The Complete Guide for PCB Testing Methods in 2023

For obvious reasons, extensive testing is crucial for the production of high-quality electrical components. PCB fabrication is no exception because the seamless operation of circuit boards is what ensures the performance of all the electronics in the first place.

Let’s deepen into all the industrial methods of PCB testing which are valid and are widely used as of 2022.

Why is PCB Testing So Important?

mother board

Quality Assurance of sub-products, finished components, and the technology of fabrication at each stage of production is of great importance. The reason is that without careful testing, the risks of a high percentage or entire batches of malfunctioning PCBs increase drastically. 

Typical consequences of neglecting PCB testing are failures and defects in all the electronics produced. It absolutely leads to customer dissatisfaction, deterioration of brand image, and other issues that are extremely hard to resolve afterward. 

Technically, entire PCB testing can be divided into manual and automated and electrical testing of both the laminated materials and the following components:

  • Capacitors.
  • Resistors.
  • Diodes.
  • Fuses.
  • Inductors.
  • Integrated Circuits (IC).
  • Transistors.
  • Relays.

So, the PCB testing is essential for ensuring the required-quality laminate materials and mounted components as well as their assembly.

Benefits of PCB Testing

Let’s take a brief look at the actual benefits of having solid QA departments and robust methodologies in practice:

  • Minimize waste. Reduced amount of sub-quality components = less raw material wasted, which results in more efficient production outcomes.
  • Higher profitability. If a manufacturer succeeds in not producing poor-quality components, it saves on energy, water, working hours, and other types of expenditures but raw materials. 
  • Reduce returned products. Not producing failing PCBs result in close to zero returns and/or complaints from customers/ end consumers.
  • Optimize safety. Some issues in the production process may result in hazardous situations employees may be subject to (intoxication, electric shock, etc.). End consumers may also be injured because of sub-quality PCB since electronics is a dangerous type of goods by default.

Apparently, you have all the reasons to have the best Quality Assurance practices integrated.

Types of PCB Testing

Now let’s pay close attention to existing types of PCB testing. 

Quick note: you may have several key ones integrated into the production process (manual ones, or others that are done by machines installed as a part of the manufacturing line). Yet, you must have a QA department with other equipment units for testing included. 

Alternatively, you may send small selections of PCBs for testing in an independent QA agency to get the appropriate certification. Most likely, you will have to opt for both options.

1. Visual Inspection

It’s simply manual testing when a technician uses a magnifier and its bare eyes to inspect a small selection of PCBs for visible defects. These may include solder defects, scratches, missing components, board stains, etc. This inspection method is an absolute must-have in place.

Pros:

  • Affordable, easy to implement, and does not require a lengthy setup.
  • Capable of identifying the major PCB issues and solder defects.

Cons:

  • Prone to human error and fully relies on a technician’s skill.
  • A labor-intensive and time-consuming method that cannot be optimized. It also tends to be inconsistent for obvious reasons. 
  • Can only inspect visible solder joints and deteriorations.

2. In-Circuit Test (ICT)

It’s that type of testing considered the most reliable and robust one, though nearly the most expensive. Which, actually, depends on board size, design complexity, and other factors. 

ICT is more known as a bed-of-nails test. It’s powering up and actuating the individual circuity on the board. The “screening” ensures like 85-90% coverage of the board’s entire circuit pattern. This way, you are already 85-90% free of human error.

The test’s working principles include the use of fixed probes that are mounted in a way that matches the PCB design. These check the integrity of the solder connection. So, the “bed of nails” is actually the composition of probes on which a PCB lies. 

Actually, the access points that are normally predesigned on all the industrial PCBs serve exactly this test execution. They ensure the connection of probes with the circuit. It’s hardly possible to physically put probes under each connection (that’s why 85-90% coverage instead of 100% coverage), but the core joints can be tested successfully. 

As you may have already understood, you should have design-for-manufacturing in place in order for this test to be even possible to execute (in another way, you just did not plan access points).

Strengths of the method:

  • The coverage area can reach 98% of the total PCB circuit pattern.
  • An accurate and effective method of testing PCBs intended for mass production.

Weaknesses of the method:

  • Still does not ensure 100% coverage leaving room for an error.
  • Expensive method overall.
  • Require planned design.
  • Not suitable for half-finished prototypes.
  • Cannot access some defects like voids, excessive soldering, etc.

3. Flying Probe Testing (FPT)

The flying needle testing is seen as a modification or an improvement of ICT. In this method, there is no need for a power supply. A flying probe machine can access test points, untented vias, and the ends of components. 

It additionally can be programmed to check the value of passive parts. This way it allows for testing individual elements, including shorts, opens, and diodes, and distinct indications such as capacitance, resistance, and inductance. Tailor-made fixtures and design changes are of no need here.

Strengths of the method:

  • Cheaper than ICT and faster to implement.
  • High test coverage and ability to access additional elements and test indications.
  • No test points needed = more space available for routing.

Weaknesses of the method:

  • Does not suit mass testing as the method is too slow.
  • Some defects are inaccessible.

