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1、板级测试指南 英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. 在过去，一块印刷电路板就是一份艺术品，印刷电路板的设计人员不仅仅是一名技术人员还是一位艺术家。器件有不同的形
2、状、大小和颜色，而且连接器件的导线形式优美。Todays 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. 相比之下，现在的电路板令人厌烦。例如，在一块标准的数字印刷电路板上，所有的器件都是有棱有角的，布线的精度也只有计算机才可以完
3、成。Test methods for PCB assemblies have changed as well. In the past, nearly all testing was functional testing, and it was performed with bench top instruments or a custom tester. Today, the test engineer has many more options, including in-circuit testers, manufacturing defects analyzers, and sever
4、al types of functional testers. 对印刷电路板装配的测试方法也已经改变了。以前，几乎所有的测试都是功能性测试，使用传统的测试仪器或台式设备就可以完成。今天，测试工程师有很多选择，包括在线测试设备，制造缺陷分析设备和一些功能测试设备。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
5、components under test are in a circuit.测试工程师的第一个选择就是在线测试设备，之所以称之为在线测试仪器是因为它所测试的器件是电路板装配的一部分，也就是受测设备是“在电路中” 。Shorts-and-Opens Test短路、开路测试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 w
6、ith 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. 在线测试完成的第一项测试任务就是短路、开路测试。焊接导致短路是很普遍的制造缺陷，必须在器件测试开始之前发现这一问题。短路可能导致印刷电路板不能通过其它测试，在对电路板上电后，会损坏测试设备或电路板。After performing the shorts and opens test, t
7、he 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
8、 ICs. 短路、开路测试完成以后，在线测试仪需要在同一时间对印刷电路板上的每一个器件进行测试。通常，首先对无源器件测试，如电阻、电容和电感。如果所有的无源器件都通过测试，然后对有源器件测试，如晶体管和集成电路。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, its an A
9、C voltage. To measure component values accurately, most testers can make two-wire, three-wire, or four-wire measurements. 在测试无源器件时，测试设备对被测试的器件施加一个测试电压后，测量电流。对于电阻来说，这个电压是直流的，对于电容和电感来说，这个电压是交流的。为了使测试更加精确，很多测试设备可以构成两线、三线或四线测量。An in-circuit tester tests a digital IC using the ICs truth table. The tester
10、 sets the inputs to all combinations of 1s and 0s and looks for the correct logic values on the outputs. The tester does this with a process called back driving, 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 tes
11、ter must also capture the output value of the IC.在线测试设备采用真值表来测试数字集成电路。测试设备对输入设置连续的 1 和 0，检测输出端的正确逻辑值。这一过程被称为反向驱动。在这个过程中，对输入端施加一个短时的比逻辑值高的电流，同时测试仪器要捕捉到集成电路的输出值。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
12、 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 t
13、o spring-loaded probes as nails and refer to a fixture using spring-loaded probes as a bed-of-nails fixture. 为了完成在线测试，你需要一个测试固定装置，它可以将被测电路板上的节点连接到测试设备上。很多在线测试固定装置是带有弹簧的探针或者采用真空启动的。连接测试设备和探针的导线来回的传递测试信号。技术人员将受测设备放到固定装置上并启动真空装置，这样，电路板就被固定了。测试工程师把带有弹簧的探针当作“钉子” ，把带有这种探针的固定装置当作“针床” 。To keep up with chang
14、ing 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. 为了跟上变化的印刷电路板技术，近些年来，在线测试设备已经增加很多功能。很多测试设备
15、具有分界扫描的功能，这样，测试人员就可以直接测试印刷电路板上的互连，而没用必要使用探针去测试每一个节点了。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 test
16、er 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. 对于表面安装设备来说，焊接开路是一个很普遍的制造缺陷，在线测试设备可以将其测试出来。为了完成这项测试，你需要将一个金属板放置到被测电路的上面。测试设备测量金属板和受测电路的每一条导线。由于开路的导线比连接线的电容要低，所以测试仪器可以检测出哪条导线连接正常，哪条导线是开路。Other Test Options其它测试项目An alternative to the in-circuit tester is the manufacturing defects analyzer (MDA).
