More electronics companies than ever before are turning to low-cost ATE to satisfy their production test needs. It’s more than just the obvious appeal of lower acquisition and operating costs. They turn to low-cost ATE because many new systems offer performance surpassing that of earlier, larger ATE.
The new breed of ATE is better for several reasons. Topping the list are the new test techniques, such as vectorless testing, and greater use of embedded UUT resources.
Most low-cost ATE is available in a base configuration that can be upgraded to accommodate a variety of test resources. Many of these test systems also can be equipped to perform dual tasks, such as process fault finding and end-product functional evaluation.
Finding Faults Without Vectors
Electronic equipment manufacturers have long relied on in-circuit ATE to identify manufacturing and component defects on PCBs. With increasing device complexity, however, it is more difficult to obtain or generate device models, and dreadfully time-consuming to produce test-vector sets providing adequate fault coverage.
The fault spectrum also is changing with the advent of SMT. The most prominent fault class in today’s SMD-populated PCBs are open connections, usually solder voids between the board and the device lead. Since it is always best to find the most defects with the fastest and least expensive technique, the search began for a new solution to identify and locate “open” faults.
After examining a variety of approaches, Hewlett-Packard introduced the TestJet™ technology in early 1993. It was initially used in the HP 3172 Process Test System. Later, it was also licensed to other companies and implemented in various test systems, such as the TR-8 MDA from CheckSum and the Testronics Model 405 Series.
TestJet measures the capacitance between the lead frame of a device and a transducer that is touching the IC case. If the lead of the IC is not soldered to the board, the capacitance value will be an order of magnitude smaller than a properly soldered joint.
Since there is very little variation in lead frames from one package to another, most package types can be tested with TestJet. It also can detect most open solder joints with no false failures.1
TestJet consists of a dedicated clamshell fixture and supporting tester software. The bottom part of the fixture contains the nails which contact the individual nodes of the ICs similar to an in-circuit test fixture. The top houses individual transducers/probes to measure the lead-frame capacitance of each device. A signal enhancement module amplifies the low-level signals derived from the probes.
GenRad’s Opens ExpressTM feature is based on a similar technique and implemented in the GR228X Series Production Test Systems. It identifies opens on a range of semiconductor devices and indicates if polarized capacitors are properly oriented.
Teradyne developed MultiScan™, a set of complementary tools for the Z1800-Series testers. MultiScan includes DeltaScan, which performs analog junction tests; WaveScan, an RF inductive test technique; and FrameScan, a capacitive coupling test technique.
“The MultiScan vectorless tool set is standard with new Z1800 models,” said Craig Pynn, Marketing Manager, Teradyne Assembly Test Division/Walnut Creek. “Together, they provide comprehensive fault coverage over the SMT process fault spectrum and across different device types, package types and circuit configurations.”
“In the past, fault coverage achievable with classical MDAs was only 60% to 70%,” said Cliff Call, Applications Engineer at Testronics. “With advances in test techniques and computing power, fault coverage in excess of 90% is not uncommon with today’s MDA testers.
“We employ several vectorless techniques including multiple-level impedance scanning, IC junction characterization algorithms and HP TestJet technology. Application of these and similar techniques are some of the reasons why you can get much more PCB fault coverage with today’s low-cost ATE test systems,” Mr. Call concluded.
Combining MPT and PFT
While vectorless test techniques are suitable for many applications, a vast number of PCBs still are most efficiently tested by traditional techniques. These may include in-circuit tests, combinational or combined manufacturing process test (MPT) and product functional test (PFT).
“Traditionally, MPT and PFT have been performed as separate test activities on different test systems,” said Mr. Pynn. “Most PFT equipment is a custom setup of programmable commercial instruments with home-grown fixturing and switching, controlled by a PC.
“Today, many electronics manufacturers are moving PFT steps onto the process tester. Our customer surveys show that the capability to perform single-stage test—combining MPT and PFT steps—is more important to test managers than higher performance, such as bandwidth or pattern rates,” said Mr. Pynn.
Using an MPT system as a PFT station has several advantages: · The MPT may already contain applicable test software and hardware, including test fixtures, flexible switching, power supplies, analog stimulus/measurement and digital pattern sources that are useful for PFT.
· It saves handling time and effort by being able to perform two operations at a single site.
· It may provide utilities that can be applied to some aspects of PFT.
· It shortens the diagnostic loop when a board is found to be “good” at MPT and “bad” at PFT.
“We estimate that 75% to 90% of the hardware and software needed to implement most forms of PFT is already present on the MPT tester,” said Mr. Pynn.
Telecommunications equipment manufacturers were among the first to implement single-stage testing. But other high-volume electronic equipment producers are showing a keen interest in applying it in a variety of forms.
“We now see a growing trend toward in-line testing applying a combination of MDA and functional test techniques,” confirmed Bill Gettelman, Marketing Manager at Digalog. “This approach can reduce test time, eliminate excess board handling, and require less manpower in high-production electronic manufacturing operations.”
Using Embedded Resources
To perform functional tests, often it was necessary to employ extensive high-speed test resources and voluminous test programs. The cost of these resources and the time penalties encountered when generating comprehensive test programs prompted many test professionals to look for other solutions.
One attractive alternative uses the UUT’s internal resources, such as built-in-test facilities residing in an ASIC, as the test stimulus and response evaluator. Emulation is another solution appropriate for UUTs that contain or are controlled by microprocessors.
Since more and more boards are microprocessor-controlled, emulation is becoming more popular. Emulator test systems may be stand-alone equipment provided with extensive programming and troubleshooting aids, such as the Talon Model BE64 Bus Emulator, or an add-on feature to an MPT.
More varieties of emulation-based techniques and implementations are becoming available. For instance, “We recently invented a ROM emulation-controlled analog/digital test vector generation facility called Synchronous Self Stimulus (SSC),” said Bill Shepard, President/CEO of Array Analysis. “ROM emulation, SSC and Adaptive Inference, an Array Analysis proprietary diagnostic technique, significantly reduce the time it takes to develop functional tests.”
Many of today’s low-cost test systems are modular and can be easily configured to perform a variety of testing applications. They can be expanded to accommodate additional PCB categories or reconfigured to support a number of tasks.
An example of the multitasking capability is provided by the Corelis scan test system. It can perform scan-test-vector generation, be upgraded to perform manufacturing test and then converted into a field-depot maintenance tester.
To keep acquisition costs low, almost all test systems today can be purchased in a minimal configuration. Later, they can be equipped with features to accommodate a greater variety of test facilities needed by new UUTs.
The accompanying chart lists the basic and upgrade test capability data for a variety of low-cost ATE. This information can help you identify suppliers whose ATE provides the test facilities you require.
An alternate upgrade approach is available with the HP3070 Series of pay-per-feature systems, delivered with capabilities ranging from unpowered test to full functional test. A coin inserted into the system contains a fixed number of test credits. Depending on the extensiveness and types of test facilities you use, debit charges are deducted from the coin. You pay only for the test facilities normally required, but you have additional capabilities available whenever your UUT demands them.
If the ATE has an open architecture, it is easy to upgrade it. Open architecture not only makes it easier to incorporate third-party instruments, but it also eliminates time-consuming retrofits when adding capabilities.
“It must be easy to add hardware and software that has not been supplied by the ATE manufacturer. The ability to integrate test routines and separate code into the tester operating environment is crucial,” Mr. Pynn said.
1. “New Technique Reduces Test Development Time for SMT Opens,” Evaluation Engineering, February 1993, pp. 62 – 71.
Copyright 1995 Nelson Publishing Inc.