PCB Flying Probe Test (FPT):
We all are familiar with Printed Circuit Boards (PCBs) to be the core of electronic products functionality. These PCBs are consisted of the hard green color board commonly seen in all electronic equipment when it is opened inside. These PCBs are fully loaded with electronic components like resistors, capacitors, ICs, inductors, transformers, chokes, jumpers and connectors and many other stuff. The components along with PCB copper tracks are connected with each other and designed to carry out one or more than one functions. However it is utmost important that the PCB itself and the components residing on the PCB are not faulty and are not defected and connected correctly.
To check this, there are various tests that are performed by PCB manufactures or Contract Electronics Manufacturers (CEM) also called Electronic manufacturing Supplier (EMS). These tests are 1- In Circuit Test 2- Bed of Nails Test 3- Flying probe test. These tests are also called “Electrical Performance Test”.
These electrical test ensures that the bare PCBs or stuffed PCBs that leave the factory are comprehensively tested in the domain of electrical parameters. There are various types of electrical parameters like short circuit, open circuit, voltage, current, resistance and capacitance etc.
ICT vs FPT
There are strength and weaknesses of each of the two electrical testing methods for PCBs. ICT has its own advantages while FPT has its own, it depends upon the user requirement how the user want the PCB to be tested and what budget constraints of user are.
What is In-Circuit Testing ICT..?
Actually the ICT testing requires a solid “fixture” that the CEM will develop to access the test point in the circuit / PCB. This fixture is different for every different PCB design and hence the CEM has to develop this fixture and it take a lot of time in development of fixture. But once the fixture is made it can be attached to the in-circuit test system to simply perform the ICT within few minutes. The fixture is the interface between the circuit tester and the PCB.
The ICT is also called the Bed of Nails test because the fixture is actually the nails or points that go directly in the circuit board. The in circuit tester is the matrix or array of sensors and drivers setup to perform measurement. The measurements like resistance, inductance and capacitance between two points in the PCB can be done using ICT. ICT can also detect open and shorts.
The ICT testing is suitable large run or mass production of a particular PCB. This is because the initial design and development cost of ICT fixture is very high and the design fixture is different for different PCB designs. The ICT testing can access most of the nodes of the PCB to test and measure but some nodes that are hidden/shielded or under a larger component so these nodes remain untested and hence confidence will be less that the board still have some faults located in nodes that remained unreached due to incapacity of nails/ICT fixture.
Why to use Flying Probe Test (FPT):
The flying probe test also called “Fixtureless Testing” on the other hand do not have a bed of nails fixture except it has a fixture to simply hold the bare or assembled PCB board. The PCB is moved on the conveyer belt and brought directly under the robotic architecture of flying mechanism of testing system. The flying probe test (FPT) is basically the system of automated robotic hands that can move in x, y coordinates to gain access to each and every point on entire PCB and use z axis to move the probe vertically towards and away from the PCB. The robotic arm end effector is actually the probe that is connected to the back end testing system. The testing system is pre-programmed to carry out different measurements like voltage, current measurement, and impedance measurement, components value like resistance, capacitance and inductance. The component orientation and component polarity detection is done by a high definition camera installed on the flying probe setup. There are multiple probes (as few as 4 and as many as 20 needles) that can take measurement simultaneously of different parameters (voltage, current, resistance and continuity) between any two points on the PCB.
The PCB is fixed on the particular stand or fixture and the probes are “flying” at very high speed due to robotic arm both on top and bottom of PCB. The name flying is because the test probes are flying all the way on the PCB and can get access to any point on PCB as the FPT system is programmed.
The system is highly accurate the probes simply touch the test points on the PCB precisely and take measurements and then probes fly away from that point and take readings on other point of PCB. These probes are very sharp needle and interestingly do not damage the PCB while testing. With FPT, costs-per-unit are higher compared to ICT because of longer test cycle time periods per board (up to 15 minutes).
Advantages of FPT:
- 1- There is no need of custom tooling for fixture development in FPT as in the case of ICT
- 2- Programming the software to carry out test, takes less time than ICT
- 3- Can check open circuits, shorts, capacitance, inductance, impedance, tolerances of components
- 4- The initial or upfront cost of FPT is lower than ICT
- 5- Can access those points on PCBs which cannot be accessed by ICT fixture.
- 6- Ability to do on board verification of FPGAs.
- 7- Can check component polarity, orientation and misalignment.
- 8- Ability to focus and test Individual components
- 9- Suitable for highly complex prototype PCB electrical testing
- 1- Longer testing time and increased per unit cost
- 2- Not suitable for large production run or mass manufacturing testing of PCBs
- 3- Connector and in-active components testing inability
- 4- Components operating together cannot be tested with FPT
- 5- Testing only one point at a time unlike ICT where a fixture can directly insert nails into multiple point on PCB to test simultaneously.
Factors that determine which to select:
Both ICT and FPT have their pros and cons, it is up-to to the customer to decide which test methodology to select. These are 4 main factors that helps which method to select
- 1- Budget
- 2- Lead Time
- 3- Expected Volume
- 4- PCB complexity
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