PCB Testing-An Ultimate Guide To Prototype & PCB Assembly Testing

PCB applications have become increasingly popular and diverse. At the moment, PCBs manufactured by engineers are used to power circuits that run on military gadgets just like those that support human lives in hospitals. Moreover, because people’s lives are clearly at stake, mistakes in PCB productions are not only discouraged but forbidden. Thus, PCBs testing becomes essential in ensuring that the boards perform flawlessly and as expected from their designs.

 

Then again, attaining perfection in human-made productions is easier said than done; man is to error. At times, even as an established engineer, you might blunder a bit and develop some cold solder joints or other PCB production-related defects. At the moment, very few Contract Manufacturers test their boards extensively. As WellPCB, we take pride in being among the best in PCB testing niche.

 

So, how will you get to know where you screwed up a bit? Moreover, how can you cut on the high cost of PCB testing but still deliver quality electronics? To understand more about these age-old worries, consider reading through our comprehensive guide on PCB testing for unit productions and PCB assembly.

1、PCB Flaws In A Nutshell

1.1 A Billion Dollar Mistakes That Could Have Been Avoided

In 2014, General Motors spent close to USD 4.1 billion to repair and compensate a defect in the circuit of one of their car model’s ignition switch that their engineers had (apparently) overlooked. Now, think about it again. How big is an ignition key when compared to a functioning car?

 

Here was a wholly functional vehicle system tested in all standards (even for the costly vehicle crash tests) but was let down by a circuit mistake in the ignition board. Probably, not even the entire circuit; maybe just a defective transistor could have been the cause. Also, (gulp) there was a $4.1 Billion mistake from a PCB!

 

Was the case of General Motors an isolated case? No. About a year ago, Samsung spent close to $5.3 billion to take back and compensate a defect of the Samsung note seven smartphone’s charging system. So yet not Samsung alone; 33 million McDonald's fitness bands were recalled just about the same time (the cost was never disclosed).  

 

Generally, stories of PCB defects that cost billions of money to repair are becoming the bread and butter of the modern electronic design stories. Startups do them just as much as established companies do them. The bitter truth is that these mistakes aren’t pardonable by the market. So, how will you know when a PCB has defects?

 

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1.2  PCB Testing Stages And Indicators Of Poorly Tested PCBs

Sometimes, it is effortless to detect faults in PCBs. For instance, by visually inspecting a PCB, you can notice the common defects like those of the brown-colored overheated joints. Solder joints with too much solder, or insufficient wetting and unstable components with disturbed joints (you can read more about these defects in cold solder joints). This method of detecting defects in known as Manual Visual Inspection (MVI).

 

To increase accuracy in MVI, you might want to use a hand lens to check through the joints and ensure that there are no openings on the joints. You can do this inspection through checking for light penetration at the joints with the aid of a lens.

 

However, sometimes eyes too might fail to detect some PCB defects that can affect a PCB’s efficiency. Most of such defects are only notable through testing the effectiveness of the device components. Such extended evaluation requires improved and specialized equipment. It is also done under rigorous variables assessments like currency, voltage, capacitance, and resistance. These variables too could dictate using other specialized equipment as we shall discuss in the later chapters.

 

By comparing the expected input and output on various components against the actual measurements, you can be able to know if a few parts of the board are defective.  This process is known as In-Circuit Testing (ICT). ICT, in itself, would become complicated when more components are used in a circuit. Because of these complications, computerized equipment has been developed to ease the process of testing individual PCB components. Most of these devices will be discussed in the later as well.

 

The last indicator of a poorly tested PCB is an increase of complaints from customers. Now, primarily, all devices attract some terrible reviews from users. It is very normal. Some customers may critic a physical design; some may critic a feature, and any other such thing. However, when complaints about a particular electronic persist over its efficiency or performance, then that could be an indicator of a PCB defect. Such types of PCB flaws are the most undesirable, hardest to detect and most expensive to organizations. They rarely occur. When they happen, they have the most significant impact on the affected companies.

 

Now, let’s look at the two common types of defects available in circuit boards.

 

1.3: Types of PCB Faults

There are two main types of PCB defects: structural defects and electrical defects. Generally, almost all flaws in PCBs can be classified as falling into either of the two categories.

