March, 2013
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Page 55 Dog-Bones and Daisy Chain for Test Labs By Martin Hart, President, TopLine Corporation, Milledgeville, GA
design community. Daisy chain provides engineers with a simple and cost-effective testing mechanism to glean valuable data about process failures. Such failure analysis provides scientists and engineers with a pathway to improve SMT assembly process- es and raw materials sets. Research starts by mounting daisy chain
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devices onto a test vehicle board with complemen- tary dog-bone daisy chain circuitry. This combina- tion creates a short circuit with near zero ohms resistance measured at test points on the test vehi- cle board. Under controlled conditions, the test vehicle board is subjected to harsh environmental conditions, such as dropping, vibrat- ing, freezing, and heating with humidity up to the point of failure when a connection gradually deterio- rates, and ultimately breaks.
Easy Measurements Failures are easily measured
when the daisy chain becomes an open circuit (infinite ohms). The fail- ure is usually observed, measured and recorded with a simple ohmmeter. Daisy chain components are
better than using “live” devices when performing SMT assembly and process improvements. Daisy chain allows a simple Pass/Fail test with easy to detected “Go”, “No Go” condi- tions. On the other hand, “live” com- ponents inherently offer too many
It is impossible to make simple Pass/Fail tests with live components, and that’s where daisy chain compo- nents are used.
variables that may obfuscate and complicate the end result. Using forensic analysis and experimenta- tion, researchers can make improve- ments in the assembly process. After the assembly line, the machine or material set is optimized using dummy components, then the assem- bly line can move forward using live components. Daisy chain is available in a
wide variety of components such as BGA, CS-WLP (wafer level package), bumped die, bare die, flip chip, QFP, TQFP, LQFP, QFN, DFN, TSOP, SOIC, SSOP, SOD, TSSOP, QSOP, MSOP, SOT, PLCC, Flat Pack, CERQUAD, LCC, CLCC, DPAK and virtually every other alphabet soup acronym possible. In addition, cus- tomized daisy chain configurations are routinely available. Leadframe type daisy chain test
components — QFP, QFN, TSOP, SOIC — are manufactured by wire bonding gold wire directly to pairs of pins on the component’s copper lead frame. Daisy chain array products, such as BGA, are typically made with copper dog bone patterns placed on the ball side of the substrate. WLP wafer level die and flip chips have RDL (redistribution layers) running between the I/O pads. Adding a dummy silicon die inside a daisy chain component is highly recom- mended to give thermal mass to the package, and to make the daisy chain test IC mechanically identical to its live IC counterpart. The topside of the daisy chain component is identi- fied with a symbol to indicate that the device is a test vehicle, rather than a live component.
ngineers routinely use dummy IC packages with internal daisy chain circuitry — fondly known as “Dog-Bones” within the PC board
Daisy Chain Simplified The sum total resistance of a 2-port daisy
chain dog-bone is typically less than 50mW as measured between test points T1 and T2. The actual amount of resistance depends on the length of the circuitry as well as the resistivity of materi- als such as Cu (copper), Au (gold), Ag (silver), Al (aluminum), solder (SAC or SnPb), and so forth. Typically, ohmic testing is performed using direct current. This is because continuity testing, meas- urements of inductance and capacitance are not of prime consideration here, and they would other- wise complicate testing if the daisy chain circuit is powered by alternating current. Testing is often performed at low voltage and current, however, in
Simplified two-port daisy chain circuit. Continued on next page
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