Cutting coupons

All photos courtesy of Buehler

Printed circuit boards (PCBs) need to operate reliably in harsh aerospace environments withstanding extreme temperatures, humidity, vibration, and in some cases, solar radiation.

Therefore, the manufacturing process must be accurate and reliable, providing an industry standard 15-to-20-years of failure-free operation for electronics.

Ensuring reliability in PCBs starts with their fabrication and design, following every step of the way with rigorous quality control procedures.

PCB manufacturing

A PCB provides the basic building block of an electronic device. They consist of alternating layers of material to provide an insulating overlay and conductive paths. Board manufacturers can make cheaper PCBs of less durable materials, but higher-quality boards are mostly comprised of fiberglass, such as FR4. Producers manufacture multiple layers of the board sequentially. The processing can be expensive and adds value to the components. Finding a problem in production late in the process is not preferable, so quality control checks must be done quickly and efficiently at crucial stages. Test coupons and the part are typically manufactured within the PCB to achieve this reliability.

Documentation

Test coupons allow manufacturers to analyze PCBs in the quality assurance lab. The coupon is subject to the same manufacturing processes and sequences as the PCB and can be evaluated for the specific characteristics present on the entire board before the manufacturer mounts other components. With the same qualities and flaws as the PCB, the coupon will show any quality problems.

Even without the components that are eventually mounted on it, a circuit board is an intricate part. The board usually contains a filler, such as silica. It also can consist of aluminum, which manufacturers use as a dielectric for high-frequency boards.

Some manufacturers design test coupons that easily press-out from the board, ensuring removal without damaging the overall board. Where this is not done, they must be removed using an appropriate cutting tool, a critical step given the board’s brittle material.

Coupons can check bare board quality, issues such as etch definition, layer registration, plating quality, or solder mask definition. They can also monitor the assembly processes for surface mount technologies (SMT) and conventional plated through-hole (PTH) assembly types – cut and clinch quality, paste printing quality, insertion accuracy, reflow/flow soldering quality and assembly cleanliness.

Manufacturing the coupon at the same time as the main PCB gives both the same impedance, which depends upon the PCB’s dimensions and electrical properties.

Primary board inspection concerns are the copper plating thickness and the width of the component mounting through-holes. A scanning electron microscope (SEM) examines the holes to determine if they meet specification. If the holes are too wide, the lab rejects the board.

Copper plating can be contaminated during board coating. Also, the boards must be tested for the surface coating’s integrity. If encapsulation is insufficient, the board oxidizes, shortening the component’s lifetime. IPC-222 standards define test coupon design requirements, and IPC-601 standards identify the minimum and maximum dimensions for all internal and external features, including laminate layers, plating, foils, holes, and spaces.

Maintaining coupon integrity

Crucial to inspection, the lab needs to remove the coupons from the PCBs without damaging the coupon’s structure. The piece must be cut to a specific size while preventing the board from cracking. With the brittle materials contained in many boards, this can be challenging.

Some labs use hand saws, which can be imprecise and can damage the coupon or the board. Cutting these coupons is not merely lopping them off the PCB. There are many inspection areas and cutting damage can make a quality inspection impossible.

Also, there must be some margin between the edge of the board and where the copper coating begins. Finally, cutting must not splinter the edge of the board to avoid secondary damage.

Abrasive and precision cutting machines and sectioning saws can support metallography testing. Specimen preparation for microstructural examination of PCBs starts with a quality cut. Selecting the proper sectioning blades and precision cutting saws saves time by limiting the number of steps needed to analyze coupon samples.

Proper coupon sectioning technique depends on the boards’ thickness, size, and silica composition. Based on the board’s hardness, the technician can select a cutting blade with a low concentration of diamond cutting points that reduce damage for hard, brittle materials.

Recent developments in precision sectioning saws are improving the precision and efficiency for cutting coupons and can provide the versatility needed for various PCBs.

To achievie efficiency, semi-automated sectioning saw controls easily position samples to minimize setup time and can accommodate various coupon sizes. The controls enable quick cutting-parameter setup. Protective or smart-cut functions limit cutting loads on precision saws, ensuring consistent quality.

Precision is crucial. Accurate positioning, higher reproducibility, and minimal sample deformation are required for delicate PCBs. For further precision, advanced units can fine-tune adjustments after clamping.

Automated serial cutting – the ability to cut parallel lines accurately without re-clamping – adds efficiency, reducing operator variability and ensuring high-quality coupons. Using specialized precision cutters, the lab technician can cut 10-to-20 samples per hour with minimal deformation. With the high volume of boards coming off the plant floor into the quality assurance labs, these systems let facilities handle a greater number of samples to keep the line moving.

Through-hole inspection

Inspecting PCB test coupon plated through-holes or via’s requires metallographic preparation of the sample, a process that requires mounting the coupon in plastic to protect the sample, then grinding and polishing to the center of the feature of interest.

Due to the continued miniaturization of electronics, these features can be extremely small – yet targeting the center is crucial for accurate quality control checks. In high-volume laboratories, where speed and quality are equally essential, this can be achieved using test coupons with specific dimensions.

The coupons can be held in a precision target grinding fixture that uses reference pins to hold the coupons in position. Diamond stops in the fixture then allow grinding to the features of interest, with an accuracy up to 0.004" (0.102mm).

Quality assurance

For quality assurance labs handling a wide range of samples, precision saws can section a wide range of materials from PCBs to rubber, nickel superalloys, and carbon fiber. Saw manufacturers can guide you as to which cutting blades match the kinds of boards or other material you are inspecting.

The quality lab’s ability to keep up with inspections should not slow things down. Nor should rushing this process compromise quality control or threaten the reliability of the aircraft using the PCBs.

Buehler

About the author: Nanu Vishora is a materials engineer at Buehler, an Illinois Tool Works (ITW) company. He can be reached at info@buehler.com.