Microscopic Verification of PCB Cross-Sections: A Visual Inspection of Printed Circuit Boards
PCB Test Coupons: Ensuring Quality and Reliability in PCB Manufacturing
PCB Test Coupons are standardized circuit samples included in the PCB manufacturing process to evaluate the internal structural and electrical properties of the board. These samples are crucial for detecting internal defects and verifying compliance with quality and reliability standards before full production release.
The Role of PCB Test Coupons
The primary purpose of using PCB Test Coupons in cross-section analysis is:
- Evaluating internal structure: checking copper plating thickness in vias and inner layers to meet IPC standards.
- Detecting defects: identifying delamination (layer separation), voids, cracks, and insufficient barrel fill (solder in plated-through holes).
- Assessing thermal stress impacts particularly for high-reliability classes (e.g., IPC Class 3).
- Verifying manufacturing consistency: ensuring the board’s internal mechanical and electrical integrity matches design specifications.
- Predicting reliability in long-term operations by uncovering hidden weaknesses that could cause open or short circuits.
The Process of Using Test Coupons
The process for using test coupons in cross-section or micro-section analysis typically involves:
- Inclusion of test coupons: Coupons with features mirroring the actual PCB (holes, pads, traces) are added to the panel during fabrication.
- Coupon removal: At the testing stage, coupons are separated from the panel for destructive analysis.
- Sample preparation: Coupons are cut vertically through areas of interest (e.g., vias, traces), then polished to a mirror finish to expose internal layers.
- Microscopic examination: Using optical or electron microscopes to observe:
- Copper plating thickness and uniformity.
- Voids, cracks, or delamination between layers.
- Barrel fill quality within plated-through holes.
- Measurement and comparison: Results are compared against IPC standards (e.g., IPC-2220 series design rules for test coupons or IPC-6012 criteria for quality).
- Reporting: Findings guide quality control decisions, helping to adjust manufacturing parameters if defects or non-conformance are found.
Preparing the Sample
Before the cross-section analysis, the sample is isolated using a diamond saw and the edges are smoothened. The sample is washed with Isopropyl Alcohol (IPA) and dried. No moisture content should be present before the specimen is subjected to the solder bath.
The Solder Bath Process
The solder bath process is started by setting up the temperature to 288°C. The sample is dipped in the molten solder for precisely 10 seconds. This process may be repeated 3 times or 6 times as per the request. The hardened specimen is finely ground and polished to form a smooth surface finish.
Additional Tests
The peel strength test is conducted only on 3mm conductor width. Different coupons are used for different kinds of tests, such as through hole coupons used to determine surface plating and hole wall plating thickness, separation of inner layers, registration of the drill to the outer and inner layer images, inner layer connectivity, condition of the laminated materials around a hole, solderability, peel strength, electrical characteristics of inner layer traces, solder mask, legend inks, PTH assessments, via assessments, hole (barrel) wall quality, to view the hole Barrel structure, quality of the copper plating and plating thickness, and voids.
Cross-section Analysis and Quality Control
Through hole coupons are used to perform tests and verify the quality of the board without destructing the desired PCB. The cross-section analysis is an integral part of the PCB manufacturing process, and an efficient way to analyze board quality is to fabricate PCB Test Coupons.
After the analysis, the sample board enveloped with the polymer is called a puck. The puck may undergo additional processes like baking at 125°C for a duration of 10 hours with an air circulating oven before further examination.
In summary, PCB Test Coupons serve as a diagnostic tool during manufacturing to perform detailed internal inspections via cross-section analysis, ensuring the PCB meets stringent quality, mechanical, and thermal reliability requirements before deployment.
[1] Cross-section Analysis of PCBs: A Comprehensive Guide [2] Understanding the Importance of PCB Test Coupons [3] PCB Test Coupons: A Key to Quality Assurance [4] Simultaneous Electrical Testing and Cross-section Examination in PCB Manufacturing [5] The Role of Cross-section Analysis in High-reliability PCB Manufacturing
- In the process of evaluating a PCB's internal structure and ensuring compliance with IPC standards, controlled impedance technology is crucial for checking copper plating thickness in via and inner layers.
- During the analysis of PCB Test Coupons via cross-section or micro-section, assessing thermal stress impacts and verifying manufacturing consistency is facilitated by advanced technology like controlled impedance.
