Flying Probe Testing

The Significant Impact of Flying Probe Testing on PCB Assembly

The testing process is crucial in printed circuit board manufacturing. Before any board can leave the manufacturing plant, it must undergo stringent tests. Also, these tests help to detect potential problems in a circuit board. Ascertaining the functionality and performance of a board before it leaves the facility is of great importance.

In PCB assemblies, the two most common test methods are In-circuit test and flying probe test. Flying probe testing is a cost effective testing method that offers prototype verification. This testing method is very reliable. This article seeks to discuss every important detail about flying probe testing.

What is Flying Probe Testing?

Flying probe testing is simply an automated system with probes that maneuver or fly around the bottom and top of a circuit board. These probes fly simultaneously to make contact with the test points. It is a type of test in which the probes move from a test point to another. Also, flying probe tests are ideal for prototype circuit boards and low volume PCBs since it is easy to program.

Flying probe test don’t require test fixtures. Therefore, this removes the high cost programming time and enables design changes with test programs. Also, flying probes enable connection to test points. This test can be also known as fixtureless in-circuit test if no custom fixture is required.

The flying probe testers were specifically designed for use in bare board testing only. With time, they are now the standard for testing bare boards. However, flying probe testers measure inductance and capacitance. Flying probe tests can test for opens, shorts, resistance, and more.

The principle of the flying probe technology is that it utilizes a generic board holder. Also, probes move across the circuit board evaluating each node under software control. Therefore, the flying probe tester can reduce the number of test fixtures needed. Also, it is easier to introduce changes to features like pad positions.

Unlike in-circuit testing, flying probe use probes to replace a bed of nails. Flying probes use moveable and fixed probes to perform a simultaneous test of the bottom and top of your circuit board. The flying probe tester features high-precision needles.  Some testers use four needles while others may use as many as 20 needles for each PCB side. Also, the flying probe test needs no custom tooling. Therefore, it is a cost-effective method in the early stage of development.

How Does Flying Probe Test Work?

The flying probe test is ideal for testing prototype and low volume PCBs. It is important to know how this test works when being used. This is how the flying probe test works.

Design an FPT test program

Creating an FPT test program helps to test the whole circuit board. You have to create this program in an offline computer. Also, you will need an FTP test program generator software to achieve this. This requires ECAD, Gerbers, and BOM files.

Upload the program

The FPT  will place the PCB that needed testing on a conveyor belt in the FTP tester. This board will move to the area where the probe will be executed. The test software program regulates how the probes can contact the pads.

Application of power and electrical test signals

FPTs will integrate power and electrical test signals to the probe points. After this, such tests will read the measurements. Also, this processing will verify if a specific PCB part meets the expected outcome. Any deviation from the already established programs, this indicates a defect.

What is the Function of the Flying Probe Tester?

The flying probe tester is already programmed to perform functional testing and electrical tests to confirm the functionality of a PCB. This tester features enough DC and AC power supplies, various sensors and frequency counters. Also, it comprises signal generators to excite the nodes and make accurate measurements.

A flying probe tester performs signal integrity testing on assembled boards and test open pins on ICs. Also, this tester checks for opens and shorts on inductors.  It is a machine with high precision needles that carry out electrical tests to evaluate a PCB’s performance.

FPTs feature a camera to inspect component polarity. This tester is a machine that utilizes an XY grid gotten from basic CAD. These coordinates are already programmed in to match the printed circuit board. The needles on the tester come down at a 45o angle. Also, the prober requires two access points on the printed circuit boards to make connections.

The machine programmer identifies the value and sets it. After this, the machine verifies the value. One benefit of FPT is that it can be placed on small pads or solder connections with a high level of accuracy. A flying probe tester utilizes more test probes to have contact with the PCB under test. These probes move from one place to another on the PCB to perform tests of multiple components or conductors.

Flying probe tester programming requires transforming CAD data into an applicable file. After this, the generated file will go through a test program with new files. These files will be designed according to UUT test requirements and demands.

Guidelines and Recommendations for Flying Probe Testing

There are guidelines and recommendations that help to increase accuracy when using flying probes.

Board rail

The circuit board should have a minimum of 3mm wide boarder edge. The empty margin will help to handle the PCB inside the tester. The boarder edge can panel waste or space in the board’s design. Flying probe testers require reference points to properly place the probes. Also, these points are on the panel waste. However, putting them on the PCB can come in handy especially when there is no panel waste.

Probe points

Ensure you have accessible probe points at the PCB’s bottom side for both power and ground rails. Accessible points can speed up test and minimize overall cost.

Component legs

Ample space is important on the “toe” in order to probe near the component legs. This will enable a good solder joint. But, it is better not to probe the component legs during tests. This is because the pressure of the probe can push the component legs.

Component height

It is advisable to ensure the heights on the two PCB sides are 40mm and 90 mm respectively. If it is above this height, it can result in a “no fly” zone. Therefore, this can make accessing the assembly difficult.