4. Automated Optical Inspection (AOI)

Such a method is technically the automation of the manual one described at the beginning of the post. Its working principle is the use of single (2D) or two (3D) cameras to take images of PCBs in high resolution. These are then verified against validated images of PCBs that passed testing successfully. Any deviations are detected.

An AOI machine is preferably a part of the manufacturing line serving as the method of rejecting sub-quality PCBs in the first place. It’s more like a primary line of Quality Assurance. 

The AOI method includes all the checks that can be performed manually. It includes some others as well. AOI machine performs all of those with greater accuracy as it can detect components placed nanometers off the required location. See what an image taken from a PCB looks like in the image below.

Automated Optical Inspection (AOI) for PCB

Strengths of the method:

  • Can identify major and minor defects with great accuracy.
  • More consistent than manual visual inspection.
  • Perfectly suitable for mass production and can be a part of a manufacturing line.

Weaknesses of the method:

  • Belongs to “passive” detection methods that are capable of finding surface detectors only.
  • Time-consuming setup of the AOI machine.
  • Database-based matching is inflexible and does not suit designs that may have some alterations. 
  • Database-based matching highly relies on the quality of reference images.

5. X-Ray Inspection (AXI)

This method is unique on its own level. AXI technology is based on the penetration of a PCB and its components with radio waves to generate a 2D/3D image of the solder joints and underside pads. AXI is perfect for testing hidden joints and can detect issues optical methods fail to find.

AXI can hardly be integrated in the way it’s a part of the manufacturing line. Yet, it can be used alongside the production process. QA engineers may use it on a small selection of PCBs from each batch to find defects early on.

In such a manner a manufacturer can spot the need to make adjustments in a timely manner. This drastically reduces the number of costly repairs. X-Ray inspection is more well-justified for parts with hidden pads.

Strengths of the method:

  • Shows the highest defect detection rate compared to other inspection methods.
  • Can target not just solder connection but long-time solder join quality as well.

Weaknesses of the method:

  • Extremely time-consuming method.
  • Expensive method.
  • Requires well-trained and experienced operators.

6. Functional Circuit Test (FCT)

This form of testing is normally implemented in the final stage of PCB fabrication. It is more like a final quality control procedure. Its working principle is making the device under test (DUT) with a PCB undergo a set of checks. Just to ensure that it functions as intended.

The specificity of the functional testing is that it can be simple as an on-off power test. But it can be complex as a comprehensive test with strict protocols and with the use of testing software involved. 

FCT can serve as a substitute for other testing procedures. It simulated the actual operating environment for a DUT. From this perspective, FCT is one of the most straightforward methods. In the real world, 100% functional testing is becoming increasingly prevalent, especially for small batches of expensive circuits. Just to make sure that each product is operational. 

Take a look at how this testing may be performed in the image below.

Strengths of the method:

  • The most flexible and highly customizable method.
  • Support testing of PCBs of any design.
  • Normally is more affordable than highly-specialized types of testing.

Weaknesses of the method:

  • Detection rates depend on the planned types of checks.
  • Requires well-trained and experienced operators that can define the root cause of the detected defect.
  • Boards that passed the tests still may fail under stress. 

7. Burn-In Testing

This form of testing somewhat complements the functional one. Its working principle is checking the performance under conditions exceeding the ones predicted for the normal operating environment. It helps to quickly determine hidden defects.

Burn-in testing does not only check for indicators of potential failure and test load capacity. It simulates extreme conditions to provoke failures. Some operation conditions are increased temperature, voltage, current, operating frequency, physical stress, if necessary, and others. The collected data helps to determine if a PCB can survive in a real-world environment.

Strengths of the method:

  • Checks performance under conditions that are inaccessible by other methods.
  • Ensure greater product reliability.
  • Is more affordable than some other types of testing.

Weaknesses of the method:

  • The testing process may shorten a PCB’s lifespan.
  • Some PCBs imminently fail in the testing process and are to be reworked.
  • It’s a time-consuming and labor-intensive process.

8. Other Functional Tests

Besides testing a DUT under normal operating conditions or under artificially caused stresses, there are other types of checks.  

These include:

  • Micro-sectioning analysis.
  • PCB contamination test.
  • PCB solderability test.
  • Time-domain reflectometer testing (TDR).
  • Peel test.
  • Solder float test.
  • Time-domain reflectometer. 

They are rather optional but still may be required by some customers or authorities that issue certification.

Strengths of the methods:

  • Can be freely combined with other types of testing.
  • Sometimes may serve as a replacement for other types of testing.
  • Good for detecting specific forms of functional failures.

Weaknesses of the methods:

  • Are not prevalent.
  • Do not serve as comprehensive checks. 

Final Word

In essence, every manufacturer receives way more benefits from having solid Quality Assurance and PCB testing in place rather than neglecting them. This is a simple dependency: the better PCB testing is = the better brand image and business profitability.

Among types of testing that are impossible to go without are: visual inspection, automated optical inspection, and functional circuit testing. Others may be outsourced to QA agencies and authorities that issue certifications.

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