About PCB cross-section analysis. No matter how reliable a manufacturer’s PCB fabrication process may seem, it is not enough to determine the ultimate quality of the finished product. Without proper testing, PCBs may be more prone to failure. For instance, your PCB may encounter an electronic component design failure due to a lack of proper post-fabrication inspection.
Furthermore, you may experience other issues such as chemical leaks, overheated components, and flaws in the PCB soldering process. Thus, as with any other electronic equipment piece, wne must analyze and inspect PCBs to ensure their integrity.
It can be through in-circuit testing, automated optical inspection, burn-in testing, etc. In the following guide, we will discuss a form of such testing named PCB cross-section analysis. We will define it and explain what equipment is involved.
Define cross-section in PCB?
We briefly explained how there are various methods of PCB inspection in the introduction. Some of these may be destructive, while others are non-destructive. It is a little like the difference between the X-Ray and biopsy/autopsy of a human body. It will essentially help us figure out what is going wrong or what may have gone wrong.
Cross-sectioning is a destructive form of analysis where we cut out a piece of the PCB for deep material inspection. PCBs are essentially a sum of constituents and materials.
Therefore, if a singular element of the PCB fails, the entire system may fail too. PCB Cross-section (sometimes referred to as PCB microsection) allows us to inspect the PCB’s internal quality and investigate any faults.
Types of PCB Cross-Section Testing Analysis
In this section, we will discuss the basics of PCB cross-section/micro-section analysis. To perform a successful cross-section analysis, the PCB needs to be micro-sectioned. In other words, we must extract a piece of the PCB.
In the case of a failure, we need to identify and isolate the defect region to a section that is as narrow as possible.
We can do this using the PCB’s schematics and a host of other tests such as thermal imaging. Once we identify the region, we cut or section it off from the rest of the PCB. In conclusion, this is what we refer to as the PCB core cross-section.
PCB Core Cross-Section
Once we have the PCB core cross-section isolated and separated, we mount it in a potting material. This will seal and protect it against the subsequent polishing process.
After we mount the PCB core cross-section, we gradually grind it down until we meet the desired plane of interest. We perform this task with silicon carbide paper, colloidal silica, and aluminum powder.
Cross Section Heavy Copper PCB
You can commonly find heavy copper and extreme copper PCBs in the industrial sector or heavy-duty equipment operating in very harsh environments. Heavy copper PCBs have quite a few advantages. They include enhanced resistance to thermal strains, reduced risk of circuit failure, and increased current carrying capacity.
Copper and circuitry placement is vital to ensure proper heat dispersion. In addition to this, they minimize the board’s bow and twist. The inner layer planes, core thickness, copper layer thickness, and dialectical thickness are all a part of the board’s cross-sectional layout. Thus, they must be as symmetrical as possible.
Multilayer PCB Cross-Section Via Connect
We use Multilayer PCBs for compact applications. This form factor allows us to group as many components as close together as possible. As such, this creates a PCB with a dense cross-section. Generally and most notably, multilayer PCBs consist of a plethora of mostly small vias and holes. They extend from one layer to another. PCB cross-section analysis may be an effective way of determining how many layers a multilayer PCB comprise.
Solder Joint Cross Section Analysis
Solder joints may be prone to fault too. For instance, you may encounter open joints, solder skips, and hidden component damage. And this may be due to the solder hardening prematurely during the soldering process.
Micro-sectioning through a solder joint can help us extract vital information about any solder defects. Thus, we can essentially use cross-sectioning to detect any faults from the soldering process. Furthermore, we can assess the thickness of the soldering and observe any other inter-metallic compounds.
PCB Cross-Section Equipment
If you read the previous section, you would recognize cross-section analysis requires specialized equipment, particularly in the polishing phase. This section of the guide covers what other equipment is necessary for the micro-section analysis. To be clear, we use this equipment in the observational phase of the PCB cross-section analysis.
We use an optical microscope known as a metallograph. It is an essential tool in metallography. This microscope usually contains a camera that helps you capture images of a PCB’s cross-section. Incidentally, in some circles, cross-sectioning is also known as metallographic preparation.
Nevertheless, the metallograph is a metallurgical microscope with a zoom of up to 1000x. We use this device to find any defects when analyzing a cross-section from a PCB.
Scanning Electron Microscope (SEM)
A Scanning Electron Microscope uses a focused beam of electrons to create images of PCB cross-section surfaces. In essence, it allows you to extract information about a sample’s composition.
It works by interacting with the atoms in the PCB cross-section. The scanning electron microscope has various detection modes and signals.
These include secondary electrons, back-scattered electrons, characteristic X-rays, light cathodoluminescence, absorbed current, and transmitted electrons.
The most significant advantage of using an SEM is that we do not need to polish the PCB cross-section before we use the SEM on it.
Thus, we can simply use the SEM’s back-scattered electron mode to extract the specimen’s basic and topographical information. In addition to this, the SEM has a magnification that ranges from 10x to 2000x.
Efficient Defect Inspection Tool
In essence, the micro/cross-section acts as a useful failure analysis tool. Boards are likely to falter due to an internal error. Cross-sectioning may help identify these issues through deep optical inspection. Problems may be a result of over-etching, erroneous soldering, faulty pads, etc.
