The evolution of the VXIplug&play standard has greatly improved the capability of test organizations to respond to the requirements of their customers. Improvements have come in three areas: the availability of low-cost manufacturing test platforms that can incorporate VXI instruments, integrated functional test systems for flexible high-performance digital test, and integrated software for functional testing.
These developments provide test organizations with the capability to react quickly to changing requirements while implementing high-quality test. However, good equipment and software are not enough. To take advantage of the new options in VXI, a test organization needs a standard process to meet the goals of high quality, low cycle time, and low cost.
The Environment
The circuit boards used in Teradyne test systems are built and tested in two foundries in Boston and California. The Boston group focuses on lower-volume, high-mix boards, many of which require high-performance functional test. We are organized to do fast, frequent changeovers from one board type to another.
Seven Teradyne divisions, each with multiple product lines, are our customers. We build and test about 500 different board types during a typical quarter—some with as few as 500 nodes, others with as many as 5,000 nodes.
We perform in-circuit testing of 100% of our production boards. In addition, about 20% are functionally tested before shipment to our in-house customers for system integration and final product test.
Approximately 90% of our boards undergo in-circuit testing on the first prototype production to reduce the design debug cycle and allow faster ramp up from prototype to volume production. To support that effort, the test plan for each board is developed in parallel with the hardware design.
Finding Faults
To facilitate the ramp up from prototype to volume production, we must push all the fault detection back to board-level test so that faults in system-level test do not delay shipments. We could test thousands of parameters, but we don’t. The key is to strive for 100% yield at system test, not 100% board coverage. To meet that objective, we examine historical information and develop test strategies based on our best analysis of where the faults will be.
A Standard Process
To ensure that we achieve the required fault coverage for each board and to keep cycle times low, we pursue the following steps.
Determine our customer’s test requirements by meeting with the design engineer and the test technologist for the product in which the board is used. During these sessions, we discuss and document the customer’s needs. Test selection depends on the technology and performance of the board.
Review the default in-circuit test coverage, which includes tests such as shorts, opens, passive component testing, power-up analog tests, and digital pin faults. Then, we determine necessary exceptions and additions to the default in-circuit coverage.
Determine the functional test requirements. For example, functional board tests might be specified to ensure that a board will operate correctly in all configurations of the product. Another example is a board that includes a bandpass filter. In-circuit tests can confirm that all parts of the filter are in place, but functional testing is required to verify correct operation of the filter.
Prepare a formal test strategy for the board and present it in a formal testability review meeting. Considerations include the anticipated volume of the board, how it fits into current product mix (similarity to existing boards), the volatility or maturity of design, and the projected cost of test. That strategy document becomes our contract with the customer.
Determine whether the requirement can be met using existing equipment. For example, some VXI-based tests can be performed on our Spectrum 8800-Series Manufacturing Test Platform, which combines in-circuit and functional test. This approach saves cost and cycle time by reducing the number of boards that require separate functional testing.
To provide maximum flexibility in our East and West Coast operations, we try to maintain identical configurations on all our test systems, which are located at the outbound end of the assembly lines. For that reason, we might not add a very expensive instrument to all our test systems because a limited requirement for the instrument may not justify the cost across all systems. In that case, we would do a particular functional test on one of our dedicated VXI functional test systems.
If new equipment is needed, evaluate the available choices based on the following criteria:
Does it meet the customer’s measurement requirements for board-level functionality and accuracy?
Is it flexible and versatile? Can it be used for multiple applications? Test equipment used in our operation must correlate with that used in final system test. When possible, we use the same instruments in both areas to minimize debug time. Is it the lowest-cost way to meet the requirement?
Is it efficient? Does it integrate easily into our current setups (hardware and software)?
Has the supplier demonstrated the required capability—for example, how hardware and software work together with existing equipment? Although VXIplug&play has greatly improved standardization of test equipment, there still are areas of ambiguity.
For example, we once purchased a fast digital I/O card and realized afterward that it had no memory behind it. The design depended on the reconfigurable memory that was in the Slot-0 controller. We worked with both the vendors of the Slot-0 controller and the digital I/O card for about two months without getting the interface to work. In theory, the capability was there to send signals back and forth across the user bus but, in fact, it never worked correctly.
