How Does Flying Probe Test Works for PCB ?

Introduction

Testing is a critical step in ensuring the quality and reliability of printed circuit boards (PCBs). Flying probe testing is an extremely versatile technique that validates PCB assemblies without requiring fixtures or testbeds. This article provides an in-depth overview of the principles, processes, capabilities, and applications of flying probe testing for PCBs.

What is Flying Probe Testing?

Flying probe testing uses movable test probes to electrically check a PCB for:

• Shorts and opens
• Pattern defects
• Voltage and frequency response
• Passive component values
• Connectivity and signal integrity

The “flying” probes are mounted on arms or heads controlled by precision robotic motion control to target test points anywhere on the PCB surface.

Flying probes provide complete electrical testing coverage for assemblies ranging from simple boards up to complex multilayer PCBs with thousands of test points.

Advantages of Flying Probe Testers

Flying probe test offers significant benefits:

No Fixtures Required

Probes move freely to target all features, eliminating custom text fixtures.

Non-Destructive Testing

Light touch probes make non-permanent contact, avoiding damage.

High Test Coverage

Every testable net is validated for total quality assurance.

Fast Programming

Automatic test program generation from CAD data.

Flexible

Handles diverse boards without setup between changes.

Scalable

Multiple heads and probes enable high throughput.

For these reasons, flying probe is ideal for prototypes, low-volume, high-mix, or soldered assemblies where bed-of-nails fixtures are impractical.

Parts of a Flying Probe Tester

A flying prober consists of several key components:

Probing Heads

• Contain grouped flying probes on arms
• Typically 4-16 probes per head
• Probe styles optimized for precision and reliability

Motion System

• XY stage provides accurate head positioning
• Z axis control allows probing contact force

Probe Drive Unit

• Generates test signals that probes apply to the PCB
• Measures electrical responses from the board
• Checks for shorts, opens, impedances etc.

Probe Control Software

• Automatically generates probe test sequences from PCB CAD data
• Optimizes head paths and probe timing cycles
• Interfaces with test program generation tools

Flying Probe Styles

Several probe tip styles exist:

Vertical Probes

• Simple straight vertical probes
• Low cost, limited accuracy on small pads

Cobra Probes

• Angled flexible probes
• Higher accuracy on small pads

MicroSprings®

• Advanced compact spring probes
• Ultra fine pitch probing down to 25 μm pad size

How Flying Probes Test a PCB

The typical flying probe test process involves:

1. Load PCB

The PCB is loaded into the test system on precision fixtures.

2. Optically Align Board

Optical cameras align the PCB coordinate system to the tester.

3. Import Test Program

Test parameters are imported from CAD data.

4. Probe Test Points

Probes on multiple heads touch down on defined test nodes.

5. Perform Electrical Tests

The tester stimulates and measures based on programmed tests.

6. Move Probes

Heads reposition probes to cover all definable nodes.

7. Repeat Tests

Steps 4-6 are repeated until all points are verified.

8. Display Results

Pass/fail reports clearly identify defects for repair.

Flying probe testing provides a fast, flexible, and thorough validation of PCB assembly quality.

Testable Features

Nearly all PCB features can be inspected with flying probe testers:

Traces

• Opens and shorts
• Impedance measurement

Passive Components

• Resistors, capacitors, inductors
• Value and tolerance match

ICs and Active Components

• Presence and orientation
• Pin-to-pin signal verification

Board Interfaces

• Connectors, edge fingers, testpoints

Planes and Grounds

• Isolation from signals
• Resistance measurement

High Speed Signals

• Signal integrity at clock frequencies
• Time domain reflectometry

Program Generation

Creating test programs for flying probe testers involves:

CAD Data Import

• PCB CAD data (Gerber, DXF, IPC-2581) is imported
• Provides complete board connectivity model

Netlist Extraction

• Electrical test netlist is automatically extracted
• Defines connectivity between testable nodes

Test Point Definition

• Test engineer defines measurement types and pins for coverage
• Critical nets, suspect areas, known-good nets etc.

Program Optimization

• Software optimizes probe paths for speed
• Groups nearby points to minimize transitions

Test Program Export

• Completed test sequence is exported to the tester
• Typically in GenRAD or IPC-D-356 formats

This automation minimizes test programming time compared to fixture-based methods.

Test Coverage Challenges

Certain PCB features present challenges for flying probe testing:

Dense Chips

• Pitch under 200 μm prevents probing chip pads
• Requires separate loaded-board test

Internal Layers

• Cannot directly probe buried vias or layers
• Checks propagation to top layer nets

Soldered Components

• Probes can inspect only exposed terminations
• May obstruct access to pads

Uneven Surfaces

• Probes depend on a flat plane for contacts
• Steps greater than 75 μm impede probes

Supplementary fixture-based testing can verify the small subset of points unreachable by probes.

