Micro-Sectioning: What It Is and How to Do it

You've heard about sectioning, and you may even know what it is, but what about micro-sectioning?

Micro-sectioning is a more refined sectioning process that allows examining much smaller areas on a PCB.

This article will discuss micro-sectioning, its capabilities, and how to create and analyze a micro-section.

What is Micro Sectioning?

Micro-sectioning removes a small part or specific layer(s) of the multilayer printed circuit board (PCB) to investigate design issues, faulty components, or unexpected behavior.

Micrometer scale sectioning can also provide detailed information about solder joint integrity, plating thickness, material compositions, etc.

Uses of Micro-Section Analysis

Micro-sectioning is a way to look inside a PCB to check its quality.

It is a destructive test, meaning you must cut the PCB open to doing it. This procedure prevents the status of:

• PCB's foundation material
• Build checking in the boards
• the thickness of the hole-wall plating
• External and internal conductors' thickness
• Registration of both internal and external padding
• Connections between the multilayers
• Surface finishes thickness
• Solder masking or the covering material thickness

PCB manufacturers check the micro-section analysis of PCBs at different phases of production, including:

• Studying for the quality of the hole and internal registration after drilling
• Checking the thickness of the wrap and barrel after plating
• Studying for the general consistency, plating needs, final roster, and defects during the final quality analysis

PCB conductor paths

Making a Micro-Section

PCB manufacturers need to make sure their boards are of high quality, and one way to do that is by using through-hole coupons.

They need to cut coupons in an area near the center of the plated hole, and for that, they use an accurate CNC router-cutter.

A CNC router-cutter machine can create precise cross-sections of materials such as solder joints and SMT for testing.

With a hole down to 1 mil or 25 microns, this machine allows for the creation of test coupons necessary for verification, qualification, failure analysis, or standard conformance.

To make a test coupon, the fabricator cuts out a piece of the material and immerses it in soft acrylic.

Once it hardens, the test sample will form a hockey puck-like shape.

The fabricator then uses a particular machine to create a perfectly flat surface on the hockey puck and a smooth, polished finish.

This procedure helps to inspect the board's cutting side under a microscope.

Plus, they may also etch it back if needed.

A close-up section of the motherboard

The Micro-Section Analysis

Build-Checking

The inspector ensures that the PCB's multilayer build-up is effective and defects-free.

They can provide a high-quality PCB by inspecting the thickness of the copper foils, cores, prepregs, and bonding process between layers.

They can avoid defects that thermal stress may cause with careful inspection, such as voids, blistering, cracks, and delamination.

The Thickness of Wall Plating

The wall plating thickness requires six measurements (three on both sides of the hole) and then calculating the average of the six readings.

Conductors’ Thickness

The thickness of the conductors on a PCB can be affected by the cleaning processes in use.

In general, the thickness of copper on the conductor after processing should meet the minimum thickness requirements specified by IPC A 600 Class 2.

Hole Registration

Hole registration is the process of ensuring that the holes in a PCB are drilled correctly.

To do this, manufacturers guarantee that the anti-paddings and the inner copper paddings align with the drilled hole within specified tolerances.

Connections between The Multi-Layers

If you have a broken or poor connection between the layers of your board, it can cause problems with its functionality.

You can detect this through a Micro-Section analysis or electrical testing.

Surface Finishing

Micro-section analysis measures the thickness of lead-free HAL or hot air leveling.

Yet, for Electroless Nickel Immersion Gold (ENIG), X-ray measurements often measure the thickness of immersion gold.

ENIG or gold surface finish

Solder Mask

A solder mask is a thin layer used to protect the copper conductors on a printed circuit board, and Micro-Section Analysis helps calculate this layer's thickness.

Micro-Sections for Traceability

The markings on the coupon and boards help the manufacturer trace a PCB back to the machine that made it, enabling them to separate poor products from good ones.

They also make it easier to prevent future failures by identifying the processing conditions involved.

PCB manufacturing

Identifying Failures with Micro-Section Analysis

Micrometer-scale sectioning can help identify the cause of internal failures in a board.

By inspecting and analyzing a single micro-section, you can better understand what is going wrong inside the board, which is pretty helpful in fixing the issue.

Generally, the manufacturing of PCBs involves many steps, and each one has the potential to produce imperfections.

PCBs with defects, including poor solder mask thickness,over-etching, improper registration, etc., often get rejected during manufacturing.

You can look closer at the copper thickness, pads, and solder masks with specific cross-sectional views to catch these defects early on and avoid costly delays and rework.

Ultimately, manufacturers use data gathered from the micro-sectional analysis of panels to validate different steps in the fabrication process to produce high-quality PCBs.

A close-up view of a PCB

The Difficulties of Micro-Sectioning

Microsection analysis can be challenging since the materials that make up a PCB may be different in hardness, and the media used for mounting can also be challenging.

Plus, drilling a hole in a PCB is exact.

The spot must be within 10% of the PCB's center, and this accuracy level requires special equipment and personnel certified in IPC-A-600 and IPC-A-610.

Therefore, This process often occurs in a laboratory to ensure the utmost precision.

Testing a PCB in a laboratory

Additional PCB Testings

Functional PCB testing is another way to ensure your circuit board functions correctly.

These tests can include:

• X-ray inspection that provides a complete view of the board's interior.
• PCB contamination testing to identify the reason for corrosion, degradation, and metallization.
• Inspecting the board for high-frequency breakdowns using a time-domain reflectometer.
• Peel testing determines how much strength is necessary to peel off the PCB laminate.
• Solder float testing determines the amount of thermal stress the board's holes can handle.

Summary

Micro-sectioning is a process that helps manufacturers inspect PCBs for defects and understand what is going wrong inside the board.

By analyzing individual Micro-Sections, they can prevent future failures and improve the quality of their products.

I hope you found this article helpful.

While the process can be challenging, it is often essential in ensuring the success of a PCB project.

If you have any questions, feel free to contact us.