Electronic PCB Inspection and Equipment
With the advent of technology, humanity has been able to fit a computer (that used to take up whole floors) into a single chip. The circuitry for such devices has been reduced to a cardboard-thin object that hosts the entirety of the functions.
But nothing is perfect. So it bears to reason that sometimes, the PCB electronics might have some defects (which tend to happen quite a lot, given the size and nature of these products). However, it is not the defects that define a product but the manner in which such defects are identified and resolved.
However, the problem as a whole can look quite a bit daunting (I know I was amazed when asked about the nuances of testing electronic PCB). If you break the whole process down to its fundamentals, the road to excellence seems more precise than ever.
Before you can even think about using the proper testing method like the PCB type, the various steps involved are manufacturing, selecting the available tools, etc. That is why having a solid groundwork plan for tackling the problems at all their stages is essential.
First of all, we need to look at our goal, i.e., the problems we wish to address. Most often, we require only three goals out of the inspection process.
Goals Of Testing
- Pinpointing and rectifying faults such as short circuits, poor soldering, broken base, etc.
- Address any potential issues that may occur during the manufacturing or everyday use through rigorous testing. This allows the manufacturers to have an idea about how the product will react to real-world conditions).
- Reducing excess waste and money due to these faulty parts
Now that we know what we should be looking out for, we will look at the features we test PCB electronics for.
Testing Properties
Since an electronic PCB consists of many parts and characteristics, no single component or test can be used as a threshold to determine operational feasibility. That is why, ideally, we need to check for every characteristic while keeping the testing rates high and cost to a minimum.
1. Electrical Conductivity
This test ensures that the PCB conducts (or in some cases, resists) the flow of electrons correctly. Over Conducting can cause the electronic PCB to heat up and damage the whole board, while under-conducting would make them useless in any of their applications.
This characteristic is usually tested using the PCB in normal conditions ( working conditions) and pinpointing faults wherever they occur.
2. Mechanical Strength
This characteristic ensures that the electronic PCB can stand the demanding rigors of time and usage. A PCB with a flawed or fragile design might break down in the middle of an operation or during transportations, causing huge losses to both the manufacturers and consumers.
It can be tested in many ways, including but not limited to impact testing, tensile testing, etc.
3. Soldering Quality
We are all aware that it is the solder that holds the pieces of equipment in place on the board during its operation. However, if the problem arises with the solder itself, the machine can stop working right in the middle without any apparent symptoms.
I learned this the hard way when I used a bad quality solder in my school projects. While I lost out on a medal, having this problem arise in the current scenario can have far more dangerous and far-reaching consequences.
4. Cleanliness
It is a well-documented fact that the universe tends towards chaos. Entropy is our biggest enemy and one of the main reasons why pcb electronics have such lifetimes. The cleanliness of a PCB is crucial to its working. The dust particles can interfere with the voltage or current signal or cause it to overheat, thereby causing monetary and physical damage.
One of the traditional methods of using non-corrosive, non-conductive liquids is to clean a PCB after testing, thoroughly, and sealing it. This ensures that your PCB electronics are in top working condition right out of the box.
5. Target Environment
We can do as many theoretical tests as we want, but the actual value of a PCB lies in the ability to work in real conditions. For example, a PCB working in a high-temperature environment needs to have better thermal efficiency than the one that works in comparatively cooler climates.
So, one of the essential aspects of testing and inspection is to ensure that the electronic PCB runs in optimal conditions in its target environment. Otherwise, it’s just plastic.
6. Peel Length
The peel length of the rigid PCB determines its peel strength, which is the amount of force required to peel off or remove the conductor or foil from the base material. The better the peel strength, the better would be the working condition of the PCB electronics.
7. Hole Wall Quality
The quality determines how rigidly the PCB can be attached to surfaces when working. Blemished or misshapen holes can cause the PCB to slip or malfunction. These are small things that can be easily taken care of during the manufacturing process itself. However, it can turn into a massive problem if left unattended post-production.