Improving Test Access

Several PCB design techniques can maximize flying probe test coverage:

• Provide probe pads for internal layer signals
• Fanout probe points to edge of PCB
• Avoid components blocking probe access
• Include text callouts for testpoints
• Add fiducials for optical alignment
• Eliminate unnecessary copper fills

Consulting manufacturers during design avoids producibility issues.

Flying Probe Fixturing

Fixtures allow rapid and secure loading of boards into the prober:

Figure 3: Example PCB fixturing for flying probe test

• Tooling rings securely hold PCBs
• Minimize vibration or shifting
• Guide camera alignment
• Probe area remains completely exposed

indexable platens support panels with multiple boards.

Grid Probing Optimization

For panels with many small boards, grid probing can boost throughput:

• Treats panel as uniform grid of test points
• Optimizes head travel between adjacent boards
• Avoids repositioning between boards
• Can scan a panel in under a minute

Grid probing minimizes bottlenecks for high volume PCB testing.

Test Coverage Analysis

Analyzing test coverage quality is key:

• Confirms all or nearly all points are checked
• Identifies difficult to probe areas for improvement
• Provides feedback to design engineers
• Correlates faults to escapes or blindspots

Flying probes routinely achieve 98%+ test coverage on well-designed boards.

Conclusion

In summary, flying probe testing delivers fast, accurate, and comprehensive verification of PCB assembly quality while minimizing the need for custom fixtures. The techniques enabled by advanced robotic motion control, probe technologies, and automation capabilities allow flying probe testers to inspect the vast majority of assembled boards delivered from fabrication and assembly houses. By offloading tedious manual testing, flying probesScale provide quality and reliability assurance in an efficient and cost-effective manner. With continual advances expanding their scope, flying probe testing solutions will take on an increasingly pivotal role in electronics manufacturing.

FAQs

Q: What are the key benefits of flying probe testing?

A: Key benefits are no need for custom fixtures, high test coverage, non-destructive probing, and flexible test automation.

Q: What size of PCB is optimal for flying probe testing?

A: Flying probes work very well for small to medium size boards up to 18” x 24”. Larger boards can present challenges.

Q: Can flying probes accommodate thick or odd-shaped PCBs?

A: Extreme contours or thickness variations can limit flying probe access. Some edge clearance is ideal.

Q: What are the limitations of grid probe optimization?

A: It speeds throughput but probes cannot reach board edges or between tightly spaced boards.

Q: Does flying probe testing replace other electrical test methods?

A: Flying probes complement fixture-based testing. Some powered-up and functional tests are still required.

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 PCB 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 PCB 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  Flying Probe Test:

• 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 electronic component polarity, orientation and misalignment.
• 8- Ability to focus and test Individual components
• 9- Suitable for highly complex prototype PCB electrical testing

Disadvantage:

• 1- Longer testing time and increased per unit PCB 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

PCB Test and Its Importance in Circuit Board Development

Quality is a primary aim for any printed circuit board designer. Therefore a PCB fabricator needs to provide prior quality assurances when approached for assembly or fabrication services. A perfect quality assurance technique entails a PCB test process. It is therefore typical for most contract manufacturers to have varied testing methods. However, while varied, the industry always has universal testing standards.

PCB Test as a Quality Control Measure

Imagine the financial implication of getting your PCB design fabricated and having it fail immediately after it gets delivered. As an engineer or designer, it’s a chance you cannot risk. It becomes vital to have the fabricator conduct printed circuit board testing. The process comes as an integral part of the fabrication and assembly process. Consequently, plenty of dependable electronic manufacturers will provide a plethora of PCB testing techniques.

PCB testing happens at different levels. It happens at the bare printed circuit board level, the PCB raw materials level, and the printed circuit assembly product. Such tests ensure the proper functioning of the circuit as designed, and the materials used to withstand the intended application. As such, the PCB has to undergo testing to ensure qualification, conformance, and acceptance.

What does a PCB Test Entail?

Testing and inspection often go hand-in-hand when it comes to circuit boards. It often covers a range of techniques that ensures the PCBs meet the set quality standards. The standards often determine the test, and with everything aimed at ensuring the proper functioning of the circuit board. Besides this, it also adherence to the project’s specifications and eradication any errors or defects.

Electric board tester for electrical testing and other PCB board testing approaches apply in these PCB tests and inspection processes. Remember, testing procedures aim to evaluate diverse components of printed circuit boards in-depth and ensure high standards. Additionally, tests focus on general PCB attributes, specific circuit board attributes, components, etc. So what does it all entail?