As such, it gives us a visual tool to work. That allows us to iron out any issues with the fabrication process. And thus, it is essential to know the extent of your PCB manufacturer’s checks and quality analysis practices, especially if you are a customer.
PCB Trace Cross-Section Geometry
As with any other form of geometry, PCB cross-section geometry describes the cross-section’s spatial shape and alignment. A full cross-section geometry may illustrate the relationship between a PCB’s parts. It includes its layers, material, components, and traces.
Incidentally, trace cross-section geometry refers to space between and alignment of the trace. They may diagram the route and differential impedance of traces.
Coplanar, microstripline, and stripline are the most common cross-sectional geometries. Additionally, each one has its in-built variations.
Cross-Section Test Coupons and Inspection Criteria
Under the IPC 6012, the customer will commonly specify the criteria of inspection. In this section, we cover the test coupons and inspection criteria.
A PCB need not fail before you use cross-section analysis to test it. Test coupons allow you to perform a destructive test on the board. We do this without fully dissecting it and stopping it from functioning.
We can fabricate a PCB with additional extensions. Next, we can extract these extensions from the board without causing it to malfunction.
We call the cut-off extensions PCB test coupons. They will consist of the same material as the original PCB. Furthermore, they will have the same composition and specs. That includes trace width, copper, and via patterns.
We place the trace coupon extensions at every end of the PCB, at least one inch out. Once we extract them, we put them in a polymer that envelopes them. After the enclosure, the sample will resemble a hockey puck. When a test coupon reaches this level, we refer to it as a puck because of its appearance.
We then grind the puck down until we meet the interior region. Once we expose the plane of interest, an expert inspects it using a metallograph or SEM. The test coupon’s design will follow the IPC 2220 standard.
As you would expect, each test is not the same for every coupon, and thus we must specialize. A PCB consists of many different parts and constituents. Therefore, there should be different types of test coupons and assessments. These help us specify and analyze which part of the PCB has a defect.
Through Hole Coupons
We commonly utilize these types of coupons in inspection, especially if there is an issue in connectivity. It can help us test inner layer connectivity, the components’ condition around vias, and the holes/vias’ construction quality. Furthermore, we can use it to ascertain the Know the flex PCB materials.
We can also use through-hole coupons to assess the solderability, solder mask, legend inks, etc. You should note that you can only use a test coupon once since the entire analysis process is destructive. That means it ultimately renders samples useless.
When we assess a cross-section, we assess the plated through holes, vias, and barrel walls. Additionally, we use the coupon to view the barrel wall quality and structure. Furthermore, we assess the quality of the copper plating and thickness as well as voids.
Preparing and Using Test Coupons
Before we can put the coupon under a microscope, we need to prepare it.
First, we number or code the coupon according to its orientation on the PCB. For instance, we label left bottom coupons as LBC, right top coupons as RTC, and horizontal coupons as HC. That will assist us in separating and sorting the samples.
Next, we micro-etch it before the inspection and digital photography. If we find any faults in the captured images, we magnify the regions and record them. We also measure parameters such as dielectric thickness, solder plating thickness, and copper thickness during this process. In terms of copper thickness, we record basic, plating, and total thickness.
Analysis Process/PCB Cross Section Results and Outcome
We hope to divulge from the cross-section analysis process any flaws that may occur from the fabrication process. It helps us refine the fabrication process and prevent any future faults. While other analysis methods are available to us, they may not be enough to ensure PCB integrity.
For instance, inspection through thermal testing or other non-destructive tests may not be enough to detect subtle flaws in the PCB’s make-up. For example, we may not trace and detect the breaks present in the solder joints without destructive testing, such as cross-section analysis.
We can extract from cross-section analysis the structure of solder grains, the metals in the PCB, cracks, and voids.
Challenges in the Cross-Section Preparation of Vias and PTHs
Preparing a via or plated through holes presents a few challenges because of their nature. They need to hold their integrity as much as possible for accurate measurement. As such, we must ensure that the final product reflects the original structure vias after polishing.
In addition to this, we must be careful not to scratch the samples. There must be no orientation errors during the potting phase.
We need expert and careful polishing of the sample to remove any defects while ensuring no edge rounding.
Why is PCB Cross-Sectional Analysis so Important?
In summary, the entire PCB cross-sectional analysis may help you evaluate the
- The PCB’s core materials
- Build quality in multi-layer boards
- Solder-mask thickness
- Depth of the PCB’s surface finish
- Quality of connections between layers
- Integrity of pads
- The thickness of the hole-wall plating
- Via and via fill
The manufacturer may execute a micro-sectional analysis at various phases of the fabrication process. It can occur after drilling to inspect the hole quality and how it’s registered internally. When we perform the cross-sectional analysis after plating, we observe the wrap and barrel thickness. We inspect and search for any other defects, overall thickness, and plating during the final quality check.
Cross-section analysis is just one of many quality control techniques used by manufacturers. Because it is a destructive form of testing, it may allow us to find subtle defects in a PCB. By cutting a PCB down into smaller samples, we can perform detailed inspections and analysis. Nevertheless, the above passage should give you a good understanding of why and how PCB manufacturers perform cross-section analysis. We hope that you have found it to be helpful. Thank you for reading.