoes physical examinations (AOI or Flying Probe Test) to ensure assembly of correct components in correct orientation. Following the board level diagnostics and test routines, the qualified units are tested for hardware functionality and system level integration. There are various test strategies to be considered for these.
In Circuit Test (ICT) performs electrical probe tests on an assembled printed circuit board (PCB). In this manner electrical signals are directly applied to the on-board components or device terminals and the output responses are recorded. Generally, ICT is performed to find electrical shorts between device terminals, continuity between nodes, incorrect assembly, power supply and device functionalities using JTAG/boundary scan technology etc. The units are tested on bed of nails with spring loaded pogo-pins making contact with numerous test points on the PCBs. In Circuit Test methodology is a very powerful tool, however it has some limitations. It cannot test the quality of electrical contacts and if multiple components are assembled in parallel, it cannot determine individual failures. Actually in circuit testing is not meant to verify critical circuit parameters and functions.
Functional testing analyzes the PCBs as a complete circuitry or system. The units are simulated by providing signals though connectors and the outputs are recorded. The operator or an automatic tool analyzes the responses and determine whether the circuit is working properly or not. The test also covers regression test, integration test. Functional testing usually verifies what the systems are intended to do.
Functional testing differs from system testing in that functional testing “verifies a program by checking it against … design document(s) or specification(s)”, while system testing “validate[s] a program by checking it against the published user or system requirements” (Kaner, Falk, Nguyen 1999, p. 52).
In circuit test and functional test both cover the testing of the entire assembly in the system; but the complexity of the system determine which approach should be followed. The test development engineer should determine whether one of them is sufficient to the test the hardware.
Adjunct to in circuit test or functional test several few other test like burn in test, HAST are sometimes carried on the system hardware to determine the system quality and reliability over a specified life span. In case of Burn-in test the components and devices are electrically stressed at an increased temperature to determine the infant mortality of the system. HAST (Highly Accelerated Test, referred to as “shake and bake test” determine the robustness of the system against elevated temperature and vibration conditions. However, these tests are performed on a sacrificial board in order to collect information about the reliability and expected product life.
Developing a test strategy should be done in parallel with system design, because design should be done considering the testability (Design for Test). In the process of developing the manufacturing test methodology one should consider the suggestions from board fabricator, assembly process engineer, system design engineer as well hardware board designer. The review team consisting each one of them at minimum should consider the test coverage, the testing concepts and that should economically facilitate the fault detection during manufacturing. At the design phase, the manufacturing test development engineer should work closely with hardware board designer and suggest changes to improve the testability of the system. On the other hand, any changes done in the system that impact the test program should be apprised to the test engineer to determine the best compromise.