板级测试指南

Printed-circuit boards used to be works of art, and the PCB designer was an artist as well as a technician. Components had many different shapes, sizes, and colors, and the traces connecting them were graceful arcs in beautiful patterns.

Today’s boards, by contrast, are boring. On a typical modern digital PCB assembly, for example, the parts are all squat rectangular components, and the traces are laid out with a precision only a computer could love.
Test methods for PCB assemblies have changed as well. .In the past, nearly all testing was functional testing, and it was performed with benchtop instruments or a custom tester. Today, the test engineer has many more options, including in-circuit testers, manufacturing defects analyzers, and several types of functional testers.
The first choice for many test engineers is the in-circuit tester. This tester gets its name because it tests components that are part of a board assembly; that is, the components under test are "in a circuit."

Shorts-and-Opens Test

The first test an in-circuit tester performs is a shorts-and-opens test. Solder shorts are a common manufacturing defect, and you must find them before proceeding with the component tests. Shorts may cause a PCB to fail other tests and may damage the tester or the board under test if you must apply power to the board.

After performing the shorts and opens test, the in-circuit tester tests each component on a PCB assembly one at a time. Usually, the test program will first test the passive components, such as resistors, capacitors, and inductors. If none of the passive-component tests fail, the tester will then test active components, such as transistors and ICs.

To test passive components, the tester places a test voltage across the component under test and measures the current. For resistors, this is a DC voltage; for capacitors and inductors, it’s an AC voltage. To measure component values accurately, most testers can make two-wire, three-wire, or four-wire measurements.

An in-circuit tester tests a digital IC using the IC’s truth table. The tester sets the inputs to all combinations of 1’s and 0’s and looks for the correct logic values on the outputs. The tester does this with a process called backdriving, in which it applies a high-current pulse for a short period that overrides the logic value on the input. During that short period, the tester must also capture the output value of the IC.

To perform an in-circuit test, you need a test fixture that connects the circuit nodes on the board under test to the tester. Most in-circuit test fixtures use spring-loaded probes and are vacuum actuated. Wires running from the tester interface to the probes carry test signals back and forth. A technician places the board under test on the fixture and activates the vacuum, which pulls the board down onto the pins. Test engineers refer to spring-loaded probes as "nails" and refer to a fixture using spring-loaded probes as a "bed-of-nails" fixture.

To keep up with changing PCB technology, manufacturers of in-circuit testers have added features in recent years. Many in-circuit testers now have boundary-scan test capability. With boundary-scan testing, you can test interconnections on PCBs without using a probe on each node.

In-circuit testers can now also test for solder opens on surface-mounted devices (SMDs), a common manufacturing defect on boards with SMDs. To perform this test, you need a fixture that positions a metal plate over the circuit under test. The tester measures the capacitance between the plate and each lead of the circuit under test. Since open leads will have a lower capacitance than connected leads, the tester can tell which leads are soldered correctly and which leads are open.

Other Test Options

An alternative to the in-circuit tester is the manufacturing defects analyzer (MDA). MDAs detect manufacturing defects, such as shorts and missing components, but they can’t test digital ICs. Like in-circuit testers, MDAs use a bed-of-nails fixture to connect a board under test to the tester.

MDA manufacturers feel that their testers offer a cost-effective test strategy. These testers cost less than in-circuit testers, and some manufacturers claim that tests run faster on MDAs. Because most of the faults found at board test are manufacturing defects, fault coverage for MDA tests is nearly as good as fault coverage for in-circuit tests. And, when coupled with functional testing, the fault coverage for the entire process is as good as or better than in-circuit testing alone.

What’s the Problem

Because in-circuit testers and MDAs test components individually, a PCB assembly can pass an in-circuit or MDA test and still not function properly. The problem might be an interaction between components or an IC that fails when run at normal operating clock rates. For simple assemblies, the number of boards with these problems is small, so many companies pass the boards on to final assembly without a functional test. Finding these problems at final assembly costs less than performing a functional test on each board.

For complex boards or boards that aren’t tested with an in-circuit tester, most companies perform a functional test before final assembly. As the name implies, a functional test simulates an operating environment and tests a board against its functional specification. There are many types of functional testers.

For microprocessor-based boards or boards that fit into a standard bus, such as the PC bus or VME bus, an emulation tester might be a good choice. Emulation testers work by emulating a microprocessor or computer bus to control the circuitry on a board under test.

On microprocessor boards, the emulator takes the place of the microprocessor and places the appropriate data, address, and control signals on the microprocessor pins. Because the emulator works exactly like the microprocessor, you write test programs in the assembly language of the microprocessor or in a language that can be compiled to run on the microprocessor.

For analog or mixed-signal boards, many engineers design and build custom testers. They select a set of instruments to provide test signals and make measurements and then bolt the instruments into a standard, 19-in. rack. Test engineers call these test systems "rack-and-stack" because of the way they look. If the instruments have a GPIB or serial interface, you can control them with a computer equipped with a similar interface.

Rack-and-stack systems can be large if the test requires many different instruments. These systems can also be slow, because the instruments are only loosely coupled to the computer. To shrink such systems, and improve their performance, many test engineers now design test systems using VXIbus instruments.

VXIbus instruments are designed specifically for computer-controlled test applications. They are smaller than their benchtop equivalents because they have no front panels or power supplies. They can achieve higher performance because they communicate with the controlling computer and each other through the VXIbus (a version of the VME computer bus). You can find VXIbus instruments that perform nearly all the functions performed by benchtop instruments.

Which tester is right for you? The answer is, "It depends." It depends on the type of PCB assembly, the number of assemblies you must produce each day, the components on the board, and the resources you have to program and maintain your ATE equipment. Before buying or building a board tester, you should draft a test plan that takes all these factors into account