Testing a new IC design is a critical step in the transition from product development to production. The device-under-test (DUT) board is a key component of the test interface unit (see sidebar), and it serves as the interconnection between the test socket or contactor and the ATE test head.
Because the DUT board is at the center of the test system, designing and producing one that delivers accurate and reliable test results required complete knowledge of:
The IC package to be tested.
The various test sockets or contactors that can be mounted on the board to interface with the IC package.
The various test heads to which the DUT board can connect.
The automated handling equipment used in the ATE system.
For example, there are several manufacturers of IC testers, and each offers from five to 10 different models. As a result, the DUT board manufacturer must know the functional and operational characteristics of the standard setup for every make and model presently in use. However, about 80% of all end users require some degree of custom configuration to the standard setup, so knowledge of a unique requirement is equally important for the DUT board design.
Mechanically, the DUT board design includes attachment to a metal frame/fixture to increase the structural rigidity of the board and an interlock mechanism for docking the board to the test head. Maintaining structural integrity is important since the boards can vary greatly in shape (square, rectangular, hexagonal, round) and size (6″ to 24″ wide).
A primary design consideration for the frame/fixture is whether the DUT board will be docked to a device handler or temperature-forcing equipment. Another important mechanical-interface consideration is whether peripheral circuit components, such as capacitors, resistors or relays, will be mounted to the DUT board. If so, the board design must place the components where there is adequate clearance with the test head and handler to prevent physical interference.
Electrical design considerations must be addressed in three areas: the specifications of the tester, the characteristics of the IC package and test socket or contactor, and the end user’s test parameters. Some of the electrical design considerations for the tester include the signal type such as digital, analog, or mixed; calibration capability; speed from 20 MHz to 1.2 GHz; resolution as fine as 10 ps; and an interconnection configuration for a power supply, I/O channels, and control signals.
Some of the electrical characteristics of the DUT and test socket or contactor include the device function, package type such as BGA or PGA, the I/O count, speed from 20 MHz to 2.4 GHz, and power requirements up to 250 W. Test parameters for the end user include the number of testers that will be used with a specific DUT board, typically from 1 to 25; differences in configurations from one tester to another; and the number of devices that may be tested on a single board.
Many of the electrical considerations are interrelated and must be addressed simultaneously. Impedance matched to ±3%, signal loss, crosstalk, and propagation delay matched to 10 ps are related to the IC package, the socket or contactor, the tester, and the end user’s test parameters, and all must be factored into the DUT board design.
Often these multiple considerations require external test and verification circuitry on the DUT board to augment test capability and enhance test results. A specific example is the transition of a signal from the DUT board to the test socket or contactor, where IC and tester impedance must be matched to reduce signal degradation or loss. To accomplish this, termination circuitry and decoupling components may be placed on the DUT board to improve the signal integrity.
Finally, both the mechanical and electrical considerations are factored into fabrication decisions. These include the selection of a dielectric material such as FR-4, Polymide, or Teflon; the various metals that will be deposited, such as copper, nickel, or gold; and the optimum layer stack-up, typically six to 28 layers.
The Process
The procedure for designing and producing a DUT board starts with the design spec—the document that addresses most of the electrical and mechanical design considerations. If a completed design spec is not available, OZ TEK provides a design template that can be used to define the mechanical and electrical requirements. Because most test setups have some degree of custom configuration, it is important that the DUT board producer thoroughly investigate any unique requirements that may not be specified or clearly articulated in the design spec.
The information in the design spec is used to generate the DUT board layout (Figure 1). After component placement is complete, the layout is generated automatically using computer-aided design (CAD). However, manual routing is used for critical signal paths to increase density and reduce signal loss or degradation. While it is possible to perform a complete layout with CAD, experience has proven that DUT board performance is optimized by manual techniques that control impedance, match differential pairs, minimize transitions, and reduce signal loss.
A mechanical CAD program also is used to model and simulate the frame/fixture design. The generated CAD models are fed directly to computer-aided machines (CAM) for fabrication of the DUT board (Figure 2) and the frame/fixture (Figure 3).
Two tests are performed on the DUT board before it is assembled as part of the test interface unit (TIU). The first is a continuity test to detect any shorts or opens in the board’s interconnection paths. A flying probe tester typically is used because it eliminates the need for a special fixture.
The second critical test is performed using a time domain reflectometer. This test confirms impedance and verifies the acceptable range of propagation delays on I/O channels. Upon successful completion of these tests, the DUT board is ready for assembly as part of the TIU.
Typically, the cost for a completed and tested DUT board ranges from $600 to $5,000.
About the Author
Iraj Barabi has been a vice president of OZ Technologies since cofounding the company in 1986. He obtained a B.S. in electronics engineering technology in 1981 from Cogswell College. OZ Technologies, 3387 Investment Blvd., Hayward, CA 94545, (510) 782-2654.
Test Interface Unit
The TIU is the interface between the DUT, the tester, and the handler. It is composed of a test socket or contactor that holds the DUT on the DUT board. The DUT board mounts onto the test head via a mechanical frame/fixture. Figure 4 shows an assembled TIU.
The component parts are tested separately, assembled to form the TIU, and tested again to verify performance of the integrated unit. Testing the complete TIU assembly on a simulated test-head fixture with a device simulator in the socket and a manual actuator ensures proper mechanical fit and electrical performance.
Because IC test time ranges in price from $350 to $450 per hour, verifying the performance of a TIU before using it in a live test situation saves time and money. For example, there is no immediate way to know if a short or open revealed during the test of a new IC is from the IC or the TIU unless the integrity of the TIU has been tested and verified previously.
Initially, semiconductor manufacturers purchased component parts for the TIU from many different vendors and then assembled and tested the completed unit themselves. Now, many semiconductor manufacturers want turnkey solutions and use one company that has the capabilities to design and produce each of the component parts, test them separately, assemble them into an integrated unit, and test the assembled unit to verify TIU performance. This approach has advantages in terms of shorter delivery times, increased reliability, and reduced costs.
Copyright 1999 Nelson Publishing Inc.
August 1999
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