How To Test Circuit Boards: Testing Methods and What To Test

Knowing how to test circuit boards to produce working products for your electronics is critical.

Testing is a critical part of any production process, especially when dealing with electronics, and it has several benefits.

For instance, you’ll identify faults, errors, and bugs early, resulting in few/zero returns.

These faults can also cause customer frustration, something you don’t want to encounter in business.

So we will explain what you need to check and the testing methods.

But first, here’s a detailed look at the benefits of circuit board testing.

Circuit Board Testing Benefits

In addition to the benefits explained earlier, circuit board testing has the following advantages.

Cost Savings

Testing in early production stages prevents wasteful manufacturing. Usually, manufacturers develop a few prototypes, then subject them to intensive testing to check all aspects.

And remember, testing begins in the design phase, so every step reduces errors before mass production.

These testing processes lead to cost savings because the final product that undergoes mass production is ready-to-use certified.

Time Savings

Cost savings usually goes hand-in-hand with time savings.

Testing during the early production stages sorts most issues, enabling you to move to full-scale production faster.

But testing after production will increase the lead time.

Better Safety

Tested products are safer to use, more so when dealing with circuit boards.

Faulty PCBs can short-circuit, leading to fires or electrical hazards. Such issues can put the user in direct danger.

A circuit board on fire after short-circuiting

What To Test in PCBs

Whenever PCB manufacturers test circuit boards, they look at the following aspects.

Copper Plating

Testing the copper plating involves checking its elongation and tensile strength.

Although laminated, it is critical to look at this layer.

Lamination

A circuit board’s lamination quality determines its lifespan.

It should resist peeling after applying heat or force because it protects the inner layers.

Via Wall

Via-hole walls should not delaminate or crack when the circuit board is operational.

So testing these parts checks their performance in high-stress environments.

For instance, the hole walls should remain intact in an environment with quick-changing temperatures.

A circuit board with several via holes

Operating Environment

Most circuit boards will eventually operate in humid environments. So it is vital to test if the material will absorb moisture.

The board should weigh the same before and after being placed in a humid operating environment.

So an increase in weight means the PCB has failed the test.

Electrical Conductivity

The primary function of a circuit board is to provide electrical connectivity between components in a compact package.

So this test is critical because it checks if the board can conduct electricity with little to no leakage.

Solderability

Boards require assembly after production.

So you must test for solderability to check if the components will attach firmly to the soldering pads.

For instance, you need to check the wetting property because it determines the quality of the solder joints.

Cleanliness

Cleanliness testing measures the PCB’s ability to resist humidity, corrosion, and other environmental factors.

PCB Testing Methods

Since you know what to test, here is a list of the typical tests done on printed circuit boards.

In-Circuit Testing

In-circuit testing is a robust PCB assessing process because it can locate 98% of faults.

Also known as the bed of nails test, the procedure uses the following electronic testing equipment to test the board.

In-Circuit Tester

This system comprises thousands of sensors and drivers to perform test measurements.

Software

The tester’s software instructs the system on what tests to perform on each board.

Also, the program indicates the parameters constituting a pass or failure after testing.

Fixture

The fixture is the hardware that connects the in-circuit tester to the board. It looks like a bed of nails, hence the test’s name (bed of nails test).

Each of these nails is a sensor, so the bed of nails connects to specific parts on the PCB.

Therefore, the board must have pre-designed access points for the sensors to make contact.

And the fixture applies pressure to ensure the nails have constant contact during testing.

These nails then feed information back to the in-circuit tester.

Generally, sensors are expensive hardware, making this component the most costly part of the ICT system.

The only issue with ICT testing is that it is best for analog circuits because it measures analog parameters like:

Capacitance
Resistance
Inductance
Shorts
Opens
Diode issues

Also, the equipment is not cheap. So it is best for testing during large-scale production, which usually occurs in the later production stages.

Flying Probe Testing

Also known as a Fixtureless In-Circuit Test, flying probe testing is a type of ICT.

And as the name suggests, it does not have the fixture component. So the hardware is significantly less expensive than that of ICT.

