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May 2014 • SMT Magazine 63 Does DFT enable good diagnostics? Design teams need to have a comprehensive understanding of the available test methods, fault spectrums and plan for manufacturing tests with diagnostics effectiveness in mind. Many R&D engineers confuse coverage and overall test with diagnostics, and they do not plan for the right level of access. Critical de- sign challenges of smaller size components and complexity will drive a continued war on PCB real estate and design engineers need to col- laborate with engineering to identify ways to maximize diagnostics coverage. Can the faults be found earlier? As shown in Table 1, test methods from MDA, flying probers and ICT, followed by func- tional test have been prevalent in the industry. The key challenge is to decide what test meth- ods to use. From a ROI perspective, finding faults during earlier manufacturing steps can reduce costs by as much as 10X. It is critical to understand projected bone piles and potential scrap based on each test method implemented. Currently, in terms of test coverage with diag- nostic effectiveness, ICTs lead the effort, fol- lowed by MDA and flying probers. To build a successful test management prac- tice, design engineers and test engineers need to understand the following: • The fault spectrum in manufacturing • Each opportunity for defect creation • How to define the test coverage • The impact of catching the defect earlier New Challenges with SOCs & HDIs There is increased usage of HDI and 3D SOCs, which are prevalent on modern PCB as- semblies like tablets and smartphones. The use of SSDs causes a lack of access on board real estate, so there may not be enough access to define a good test strategy. This has led to the increased use of functional test as the key gate- keeper for screening manufacturing defects— however, testing at such a downstream stage of the manufacturing process is also normally the most expensive stage. Advocating the principle of earlier detection of such defects as a lower cost method, some ICT test suppliers have risen to the challenge by mod- ularizing test strategies and integrating embed- ded test in manufacturing. Test solutions are now available for integrating a variety of test meth- ods, from basic ICT to complex embedded test in manufacturing, with the objective of diagnosing defects before they reach functional test. Successful Test Management Practices • The test engineering community needs to collaborate with design teams to ensure 100% (nets and components) coverage. While this is difficult to achieve in some cases, it is critical to engage with R & D teams on these discussions early in the process • If access is an issue, review the design and try to answer this question: What is the cost im- pact of not accessing a particular net? • Review implementation of limited access test technologies like advanced boundary scan, Cover-Extend, embedded test and bead probes to regain coverage of the nets which are other- wise not accessed • Test engineers should simulate PCA test coverage upfront in the design process and en- able the use of all technologies to ensure test coverage before functional test. There are a number of software tools available to simulate test strategies • Review DFT processes and recommenda- tions. There are a fair amount of tools available in the industry, including testability guidelines by TMAG/SMTA SmT References 1. Duane Lowenstien, Simplifying Econom- ics of Test and Repair in Factory and Depot 2. Bob Neal, Design For Testability 3. Rosa D. Reinosa and Carl Michel, Manufacturing Test Strategy Cost Model 4. Stig Oresjo, A New Test Strategy for Complex Printed Circuit Board Assemblies nK Chari is director of marketing for the Measurement Systems Division of Agilent Technologies. He leads the MSD marketing team covering in-circuit test and board/automotive functional test solutions. PCBa TEST: ENaBLING THE RIGHT maNaGEmENT PRaCTICE continues feATuRe