General PCB Attributes Testing

• Copper plating. Copper as an integral constituent of the circuit board gets tested for elongation and tensile strength. It often gets laminated on the circuit board to offer conductivity.
• The lifespan of a printed circuit board relies heavily on the quality of the lamination. For instance, when the laminate peels off, it can lead to adverse functionality issues—tests such as laminate’s resistance regarding peeling when subjected to heat or force that apply.
• It is an aspect that ensures proper attachment of components on the PCB board. Tests on solderability aim to eliminate defects on the ultimate product besides ensuring the proper attachment of components. Some of the vital factors analyzed include wetting, which determines the receptivity of the surface to liquid solder.
• A key aspect of any printed circuit board entails electrical conductivity and thus forms a test subject. How well a PCB conducts electric current devoid of excessive leakage becomes essential.
• Hole wall value. It is a vital quality aspect of the printed circuit board. The hole wall value or quality prevents cracking or delamination when the PCB is deployed in the field.  It is a parameter that needs analysis for cycling and rapidly changing temperatures to gauge reaction to thermal stress.
• It primarily happens for circuit boards with applications that entail humid environments. In such a test, the PCB gets tested for its water absorption capacity. The PCB gets weighed pre and post-subjection in a humid setting, and any significant weight change leads to its failure to make a grade.       
• It entails the analysis of PCBs for their capability to resists environmental conditions such as humidity and corrosion. The tests follow a similar route to the environmental test. The analysis gets performed before and after the exposure to determine the PCB’s resistance.

PCB Components Subjected to Tests

Printed circuit boards often have diverse and complex components that allow for their proper functioning. It entails components like capacitors, transistors, fuses, diodes, and resistors. During PCB development, the circuit board, complete with its components, undergoes a testing phase. Components that get tested include the following.

• It is a vital circuit board component responsible for storing energy as an electrostatic field. A capacitor blocks the direct flow of current by enabling indirect flow while storing energy. It becomes prudent, therefore, to test capacitors. Here, the voltage gets applied to analyze its proper functioning based on the expectations. Aspects such as leakage, shorts, or malfunctioning capacitors get exposed during this process.
• It comes as a tiny electronic device responsible for unidirectional current transmission. It functions by blocking the opposite transmission of current when transmitting. A diode proves sensitive, and thus, careful management when testing is prudent.  Always consult a professional before testing delicate components.
• It forms a key component of printed circuit boards. Resistors come very small and with two terminals responsible for converting current into voltage. Testing of resistors needs an ohmmeter. In instances where you want to self-test your resistors, isolate it and deploy a digital multimeter. You connect the leads on the resistors and then observe the readings. High readings indicate a problem like an open resistor.

As much as each component comes in isolation, they often work complimentarily in a circuit board. However, monitoring and testing each component becomes vital to ensure optimum functionality of the PCB. But if you need testing or monitoring services, do not hesitate to contact us via our RayMing PCB and Assembly website.

Specific PCB Tests

General testing of PCB aspects is important, and the same happens with its diverse components. However, all these often entail specific tests to ensure adherence to industry specifications. Some vital circuit board testing specifications entail MIL-PRF-31032, MIL-P-50884, MIL-PRF-55110, IPC-6018, IPC-6016, IPC-6013, and IPC-6012.  Some of the specific tests to achieve these specifications include the following.

• Continuity testing
• Electrical testing
• Dimensional verification and inspection
• Dielectric withstanding voltage
• Folding flexibility
• Flexural endurance
• Ionic cleanliness or/and ROSE (resistivity of solvent extract)
• Microsection analysis as received, after thermal shock, thermal stress, or rework simulation
• Moisture insulation resistance and IR (insulation resistance)
• Peel strength
• Marking ink test for solvents resistance
• Solderability as per J-STD-003
• Visual exam according to both IPC-A-600 and IPC-A-610
• Thermal cycling

Other board material testing include the following

• Advanced instrumentation that encompasses FTIR, real-time X-Ray, and SEM/EDS,
• CAF (conductive anodic filament)
• Copper testing as already stipulated (including purity, elongation, and tensile strength)
• EMC or electrochemical migration
• Environmental testing
• Flammability IPC-4101 qualification analysis that primarily deals with the laminate Material
• Conformal coating tests for MIL-I-46058 or IPC-CC-830 standards
• Soldermask testing for IPC-SM-840 standard
• SIR or surface insulation resistance
• Thermal tests through glass transition temperature (Tg), coefficient of thermal expansion (CTE), and DSC