Via

It is possible to probe on the edge of printed circuit board vias. However, exposing the vias will provide better results. Therefore, include the “non-tented” specification in the PCB design.

Cleaning

Make sure you clean the probe areas before testing. Remove any influx and ensure you use pin-probeable flux. Cleaning the assembly before testing guarantees reliable results. Failure to clean the assembly can result in a false fail. This can increase test time in the long run.

Difference Between ICT and Flying Probe Testing

Flying probe testing and ICT are great testing methods for PCBA. The major goal of these two testing methods is to detect any defect in a PCBA. However, there are differences in the way each test method operates.

The In-circuit testing (ICT) involves the use of a nail bed fixture. Also, it tests the functionality of a PCBA i.e white box testing. While in-circuit testers uses nail bed fixture, FPT uses flying probe. The ICT fixture comprises spring-loaded pogo pins that make contact with the PCBA at the area of the designed test points. Every pogo pin links one test point or node in the PCBA under test.

Flying probes don’t use the bed of nail fixture. Rather, it uses fixed and moveable probes to perform simultaneous in-circuit test.  This testing uses fly probes that move from one test point to another according to instructions provided by the specific program.

FPT is a cost-effective solution for a low volume circuit board. The In-circuit testing is ideal for high-volume and more advanced PCBs. However, the lead time and up-front costs with ICT testing  are longer and higher. The flying probe performs better than the bed of nails tester.  Also, the flying probe performs programming more quickly and more easily than in-circuit testing.

In-circuit testing can perform on board verification FPGAs. Also, this testing can check the integrity of soldering of BTC components. While ICT requires custom tooling, flying probe doesn’t. Also, ICT features bed of nails fixture with pins that contact the PCBA at test points. Testing probes offer improved test coverage.

Features of Flying Probe Testing

Flying probe test has some advanced features which are

Phase different measurement unit (PDM)

This unit transmits a high frequency signal between the signal line’s sections and reference line. Also, it evaluates the signal phase at the other endpoint. Therefore, it estimates their phase difference.

High voltage stress test (HVS)

This test evaluates high resistance unidentified isolation defects by PDM. HVS integrates high voltage pulses between signal lines to identify high resistance problems. This test provides the option of using about 500V to 1000V.

Micro shorts detection

The use of sudden high voltage can make parts with micro shorts to burn out. Therefore, detection of micro shorts enables the use of low voltages and increases it gradually to prevent any damages.

Benefits of Flying Probe Test

Flying probes are commonly used for testing the performance of circuit boards. This test is widely used in the printed circuit board industry. However, there are benefits and limitations of using this probe test.

Reduced test development time

The program used for flying probes is quick to develop from the PCB design files. Flying probes don’t require mechanical fixtures. The designing of the test programmer for FPT demands the PCB files. Also, flying probe tests reduces cost and time compared to permanent test fixtures.

Cost effective

Test probes is a cost-effective solution for low volume printed circuit boards. Also, it is ideal for testing prototype circuit boards. This type of testing method doesn’t require any custom fixture. Therefore, this fixtureless in-circuit test is highly cost-effective.

Easy to make changes

You can change the software to make any changes to pad components. Also, the software responsible for the movement of probes is easy to change. In addition, it is easier to adjust the probe landing location through software change.

Limitations of Flying Probe Testing

Slow speed of operation

When you compare flying probe testing to other testing, you will realize the flying probe is slower. This is because these probes move physically to every position in turn. Also, the limited number of probes can make testing take a longer time.

It isn’t ideal for some components

Flying probe testers may not test other components aside from passive components. These testers don’t test components working together.  To get greater levels of defect identification technologies like boundary scan. Also, on-board memory allows you to undertake more complicated tests.

Crucial Factors to Consider when Choosing Flying Probe Test

Flying probe isn’t ideal for testing some areas of a circuit board. There are factors that help you decide whether to use flying probes or not.

Product design

The PCB complexity and accessibility to the board will determine if FPT machines are an option. Flying probe is excellent when probing the ends of component pads. Also, they can evaluate electrical networks in exposed vias.

Expected volumes

When you are testing a large volume of circuit board, ICT is the best option. However, FPT is ideal for PCB prototypes and small volume PCB.

Lead development time

The lead development time for the flying probe is short. This is because this testing requires no custom test fixtures. Generally, the development lead time for a flying probe is less than 7 days.

Cost

Flying probe doesn’t cost as much as an in-circuit test. An ICT fixture costs much more than the programming cost of FPT and ICT systems. If you are working with a tight budget, flying probe testing is a great option.

Conclusion

Flying probe testing is ideal in PCB assembly. This method of testing has several advantages like minimal lead time. Also, there are no additional costs for fixtures. FPT is ideal for low-volume PCB productions. Flying probe test is gradually gaining popularity in the PCB industry.

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