 

1.3.1: Structural PCB Faults

Structural PCB defects are a result of errors associated with the inappropriate structuring of a PCB when manufacturing PCBs. These faults are the most common types of faults in PCBs. They include defects like:

  1. Open circuits: like the name suggests, these are incomplete circuits that are at times overlooked during manufacturing. Most open circuits are solder-related, and they occur during soldering; often as a result of not melting the solder appropriately. These errors have the highest preference of occurring in PCBs with a 25% occurrence rate in PCBs.
  2. Insufficient solder: these defects are also solder-related. However, they often occur as a result of insufficient wetting of a joint. Inadequate solder defects have an occurrence preference rate of 18%
  3. Short-circuiting: even though short-circuiting can be classified in both structure and electrical defects, short-circuiting has a higher preference rate of occurring as a result of a wrong structure than as a result of a defective electrical component. The defect occurs at a rate of about 13% in the overall PCB defects.
  4. Missing electrical component: these errors occur with a preference rate close to that of short-circuiting (about 12%).
  5. Misaligned components: misaligned components can at times be noted after designing but just before assembly. These defects occur at a rate of about 8% on the overall number of defects during production.
  6. Excess solder: even though excess solder can be problematic, hardly do excess solder cause defects in production. This argument is correct because excess solder is easy to notice and rectify. However, there are times that these defects occur. These errors occur at a rate of about three percent.
  7. Missing nonelectrical parts: These errors hardly occur. They have a very low preference rate of occurring of about two percent.

 

1.3.2: Electrical Defects

Electrical defects are the types of flaws that stem from the wrongful use of the electrical components in circuit boards. In modern times these types of errors rarely occur and even when they do occur they are not as fatal as design errors.

  1. Defective electrical components: these defects occur when defective components are soldered onto circuits. They are also easy to detect and rectify. These errors have a preference occurrence rate of about eight percent.
  2. Using wrong components: sometimes, defects can occur when wrong components, e.g., resistors with different ratings that the recommended ratings are used. They occur at a rate of about five percent.
  3. A wrong orientation of components: these defects occur when components are soldered with wrong orientations. They occur at a rate of about two percent.  

 

1.4 Chapter Overview

Comprehensive testing of PCBs is often implicated in the quality of electronics produced by a manufacturer.  Well-tested PCBs where testing gives an eye to details result to high-quality electronics just as poorly tested PCBs result to flawed electronics.

 

As you might have observed from the few illustrations, PCB flaws are always expensive. To startups, PCB blunders could cost a company its reputation. To developed companies, defects could cost them money and part of their reputation. Both the two losses are undesirable

 

In the olden days, many emphases were paid to the errors that could be done at the designing phase of PCBs. However, with improved technology, there is an improvement in the design of PCBs nowadays. More emphasis on PCB testing has been taken on the process of manufacturing PCBs itself. That is why any electronics manufacturing company interested in staying in the market over a more extended period, must invest more in the testing of PCBs.  

 

Going forward, we will now have a keener look at the popular testing methods and industrial equipment that can be utilized to test PCBs.

 

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2、PCB Testing Methods

When developing local PCBs as a hobbyist, you might not need to have any consistent strategy to develop the PCBs. Consequently, you will not require a testing strategy either. Once you have a rough sketch of the circuit you expect, everything can fall in its place.

 

However, commercial PCB production and PCB testing do not gamble in its implementation procedures. It follows a laid down strategies that aid evaluation of efficiency and tolerance to faults. Currently, there are about seven approaches to testing PCBs. Of the seven, only three of the strategies are dominant in the industry. These three dominant categories of testing include Manual Visual Inspection (MVI), In-Circuit Testing (ICT) and Functional Testing (FT).  

 

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2.1 Manual Visual Inspection (MVI)

That is the primary method of testing PCBs. It involves looking for faults in a PCB with an unaided eye. MVI is the simplest, oldest and still the most popular way of inspecting PCBs. It is applicable for simple, low-volume production of PCBs.

 

However, it is not very efficient for high volume, or complex PCBs as human eyes (unaided) might not notice some of the connections that might be hidden. Also, when inspecting high-volume productions, humans might tend to get bored or tired. Because of these two main drawbacks of MVI, there are other methods one can employ to minimize errors even though closely related to MVI. Top in this list is the use of lenses.

 

2.3 Using Microscopes And Lenses

It is possible that your PCB might fail to meet its design requirements even though unaided MVI might show you that all is well. Here is when you will need to have a closer view over a lens or a microscope.

 Sometimes our eyes might not be so effective in detecting some of the minor defects in solder joints of PCBs. However, with the aid of lenses, you can magnify the boards and bring the connections on the board to a closer examination.

 

2.4 Using X-Rays

X-Ray scans on PCBs give many details about a board that cannot easily be seen even under a lens. X-Rays are particularly necessary when examining boards that have hidden connections. You will need to take a few scans of the same board under different angles and then compare the images with the expected connections from the original design specifications.

