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www.us-tech.com Dog-Bones and Daisy Chain for Test Labs Continued from previous page
some cases, high voltage and high current are pre- ferred. In complex PC board (PWB) designs with multiple daisy chain data lines, the I/O is run to a connector at one edge of the board. The list of applications for using daisy chain
is expansive. Daisy chain test ICs are suitable in a wide variety of process-related applications, such as life cycle testing, drop testing, verifying the effects of CTE (Coefficient of Thermal Expansion),
Standard dummy components (without daisy
chain) might be adequate when demonstrating mounting machines (pick-and-place mach ines). Rudi mentary applications such as machine main- tenance and machine acceptance testing don’t nec- essarily require test components with daisy chain. However, for true re search and for optimizing
the assembly process, daisy chain devices with dummy die are required to obtain the most mean- ingful re sults. Critical applications in volving environmental
life cycle testing require daisy chain packages with dummy silicon die. The daisy chain provides elec- trical continuity testing, while the dum my silicon die replicates the thermal mass of a “live” device. Daisy chain is created inside the IC package
by stitching gold bonding wire between pairs of bonding pads on the lead frame. BGA daisy chain is often made with copper traces between pairs of ball pads on the bottom side of the substrate.
Inside Connections In the schematic of a daisy chain circuit, pin 1
is connected to pin 2 within the device. Pin 3 is con- nected to pin 4. Pin 5 is connected to pin 6, and so forth until the last pin “N”. On the PCB test vehi- cle board, copper traces connect landing pads pin 2 to pin 3, pin 4 to pin 5, and pin 6 until the last pad. Electrical continuity (using DC current) is
Example of a 256-pin CBGA with
daisy chain dog-bone pattern between solder balls.
selecting the correct amount of solder paste, evalu- ation of solder paste stencils, checking for voids caused during reflow, and underfill experiments.
Side by Side Occasionally, engineers perform parallel tests
using daisy chain components with standard tin- lead (SnPb) plating alongside Pb-Free products in order to achieve a benchmark and derive compara- tive data.
made between the PC board and the DUT (Device Under Test) after soldering. A short circuit is measured using an ohmmeter from test point “T1” to test point “T2” on the PC Board. Alternately, the test points can be run to a connector located on the PC Board. “Pass” condition occurs when short circuit
(approx. 0W) is measured between T1 and T2. “Failure” occurs when an open circuit is
measured (infinite W) between T1 and T2. Solder joint reliability is verified by vibrating,
drop testing and temperature cycling the assembly using JEDEC, IPC and Mil specifications (–55 to +125°C in a humidity chamber) until a failure occurs. A “failure” means the daisy chain circuit goes from a “short” (0W) to an “open” circuit (infi- nite W). The assembly and solder joint is forensi-
Schematic of daisy chain BGA mounted
onto a PC board with complementary daisy chain circuitry.
cuits (Failures). Dispensing and encapsulation evaluations also use daisy chain components. Scrap components or live components do not have daisy chain. It is impossible to make simple Pass/Fail tests with live components. It is best to use daisy chain components for such evaluations. Major companies such as Intel, IBM, Honeywell, Hewlett Packard, Cisco, Flextronics, Celestica, Sanmina-SCI, Jabil, Boeing, Northrop Grumman, Lockheed and others understand the benefits of using daisy chain components for defining and refining assembly processes. Daisy chain test die offers significant benefits for learning and improv- ing assembly processes. Contact: TopLine, 95 Highway 22 W.,
Milledgeville, GA 31061 % 800-776-9888 E-mail:
info@topline.tv Web:
www.topline.tv/DaisyChain.html
cally analyzed to determine the cause of failure. Dummy test devices with daisy chain and silicon die are an economical way to study why failures occur and how to improve the assembly process. BGA, CSP and flip chip test die with daisy
chain are used in underfill experiments. Daisy chain devices are mounted on a test vehicle board, underfilled and then temperature cycled to a point of failure. The results are observed using an ohm- meter to record short circuits (Pass) and open cir-
March, 2013
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