PCB LAYOUT engineers must understand ICT flying probe online testing

As a PCB LAYOUT engineer, you must understand ICT flying probe online testing, so that you will know the significance of adding test points in the PCB design process. Many companies require that ICT test points be added to more than 95%, which is very difficult for PCB design. After completing all PCB LAYOUT work, it often takes a day to add test points.

Flying probe ICT basically only performs static testing, and its advantage is that it does not require fixture production and shortens program development time. Bed-of-needles ICT can perform analog device function and digital device logic function tests, with high fault coverage, but a dedicated bed-of-needles fixture must be made for each single board, and the fixture production and program development cycle is long.

Online testing, ICT, In-Circuit Test, is a standard test method that checks for manufacturing defects and component failures by testing the electrical performance and electrical connections of online components. Online testing in circuit tester is referred to as ICT (In-Circuit Test System), and its Chinese name is online testing, which is mainly used for testing assembled circuit boards (PCBA). The "online" here is a literal translation of "In-Circuit", which mainly refers to electronic components on the line (or on the circuit). Online testing is a testing technology that does not disconnect the circuit or remove the pins of components. "Online" reflects that ICT focuses on detecting assembly problems of circuit boards by testing components on the line or open and short circuit states.

It mainly tests the open and short circuits, resistances, capacitances, inductances, diodes, transistors, transistors, ICs and other components of the circuit board! It mainly checks the open and short circuit conditions of individual components online and various circuit networks. It has the characteristics of simple operation, fast and rapid operation, and accurate fault location.

In the early days, the industry also classified ATE equipment into the ICT category, but because ATE testing is relatively complex and also includes functional testing after power-on, such as TTL, OPAMP, Frequency, TREE, BSCAN, MEMORY, etc., ATE was separated into another category!

Basically, all large circuit manufacturers need to use ICT testing, such as ASUS, DELL, IBM, INTEL, BENQ, MSI, HP, etc.!

The world's largest ICT test equipment manufacturer is Agilent, and others include Teradyne, Astec T2000, Galaxy, and WINCHY.

It can detect manufacturing process defects and component defects by directly testing the electrical performance of online devices. For components, it can check out component value out-of-tolerance, failure or damage, and program errors for memory. For process, it can detect faults such as solder short circuit, wrong component insertion, reverse insertion, missing components, pin lift, cold solder joint, PCB short circuit, broken wire, etc.

Check the electrical performance of online components on the manufactured board and the connection of the circuit network. Be able to quantitatively measure components such as resistors, capacitors, inductors, crystal oscillators, etc., perform functional tests on diodes, transistors, optocouplers, transformers, relays, operational amplifiers, power modules, etc., and perform functional tests on small and medium-sized integrated circuits, such as all 74 series, memory type, common driver type, switching type, etc. ICs.

The faults detected by the test are directly located on specific components, device pins, and network points, and the fault location is accurate. Repairing the fault does not require much professional knowledge. The program-controlled automated test is simple to operate and the test is fast and rapid. The test time of a single board is generally from a few seconds to tens of seconds.

Online testing is usually the first test process in production. It can reflect the production status in time and is conducive to process improvement and enhancement. The faulty board tested by ICT can greatly improve production efficiency and reduce maintenance costs because of its accurate fault location and convenient maintenance. Because of its specific test items, it is one of the important test methods for modern mass production quality assurance.

Isolation (Guarding): In actual circuits, devices are interconnected and affect each other, making Ix ref, so they must be isolated (Guarding) during testing. Isolation is a basic technology for online testing.

In the above circuit, the connection between R1 and R2 shunts the current, so that the equation Ix=ref, Rx = Vs/V0*Rref does not hold. During the test, as long as G and F are at the same potential, no current flows through R2, and Ix=Iref, the equation of Rx remains unchanged. Grounding point G, since point F is a virtual ground, the potentials of the two points are equal, and isolation can be achieved. In actual practice, an isolation operational amplifier is used to make G and F at the same potential. ICT testers can provide many isolation points to eliminate the influence of peripheral circuits on the test.

IC testing: For digital IC, use vector testing. Vector testing is similar to truth table measurement, stimulating input vectors, measuring output vectors, and judging the quality of devices through actual logic function testing. For example, for the test of analog ICs, the voltage and current can be stimulated according to the actual function of the IC, and the corresponding output can be measured as a functional block test.