8. Component Placement
This refers to the projected placement of the components (theoretical) and the actual placements. There are always some errors during such processes. So we need to ensure that the components are placed with exactness within the allowable error limit.
9. Orientation
Electronic PCB components are some of the most precise electrical objects. Since the vast amount of circuitry has been crushed down into a single board, the complexity of the whole circuit increases by many folds. In such situations, even the smallest of orientation or polarity error can cause tremendous losses.
That is why PCBs should always be manufactured, keeping strict guidelines in mind.
Now that we have noted the various characteristics that we need to look out for, we will glance at the different methods to test out these characteristics and find the flaws.
One thing to keep in mind is that all of these tests can look for multiple flaws simultaneously. This increases their range and reduces the time but can cause some errors to slip by. However, in most cases, repetitive testing can minimize this error value to almost zero.
PCB Inspection and Testing Techniques
As we read earlier, different components work in various manners and react differently to tests. They are unique in terms of visual, structural, and characteristic functionalities. Some of the most common testing methods include:
- In-Circuit Testing (ICT)
- JTAG Scan
- Automated Optical Inspection
- Automated X-Ray Inspection
You might have noticed a trend in the above test methods. They include both automated test equipment (ATE) and manual visual inspection (MVI) because machines, like humans, can make errors. Humans, being humans, are bound to make errors. But if the two work in tandem and in harmony, the errors can be minimized easily.
But, on an industrial level, the automated tests (which are more effective) are usually reserved for assembly-level tests as they can be pretty costly.
Now, aside from these tests, there are also mechanical tests available. This is because PCBs have a wide range of applications, including aerospace, mining, drilling, etc. It is essential to ensure that the PCB can take on all of the external anomalies that it encounters aside from internal anomalies in such environments.
It also ensures that components like BGA can withstand the shock, wear, and tear caused by constant use. Although these tests are destructive in nature, they make up for that by identifying the feasibility of the product while it is in its infancy, thereby saving time and money.
Let us look at the testing methods to understand how they are performed and the results we get.
In-Circuit Testing
This test is mainly made of an in-circuit tester, a fixture, and software to read all the data. It is widely used amongst all PCB manufacturers because this test can cover most of the manufacturing defects that occur in PCBs.
The testers can be used to check for short circuits, open circuits, resistance, capacitance, inductance and can also be used to verify the correct polarities or orientation for devices such as diodes, integrated circuits (ICs), and transistors.
The in-circuit tests run all of these checks on the components depending upon the model of the design. With a theoretical potential detect rate of 98% (of faults, obviously), the ICT is one of the best methods of testing a PCB for defects in the design. Although practically speaking, 98% is not entirely possible due to the lack of access to some of the nodes, and also, it cannot detect very low capacitance and inductance values. Nevertheless, even then, it finds widespread applications in the manufacturing industry.
It has multiple benefits, such as simple defect detection and programming, user-friendly test report generation, etc. The equipment can get costly, but that is a one-time investment. However, once set, we cannot upgrade or update the equipment as they are mechanically fixed.
The two most widespread types of In-Circuit Testing are the Bed of Nails and the Flying Probe.
● Bed of Nails Technique
Also known as the Universal Grid Technique, this type of in-circuit testing relies on a large number of pins (that are spring-loaded) to make contact with the PCB. These pins resemble a large number of nails lying on a bed, and that way, they are, and might I say so aptly, named the way they are.
During the test, each of the pins makes contact with the circuit node. This method allows it to identify open and short circuits, defective components, and solder joint bridges.
An average in-circuit test comprises multiple pins spread across the board. The main reason for applying the various pins to the board is that many pins ensure tens of thousands of connections to be made simultaneously with each pass.
The length of each pin is about 35mm, and they are usually instructed at the end of a net-like hole or a mounting pad.
The test uses the produced signals and voltages in the circuit, which are then analyzed to look for any defects or deformities.
This type of test is generally a low-cost method to test mass production systems, simple and analog boards. It is, however, limited when it comes to small pitch widths, SMDs, BGA, and other similar components and characteristics.
