Electronic test - flying probe test

The manufacturing process of a Printed Circuit Board (PCB) includes a critical testing stage. Every PCB must undergo testing to ensure that the manufacturer can detect any electrical or circuit issues before the PCB leaves the facility. The most common types of testing methods include In-Circuit Testing (ICT) ,Flying Probe Testing (FPT) and 4-wire Kelvin testing (For PCBs use in automotive, or aerospace applications).

FPT is a highly precise and advantageous choice that is becoming increasingly popular, particularly with the rise of small electronic products. Now, let's learn more about Flying Probe Testing and its benefits.

What is Flying Probe Testing?

Flying Probe Testing, as the name suggests, uses "fly" test probes, which means these test probes move from one test point to another based on specific instructions provided by a software program written for the board under test to test multiple conductors or components. This method does not require custom fixtures, so it can also be referred to as fixtureless in-circuit testing when used on assembled PCBs. Therefore, it offers high cost-effectiveness for prototypes and small to medium batch production.

Flying Probe Testing used to be the standard for testing PCB assemblies. It is often compared to In-Circuit Testing (ICT), but it has been around for much longer.

Aspects involved in non-powered tests during Flying Probe Testing include:

• Resistance

• Capacitance

• Inductance

• Open circuits

• Short circuits

• Diode checking

• More

This testing method has a history of over 20 years, longer than other methods such as ICT.

What is In-Circuit Testing?

In-Circuit Testing refers to a system that utilizes a single probe to make contact with each test point on a PCB to check for manufacturing defects and faulty components by testing their electrical performance and connections. In-circuit testing (ICT) is more suitable for mass production as custom test fixtures are required for each PCB assembly. Due to the complexity of PCB assembly, these fixtures are often expensive. 

Both FPT and ICT are effective methods for testing PCB performance.

4-wire Kelvin testing

4-wire Kelvin testing, also known as 4-wire resistance measurement or 4-point probe measurement, is a technique used to accurately measure the resistance of a component or material with low resistance values. It is commonly used in applications where high precision and low measurement error are required, such as in the testing of electrical conductors, resistors, or semiconductor devices. PCBWay factory has 4-wire Kelvin testing, which is generally used in aerospace, military (PCBWay does not accept military PCBS), automotive, medical and other high-precision PCB boards

In a 4-wire Kelvin test setup, four separate electrical connections or probes are used to measure the resistance of the test sample. The setup consists of two sets of probes: the current-carrying probes (also known as the "source" probes) and the voltage-sensing probes (also known as the "sense" probes). The key principle behind this technique is to separate the current-carrying path from the voltage-sensing path, thereby eliminating the influence of lead resistances and contact resistances on the measurement.

Here's how the 4-wire Kelvin testing works:

1.Current-Carrying Probes: The two current-carrying probes are connected to the ends of the component or material being tested. These probes are responsible for passing a known and precise amount of current through the test sample.

2.Voltage-Sensing Probes: The two voltage-sensing probes are positioned on the test sample, usually at a specific distance from the current-carrying probes. These probes measure the voltage drop across the test sample accurately.

3.Measurement Process: During the measurement, a constant current is passed through the test sample using the current-carrying probes. The voltage drop across the sample is then measured using the voltage-sensing probes. Since the current-carrying and voltage-sensing probes are physically separated, the voltage measurement is not affected by the resistances of the connecting wires or contact resistances. This arrangement allows for a more accurate determination of the sample's resistance.

4.Calculation: The resistance of the test sample is calculated using Ohm's law (R = V/I), where R is the resistance, V is the voltage measured across the sample, and I is the current passing through the sample.

By utilizing the 4-wire Kelvin testing technique, the measurement accuracy is significantly improved, as it eliminates the errors introduced by the resistance of the connecting leads and contact resistances. This method is particularly useful when measuring low resistances or when high measurement accuracy is required.

How Does Flying Probe Test Work?

The process of flying probe testing is relatively simple and can be divided into the following three steps:

Step 1: Create an FPT Testing Program

This design aims to test the entire circuit board. It is typically done on an offline computer using an FPT test program generator application, which requires Gerber, BOM, and ECAD files. The BOM should be in Excel format, and the ECAD files need to be in CAD format.

Step 2: Upload the Program to the FPT Tester

Once you have created the testing program, it can be loaded onto the conveyor inside the tester, and the circuit boards will move on the conveyor to the test area where the probes work. The boards can be singular or multiple, depending on the program control.

Step 3: Probes "Fly" on the PCB as per the Testing Program

Next, the probes "fly" on the circuit board according to the testing program. They perform electrical and functional tests on various points of the PCB using test signals. The measurement results are further processed to determine if specific circuit sections meet the expected requirements.

If the circuit board doesn't achieve the expected results or deviates from the programmed settings, the probes will indicate a defect in that unit, resulting in a test failure.

The Advantages of Flying Probe Test

• Lower testing costs: FPT has lower costs compared to ICT because it does not require custom fixtures.

• Shorter development time: The runtime of FPT varies depending on the size of the PCB. In most cases, the testing time per board is approximately 5 to 15 minutes.

• Greater flexibility: The automation of FPT enhances flexibility in making test changes. While there are limitations for some more complex tests, the ability to handle variations makes this approach useful.

• Accuracy and precision: The positioning of the testing probes has high accuracy and reliability, helping prevent issues in the finished products.

• Ideal for small to medium-scale production: Flying probe testing is well-suited for small-batch production due to its low development costs and short development time. While it may not be suitable for mass production, it can still be used for samples and prototypes.

• High-tech features: FPT now includes high-tech functionalities such as phase difference measurement (PDM) units and micro-short detection.

The Disadvantages of Flying Probe Test

• Potential for physical damage: Since flying probes make direct physical contact with vias and test pads and tend to create small dimples on the board surface, some OEMs consider this a manufacturing defect. However, with the continuous advancement of science and technology, this problem will be solved by the appearance of upgraded flying probe testers.

• Potential for poor soldering: Sometimes, when a flying probe tester is working on a component that does not have a test pad, it is possible for the probe to come into contact with the component lead, resulting in loose leads or poor soldering.

• Not suitable for high-volume and large complex circuit boards:  FPT only has a small number of test probes working, and they must cover all test points on the circuit board. For smaller boards and samples, this is usually not a problem, but for large, complex boards produced in high volumes, it is not advisable.

Despite the above disadvantages, FPT is still regarded as an important testing method in PCB manufacturing and PCB assembly, and will always play a vital role in leading electronic products to achieve superior performance and high reliability.