The only problem with using X-rays is the fact that it is expensive and it may not be applicable for hobbyists and small-scale producers.

 

2.5 Using A Saw

Surely, slicing through a board is indeed a bad idea. However, there are times when a PCB might indicate questionable connections in hidden parts (often after analyzing an X-Ray image). One of the easiest ways to settle the worries is to slice through the board with a saw and inspect the underlying connections of the faults.

 

In so doing, you will be able to observe internal errors and rectify them to have better productions of other boards.

 

2.6: Automated Optical Inspection (AOI)

 

When producing large numbers of PCBs, MVI becomes tedious and boring.  AOI is an automated improvement of MVI. AOI systems take snapshots of various parts of the board and use the images to construct a representation of the actual board on a comp which is then compared with the original design.  

 

AOI is faster and more accurate than MVI. However, it is expensive to acquire and maintain for simple projects.

 

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2.7 In-Circuit Testing (ICT)

In-Circuit Testing is the most comprehensive way of testing a PCB for defects. It aims to test the components of a board to ascertain their efficiency in the entire prototype board. Usually, the PCB to be tested is subjected to a bed of nails that get connected to different components selectively to assess their performance as units.

Though it is the most recommendable way of testing, ICT is often expensive, tedious and time-consuming. Thus, it is usually reserved to test PCBs that produced on a large scale. A different and newer version of ICT testing that is cheaper and offers a more economical way to test prototypes is known as the "flying probe" method that is discussed in details when in chapter 3 when talking about tools.

 

2.8 Functional testing

It is always the last phase of testing of PCBs. Once a PCB is made, a functional test is a test comprising of set programs that are uploaded onto the test device to see if it performs its functions as expected.

 

Conclusion

Primarily, it is not that these methods are perfect; they are just less prone to errors. This assumption is true because the strategies have been subjected to constant inspection and reviews to improve their effectiveness.

 

These methods (to a great extent) also dictate the type of tools that are developed for testing PCBs. In our next chapter, we will now narrow our focus to the different tools that can be used in industries to test PCBs.

 

3、Industrial PCB Testing Equipment

From the previous chapter, we had highlighted some of the methods you can deploy to find faults in PCBs by checking them manually. In other cases, you might use to employ lenses and other devices such as a Multimeter for the same purpose.

 

Then again, when working as specialized manufacturers, these methods could become boring and tedious. Daily, you will be subjected to incredibly complex circuits with different specifications. Under such conditions, unaided MVI becomes defective. In this chapter, we will look at some of those critical tools we use in industries when testing PCBs. Below are some of the tools.

 

3.1 The Automated Optical Inspection (AOI)

Now, the fundamental requirement of PCB testing is to ensure that PCBs produced to meet all the conditions included in their designs. However, as I had indicated in the introduction, sometimes errors are overlooked during production. AOI machines are machines that help you to scan through a PCB and check for design irregularities. 

 

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AOI machines are used to aid Manual Visual Inspection (MVI) during PCB manufacturing. They consist of a system of coordinating cameras and a platform for holding the PCB to be inspected. The board is then lit from different specific angles as the cameras take image scans from the angles. Using the images, AOI machines can construct an abstract representation of the PCB and compare it with the design specifications of the original PCB design.

 

These machines do some MVI in a faster and more accurate way that could be done manually. Primarily, Automated Optical Inspection tools focus of minimizing structural errors.

 

Currently, there exist both 2D and 3D AOI machines. Of the two types, the new 3D AOI machines perform faster and better than their 2D predecessors. The 3D AOI machines are also sensitive enough to detect large power-related errors.

 

Advantages of AOI machines

  1. They are popular than other machines
  2. They detect structural errors faster
  3. They can be utilized during the process of manufacturing to minimize the overall time spent in final testing.

 

Disadvantages of AOI machines

  1. They cannot examine hidden connection or underneath BGAs
  2. They are ineffective when examining loaded boards with hidden components.

 

3.2 Flying Probe-Testing Machine

Flying Probe Testing machines (sometimes referred to as Flying Prototype machines) are devices that provide a platform for assessing the performance of prototypes. These machines differ from AOIs in that they perform some In-Circuit Testing of PCBs. They are preferred to older methods of testing because they are cost-effective and easy to operate.  

 

The Flying Probers consist of a board of pins that are interconnected to different components on a board. The components then get tested through either powering or sending signals and assessing their inputs and outputs with those expected in their designs. These machines perform a form of rapid in-circuit testing approach in testing components to ascertain the effectiveness of prototypes.

