Years ago, I stumbled upon a coffee table book dedicated to molecular gastronomy. While I’ve always appreciated chemistry, the subject matter didn’t particularly captivate me as a consumer. However, a few images managed to catch my eye. One, in particular, stood out: a cross-sectional cut of a lit charcoal grill, revealing the heat distribution before it eventually collapsed. It reminded me of the saying, “It’s what’s on the inside that counts.” These cutaway views allowed for observing physical and chemical processes that would otherwise be merely described based on the grill’s construction.
While PCB cross-section analysis may lack the dramatic visuals of sizzling metal and sparking charcoal briskets, it serves a crucial purpose in PCB inspection. Similar to X-ray analysis and other indirect inspection techniques, it enables quality control to identify and address design or manufacturing issues that may have otherwise gone unnoticed. However, what sets cross-section analysis apart from its indirect counterparts is its ability to provide a clear internal view of the PCB features, making it a valuable tool for high-level magnification and examination.
Coupons: Destructive Testing Without the Downside
Cross-section analysis plays a vital role in PCB inspection, but it is irreversible. To overcome this limitation, manufacturers utilize PCB coupons as sacrificial pieces of the substrate. Here are some key points about coupons and their analysis:
Benefits of PCB Coupons
- Allow for cross-section analysis without damaging the main board.
- Provide representative samples of the manufacturing conditions of the entire panel.
Importance of Coupon Cross-Section Analysis
- Crucial for checking the quality of plated through-holes (PTH).
- Helps identify defect modes resulting from drilling, desmearing, and plating steps.
Utilizing In-Panel Coupons
- In-panel coupons can serve as checkpoints for various manufacturing processes.
- Helps avoid wasting additional materials, energy, and labor on defective boards.
Depaneling Considerations
- Use routers for coupon depanelization to avoid damage to the sample and other boards.
- Avoid punches and cutting tools that offer less control.
Preparation Steps for Coupon Analysis
Grinding
- Removes excess material that may obscure the features of interest.
- Aims to keep the area of interest (AoI) small before the sample cleanup.
Polishing
- Enhances sample quality by eliminating scratches and visual defects.
- Produces a precise representation of the features under consideration.
Increasing Accuracy and Throughput
- Variability in sample results can occur due to the nature of the process.
- Standard reference distance from the hole center to the sample’s surface can be set for larger statistical population sizes.
With PCB coupons serving as a valuable tool for cross-section analysis, let’s now explore an alternate approach to enhance this diagnostic process.
An Alternate Tool for PCB Cross-Section Analysis
Traditionally, the “puck” method has been widely used for sample preparation in cross-section analysis. However, this method has its limitations, particularly in terms of limited viewing angles and poor parallelism to a theoretical datum plane, which are increasingly problematic as the electronics industry operates on a larger scale. As the need for improved sampling resolution techniques grows, the relevance of the tripod micro-section tool for PCB analysis is becoming more prominent.
Originally utilized for semiconductor wafer analysis in the 80s, the tripod micro-sectioning tool offers a valuable alternative. With advancements in board technology to support component miniaturization, this method proves to be excellent for preparing specimens for high-powered magnification and imaging, such as SEM inspection.
To provide a clearer comparison between the tripod and puck sample preparation methods, let’s examine their respective pros and cons:
Comparing Tripod and Puck Sample Preparation Methods |
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| Pros | Cons | |
| Tripod |
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| Puck |
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In terms of encapsulation, tripod micro-sectioning provides more flexibility than the puck method. The encapsulation process aims to capture the Area of Interest (AoI) without introducing additional stress or damage to the sample. Tripod micro-sectioning allows for pre-encapsulation before grinding, lapping, and polishing, offering greater manipulation capabilities. The workflow for tripod micro-sectioning closely follows that of the standard puck method and includes the following steps:
- Cleaning: Prepares the sample for analysis and removes large, loose debris.
- Micro-encapsulation: Protects the sample during image preparation.
- Bulk removal: Excises material from the micro-section site to reveal the AoI more clearly.
- Micro-encapsulation (if not already performed before bulk removal).
- Mounting: Fixes the position of the AoI relative to the grinder/polisher.
- Grind
- Polish
- Wash and dry: Provides a final clean of the AoI before imaging.
- Imaging: Tripod micro-sectioning works well with variable pressure SEM, as polishing only needs to go up to 1 µm grit, saving the finest polishing step (<1 µm).
With its advantages and compatibility with SEM imaging, tripod micro-sectioning presents a compelling option for PCB cross-section analysis. In addition to the tripod micro-sectioning technique, your contract manufacturer (CM) covers all sections of PCB manufacturing.
Your CM Covers All Sections of PCB Manufacturing
PCB cross-section analysis serves as an invaluable visual inspection tool, providing insights into manufacturing features that would otherwise remain hidden. While it is a destructive mode of inspection, designers and manufacturers can allocate excess panel area specifically for sampling purposes. By ensuring proper sample preparation, manufacturers can effectively study defects and identify areas of Design for Manufacturing (DFM) that require revision.
Laboratory-level testing for cross-section analysis demands significant technical expertise and specialized equipment. At our company, our dedicated engineers are committed to the development and production of electronics for our customers. We work hand in hand with our trusted manufacturing partners to deliver countless electronics that have made a significant impact on saving lives and transforming them for the better. Our unwavering commitment lies in the continuous process refinement to ensure the highest quality of every PCB we build.
