Inside A PCB: A Cross Section Analysis

It is pretty much impossible to think of electronic devices available nowadays without thinking of PCBs (Printed Circuit Boards). After all, it is PCBs that ultimately decide the measure of functionality of your electronic devices. The better your PCB is, the more proficiently your device is bound to run.

To ensure that you get PCBs to fit into your device, the newly fabricated PCBs have to go through a series of internal and external tests to measure their overall quality.

The internal quality of the PCBs is inspected and verified using IPC inspection guideline with a process called Cross Section Analysis (also known as Micro Section).

What Does the Process Entail?

To begin the analysis a small part of the PCB is cut out called ‘Puck’ named such because it is immediately placed in a very soft acrylic that hardens around it and begins to resemble an ice hockey puck. This makes it possible for the board to be viewed from the side under a microscope.

Cross section analysis can also be called a threefold inspection tool of the internal makeup of a PCB as it is effective for in process verification as well as a failure analysis. The sections are usually taken from coupon sets procured with the production boards. The criteria for the inspection are set by the product specification referred by the customer IPC 6012 class, MIL-PRF-55110/31032 or by the customer’s very own requirements.

The coupons are positioned on the production panels with the boards and are processed along with the production panel where they reflect the process effects that are made to the entire panel. This eliminates the need to remove a production part to verify that the board meets the criteria.

Through hole coupons are usually used to determine:

• Surface and Hole Wall Plating Thickness
• Separation of the Inner Layers
• Registration of the Drill to the Outer and Inner Layer Images
• Inner Layer Connectivity
• The Condition of the Laminated Materials Surrounding the Holes
• Other methods are also used to evaluate:
• Solderability
• Peel Strength of the Copper
• Electrical Characteristics of Inner Layer Traces
• Solder Mask
• Legend Inks

With this, the manufacturers can inspect the prepared sections at various stages of production. Through this, they can get clues to possible process issues and the viability of the boards on a specific panel.

The corrupted/poor product is separated from the batch which is identified by the markings on the coupons and boards on which the panel was fabricated. The markings also help to identify the processing specifics for boards that may be involved in future failures thus becoming a very important part of the entire process.

The second role of the cross section analysis is to act as an effective failure analysis tool. If there is any indication of internal failure then individual boards may be subjected to sectioning. A visual inspection of the section helps to identify the reasons for internal process failures but other defects such as extraneous copper, over etch, poor solder mask developing, and etc. can be identified with cross sectional views of the surface trace work, pads and solder mask.

Data that is collected during the sectioning of a single or various lots of boards can be used for process improvement as well validation of the various processing steps used to fabricate the PCBs.

When you reach out to a company to have your PCBs manufactured, make sure to ask them about their process of quality inspections.

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