 

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Advantages of Probe-Testing machines

  1. They are cheaper and faster than the ordinary ICT testing machines.
  2. They increase quality as they allow engineers to produce prototypes and add changes on them before final releases.
  3. They are easier to operate than traditional ICT machines.

 

Disadvantages of Probe-Testing devices

  1. They are not detail-oriented like the conventional ICT machines.

 

3.3 Impedance-Control testing Machines

The physical design of a circuit on board and the amount of current passing through the board affect any AC circuit passing close to the same circuit. A combination of these effects and the resistance through the circuit form the impedance of a circuit.

 

Impedance testing is often tedious to perform. You will need to determine the conductor length, the spacing between the circuits, width & height of the conductor as well as time appropriately to measure impedance. A Time Domain Reflector (TDR) is used to measure the impedance of a board. However, normal TDRs are not very useful in automated testing of impedance. Improved Impedance control testing machines are therefore employed when manufacturing PCBs on a larger scale.

 

3.4 Copper Thickness Testers & Hole Examination Machines

The copper thickness testers and hole examiners are reasonably related machines. These tools are usually over-the-desk devices that are used to measure either the width of a drilled or the thickness of the copper layer interconnecting components.

 

A sample Copper Thickness Tester

A Copper Thickness Tester measures the thickness of the copper. A typical Thickness Tester looks as shown in the image above.

 

 

A Hole Examination Machine

Hole examination machines are used to estimate the diameter of holes by inserting pins with different diameters. A typical hole examination machine could look like the one shown above.

 

3.5 PCB Electrical Test Systems

Once all the components get welded onto the PCB, it is paramount to perform some In-Circuit Testing and ascertain the efficiency of the integrated components. Usually, this phase should come after a thorough ICT. Without a comprehensive ICT testing, this type of testing can only indicate the probability of an existing problem without necessarily giving out a way to locate or solve the error.

 

Generally, any other system that is developed to analyze the efficiency and reliability of a circuit board could fall under this categorization.

 

3.6 Chapter Overview

So far, we have highlighted the need to have PCBs tested as well as shown some of the machines used in the industry to test PCBs.  In the remaining chapters, we will now focus on the Assembly testing of PCBs.

 

4、PCB Assembly Testing

4.1 Introduction

Assembly testing of PCBs is a type of testing that is done after the electrical components specified in a Bill Of Materials (BOM) of a PCB get fitted onto a board. PCB assembly testing is closely linked to ICT (that we have already discussed before).

 

Usually, testing is done on a bed of nails that is fixed on a flat surface with access to multiple components connected at the PCB. Depending on the type of testing that is scheduled to be undertaken, the board may or may not be powered.

PCB assembly testing is done in two approaches:

  1. Testing a single component/module on board and investigating its correlation with every other component within the network of electrical components/modules. This test is often performed for individual components and advanced to every other component within the network that shares interests with the element that is tested.
  2. Testing by sampling a few specific critical components on the circuit and investigating the outcomes of the component(s) performances with the expected outcomes.

 

4.2 Benefits Of Assembly Testing

  1. An entire circuit board is tested for power faults like shorts or incomplete circuits. The test also establishes the presence of defective components on a prototype.
  2.  This type of testing also involves power-up tests that help to minimize a customer's debug needs significantly
  3. It can detect nearly all possible faults in a PCB
  4. Has an Operating System platform-independent PCB testing platform
  5. Testing follows accepted universal IPC standards

 

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4.3 Demerits Of Assembly Testing

The main issue with Assembly testing is that the testing of a single prototype PCB takes a lot of time and financial resources. Sometimes, testing a single complex board can take between five and six weeks, and cost for testing such a board can go very high. Assembly testing is however useful and is more accurate when testing PCBs than any other method of testing. It is therefore ideal for developed brands that require high standards of quality and massive productions.

 

5、Conclusion

Testing of PCBs is a pivotal step to all PCB designers and engineers. As seen from the illustrations within this guide, proper PCB testing techniques and approaches can have a direct impact on the quality of electronics produced by an engineer. Consequently, poor testing techniques are costly to businesses.

 

Therefore, because PCB testing has a direct impact on the quality of electronics produced by any company, it is worth every cent as it is directly linked to the overall profitability or any company. The only problem is that most Contract manufacturers overcharge the testing of PCBs.

 

As WellPCB, we have equipped ourselves with the most modern methods of PCB testing equipment, and we take our time and expertise to test PCBs for our clients. In case you are uncertain of the type of testing you need, feel free to contact us and ask us more about the testing service that is appropriate for your PCBs. Alternatively, you can go ahead and give us a quotation for your PCBs to be tested.

 

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