So how does it work? Technically, the tester still has a fixture.

But instead of having fixed needles, it has a few test pins that move in an X-Y grid (flying probe).

And the system obtains this grid from the CAD software used in the design process.

Flying probe test equipment assessing a PCB

Therefore, the testing system can do the same work as ICT but quickly adapt to test new boards.

However, the moving probe makes the system slower than the traditional ICT.

So this PCB testing method is ideal for small-scale production in the early prototyping stages.

Automated Optical Inspection (AOI)

As opposed to manual visual inspection, AOI used two 3D or a single 2D camera to snap images of the board.

The testing software then compares this image with the schematic diagram obtained in the design process.

If the two don’t match, the software will flag the board and inform a technician to inspect it.

A vision-measuring instrument inspecting a PCB

But the process does not use probes or inject power into the board. So it will not uncover any hidden electrical issues.

Therefore, you should not solely rely on this testing method.

Burn-In Testing

While the flying probe and bed of probes only test by sensing, the burn-in process injects power into the circuit board.

So the process is more intense but can burn certain parts if the wiring is faulty.

And the intensity is not only about the power injection.

The process can go on for 2-7 days (48-168 hours) while injecting power at the maximum specified capacity.

This intensity can damage the board, so it is not an ideal testing method for all PCBs.

But it is essential for medical or military products that should have maximum reliability.

Boundary Scan Testing

This PCB test monitors the wire lines on the board. And it is ideal for checking integrated circuits when it is impossible to reach all circuit nodes.

The process places cells in the leads (from the silicon to the outer pins).

So you can test a PCB without reaching all nodes. Also, you can evaluate high-density, multilayer integrated circuits.

Boundary scan testing is quite versatile because you can use it for:

CPU emulation
System-level tests
Flash programming emulation
Memory testing

X-Ray Inspection

X-ray inspection does not exert any electrical pressure on the board.

But it is a critical PCB testing method because it uncovers errors with:

Internal copper traces
Solder joints
Barrels

An X-ray of a PCB and its components

The process does not even require trained operators. However, it is expensive and time-consuming.

So it is ideal for detecting faults in multilayer boards or hidden joints where optical inspection won’t work.

For instance, you can use it to inspect BGA chip packages.

Functional Circuit Testing

Functional testing comes at the end of the manufacturing process because it tests the circuit’s functions.

And it checks the parameters provided by the customer.

The process is not necessary and takes time.

However, it provides quality assurance and longevity, critical factors that save you time and money.

Others

Other testing methods worth considering are:

TDR (Time Domain Reflectometry)
Contamination testing
Solderability test
Peel test
Micro-sectioning analysis
Solder-float test

PCB testing in progress

How To Test a Circuit Board for Repairs

A circuit board can also experience damage after use, and you need to test it to identify the fault.

Here are some methods to use.

Visual Inspection

Broken traces, shorts (burns), and some faulty components are easy to spot by looking at the board.

Electrical surges are the most typical cause of PCB damage in the field.

And they cause burn spots that turn the green solder mask black.

A short circuit on a printed circuit board

Also, look for discontinuities in the connectivity & I/O, delaminated vias, tin whiskers, etc.

You might need a magnifying glass or microscope for this inspection.

Infrared Imaging

Components undergoing shorting usually heat up, and it is easy to spot them using an infrared camera.

So power the board using a low electrical current level, then ramp it up while the PCB is under the camera.

The hot components should glow red hot in the image.

A PCB under an infrared image (note the red hot glowing parts)

Use an Analog or Digital Multimeter

Multimeters are versatile testing tools that check for voltage, resistance, or amperage.

So you can use them to assess various circuit components, such as resistors, diodes, and capacitors.

Since the device uses probes, it is a manual form of ICT or flying-probe testing.

An engineer using a multimeter to test a PCB

Wrap Up

As you can see, there are several ways of testing circuit boards. And you need to use a combination of at least two to do comprehensive tests.

For instance, you can combine AOI with ICT, functional, or flying probe tests for single or dual-layer boards.

But with multilayer PCBs, try X-ray inspection instead of AOI with either of the three above.