That is an area in which the VXI spec was not explicit concerning who was responsible for what. Today, we would describe all the hardware and software we are using to the supplier and ask for a demonstration of how the instrument works. We will buy the card after we see it run.
Software Advances
One of the best things to come out of the evolution of VXI is the improvement in test software. Five years ago, all our test programs were written in C. Today, standard software like Lab Windows and LabVIEW makes test programming predictable and efficient.
A Case Study
The LA703 circuit board illustrates our test-strategy process. This board, which is a master clock in Teradyne’s Catalyst Mixed-Signal Test System, generates 10-MHz and 100-MHz signals and distributes them to several ECL and sine-wave outputs (Figure 1).
Our initial objective for this board was to verify the function of a filter used to filter out harmonics and subharmonics. Later, we also looked at other outputs to confirm that they worked at-speed. The tests and instruments used are listed in Table 1.
The instruments are integrated in a custom functional test system. It is PC-controlled under Microsoft NT. Test processes are written in LabVIEW 4.1. Before testing begins, the fixture is actuated manually to engage all connectors on the board. No vacuum or pogo pins are used. Warm-up and automated test take about 10 minutes; the duty-cycle adjustment takes 10 to 20 minutes.
Test Results
Figure 2 illustrates the type of data we collect to monitor faults at board test. Our functional test process has found faults in 16% of the boards that would otherwise have gone on to final product test. None of these failures would have been found by in-circuit test. Even the missing-part failure in this case required functional test: It was an oscillator that could not be checked at in-circuit test.
Other Lessons
In integrating VXI instruments, particularly in custom systems, you must be as rigorous about configuration management of test equipment as you are about the boards that you manufacture. We give each system a name and model number and document exactly what equipment is in each. Because we know what equipment we have and where it is, we can avoid duplication, assign resources, and maintain equipment efficiently.
That said, we recommend against building custom equipment whenever possible. It always is a lot more work than you expect, including tasks such as documenting every cable connection. Excellent VXI test platforms are available on the commercial market. Take advantage of the integration that already has been done.
About the Authors
Ken Degan joined Teradyne as test technologist after receiving a B.S.E.E. degree in 1984. He currently manages the board test engineering operations in Boston and San Jose.
Doug Perkins has been a test technologist in Teradyne’s ATE Operations group in Boston for 14 years. Before that, he worked on the Nike Ajax anti-aircraft missile system in the Army and for Raytheon on the Hawk and Patriot missile systems. Mr. Perkins has a B.S. degree in industrial sciences. Teradyne, 321 Harrison Ave., Boston, MA 02118, (617) 482-2700.
Table 1
Description
VXI Instruments Used
Test oscillator for 10 MHz ±10 Hz output at J1
RF Switch Array
Counter
Test ECL outputs individually at J9, J10, and J11 for <-0.51V at low and
>-0.51V at high
RF Switch Array
Oscilloscope
Test 100-MHz bandpass filter frequency response at J8
Insertion loss: 3.3 dB max at 100 MHz, 4.2 dB max at 75 MHz
Attenuation: 10 dB min at 65 MHz and 115 MHz, 40 dB min at 50 MHz and 150 MHz
Network Analyzer S-Parameter Test Set
RF Switch Array
Assure that 100 MHz PLL output at J8 is 10 times input at J2 ±1 Hz, test at 10 MHz
Generate 10 MHz, test for 100 MHz ±1 Hz
Generate 10 MHz + 50 Hz, test for 100 MHz + 500 Hz ±1 Hz
Generate 10 MHz -50 Hz, test for 100 MHz -500 Hz ±1 Hz
Function Generator
Counter
RF Switch Array
Adjust duty cycle of the 100 MHz square wave at J15 to 50% ±0.4% Connect J1 and J2, J6 and J7
Connect oscilloscope to J14
Displayed duty cycle is an average of 16 measurements
Adjust using through-hole resistors
RF Switch Array
Oscilloscope
Copyright 1998 Nelson Publishing Inc.
May 1998
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