● Flying Probe Test
This type of test technique utilizes an element with a smaller pitch to make contact with the required test points (such as the SMD and BGA pins). Hence it is widely used in areas that utilize small-sized contacts (around a 0.2mm test pitch).
In actual working, it utilizes several probes to make contact with the required pins, pads, and tests for opens, shorts and polarity, resistance, and capacitance.
Sometimes, testers will also include a camera to determine some of the missing components (if any) and analyze their physical properties, such as the size, shape, polarity, etc.
Automated Optical Inspection (AOI)
As the name suggests, this technique utilizes the waves lying in the visible region to find defects in the PCB. Usually, it uses one or multiple cameras to analyze the given PCB component optically. Finally, the software cross-references the images from the PCB under test with those from a similar reference board, and sometimes, even the ideal design specifications.
This test is usually done at the very end of the assembly line. In this manner, manufacturers can verify the quality of the finished product and check for faults simultaneously.
Additionally, we can also use the Automated Optical Inspection method to monitor the manufacturing process as a whole. The technology used in the pick and place machines allows the manufacturer to track the strategies and developments in real-time and also helps in correcting the assembly defects (like a potential component misalignment or misplacement) before such defects can become widespread.
For some applications, the optical inspection requires an endoscope to view the connections between the BGA and the PCB. However, this method is only applicable when the test points are actually visible to the naked eye.
Automated X-ray Inspection (AXI)
Like the Automated Optical Inspection method, the Automated X-Ray Inspection method is also a non-destructive way of testing PCBs and finding out their faults. This test has the ability to detect solder defects that are invisible to our eyes.
This type of test does not require a physical connection. It can find defects even under large IC packs, like BGA, Micro BGAs, CSPs, etc.
Generally speaking, it is good to use this testing method for areas located in the center, which are usually invisible to us. This technique relies on the property of the materials to absorb X-Rays according to their element and width (i.e., thickness and atomic number).
It is a well-known fact that absorption rate is directly proportional to the element’s atomic weight. Compounds like solder (which are pretty heavy) tend to absorb more X-Rays, and hence, are more visible in this spectrum. Conversely, the lighter elements (such as the PCB itself, the components, etc.) would appear more translucent as they absorb lesser X-Rays.
As the X-Ray can penetrate the IC package, we can use it to inspect the soldering and connections. It can also be used to identify structural defects such as the sorts and open circuits, insufficient or bad quality solder knuckling, etc.
The main capabilities for which this type of inspection method is widely used are:
- Checking for the bad or misalignment of BGA, CSP, and some other larger chips.
- Searching for asymmetrical connections
- Ensuring the integrity of the package standoff height.
- Finding and identifying popcorning (which occurs when some balls merge together to form irregular shapes and contacts)
- Analyzing the solder and checking inside it to identify defects such as bubbles, insufficient filling, etc.
The AXI method of inspection is ideal for checking the board, its components, its different layers, alignments, and orientations.
Choose The Right Test
Now that we have taken a look at the various methods of testing available and the capabilities, pros, and cons of these tests, it is time for us to determine which test to use for our purposes.
As it is well known, the PCBs vary from each other due to their various applications. As a result, it is difficult to pinpoint one test to sort out all of the problems. Sure, we can narrow down some of the tests to our likes and needs, but beyond that, economics rules.
The final goal is to create a good quality product that is free from defects and problems. To accomplish this, many standard practices have been established. For example, simple, single, or two-layer PCBs are usually tested using the traditional test methods. However, as the complexity of the design increases and the size decreases, more complicated ways of testing are required (the AXI and the AOI).
Typically, the In-Circuit tests work for most components, but components where more than one production method has been used may require multiple test methods.. But all these tests accomplish one thing: identifying and rectifying the defects before it is processed into the final products. This reduces wastage, lowers production cost, and increases product reliability.
However, keep in mind that you do not need to create the best product you can. You need to create a product without defects feasible in the market and be inspected using cost-effective methods. So it can be a good idea to use a combination of all the techniques provided to get a comprehensive analysis and find out the most cost-effective solution.