How to… use digital microscopy for material inspections

Materials World magazine
,
29 Aug 2019

An Olympus Europa Product Marketing Manager for Industrial Microscopes talks about the merits of digital microscopes in industry.

Whether it is manufacturing, quality control, in-service inspection or failure analysis, the need for clear images and micrometre precision in finished products and components stretches across industries. Digital microscopes combine the versatility to deal with a wide range of specimens with guaranteed accuracy in measurement.

The Olympus DSX1000 digital microscope offers measurement accuracy for improved compliance, 20–7,000 times magnification and six observation modes for high probability of detection, and easy, intuitive operation.

Here, I have taken a look at real-world inspection applications from different industries, highlighting the challenges of getting a clear, high-magnification image and precise measurements.

Digital purpose

Digital microscopes only show the image of a specimen on a computer screen. This means they avoid the need for eyepieces, as light from the sample is directly focused onto a digital camera. This setup lends the microscopes greater flexibility and provides a more comfortable, ergonomic experience for the user. This is particularly important in high-throughput settings, such as in quality control, where users often spend hours working at the microscope.

Many modern digital microscopes are fully computer controlled, enabling the user to place a sample and carry out the rest of the imaging and analysis from the computer. This feature benefits a range of industrial applications where traceable documentation and measurements with guaranteed accuracy are an integral part of the workflow.

The examples below demonstrate the breadth of challenges facing inspectors when using microscopy – and how to tackle these with the help of the latest generation of digital microscopes.

Metal components

Failure analysis involves examining products or components that have developed a fault to investigate the root cause of the failure and to guide future improvements. Microscopes are a key part of several failure analysis workflows, as they provide detailed images of structures and surfaces.

An important imaging challenge in failure analysis is that the surfaces and features are often irregularly shaped. Microscopes produce the clearest images when the specimen is flat, and therefore irregular specimens pose an additional obstacle to obtaining images that are well-focused across the entire field of view.

When working with conventional microscopes, operators often use low-magnification objectives to examine uneven surfaces. These objectives have a higher working distance, enabling well-focused images of uneven components. However, the trade-off is that the level of detail is lower at low–magnification. Alternatively, high-magnification objectives can be used, but these often suffer from low working distance, leading to blurred images of 3D features.

Addressing this trade-off, the latest generation of digital microscopes, such as the Olympus DSX1000, provides a combination of resolving power and working distance that makes them well-suited for industrial inspections of irregular surfaces. Due to this, it is possible to see more detail at low magnification, and to get a clear, well-focused image at high magnification, thereby improving the probability of detecting the cause of a failure.

Guaranteeing accuracy in measurements – printed circuit boards

With the continuing trend towards miniaturisation of smartphones, tablets, PCs and wearable devices, it is becoming necessary to mount components on printed circuit boards (PCB) with ever higher density. However, this means that slight inaccuracies in manufacturing may cause electronic devices not to work properly due to current or signal transmission failures.

Routine, high-throughput inspection of PCBs is usually automated, but digital microscopy is necessary when doing additional, more detailed analysis. In this application, as well as in many other industrial applications, measurements form an integral part of the inspection workflow.

When carrying out measurements, it is not only the accuracy that counts, but guaranteeing accuracy is a key parameter. Many inspection processes have audit trails where not just the measurement results have to be recorded, but also the equipment used and their specifications.

To guarantee accuracy in measurements, a digital microscope may use a telecentric optical system. This minimises the distortions that normally occur when changing the magnification or focus, for repeatability and compliance. An optional, on-site calibration of the microscope by a technician greatly contributes to ensuring this high level of correctness.

Drill tip inspection

Tiny imperfections in the tip of a drill affect processing quality and can lead to faults during manufacturing. For a fast, reliable inspection, it helps to observe the drill quickly from multiple angles without having to reposition the specimen. An additional challenge is even illumination of surface features. The smooth outer surface of the drill has high reflectivity, which can lead to low contrast in other areas, such as the tip. Glare from the specimen can also affect the image resolution. Brightfield imaging is often unsuitable in this type of application.

The 180° rotation capability of the microscope ensures that the entire tip can be inspected without the need for repositioning, which reduces inspection times.

Some structures require fine contrast adjustments in order to reveal all the necessary information. MIX illumination is a unique method that is highly suitable for illuminating samples with a complex shape. It works by illuminating samples with a combination of coaxial lighting used in brightfield imaging, and ring lighting used in darkfield imaging. This combined approach enables MIX illumination to show contrast in a way that would not be possible using brightfield or darkfield alone. This feature is helpful for defining outlines and improving contrast in threshold-based analysis and for visualising different materials present on the same sample.

Brake pads

Choosing the right illumination mode can also be challenging. Different details of a specimen will become more or less pronounced, depending on the illumination method used.

The inspection of brake pads is a good example of this need for different observation modes. Brake pads consist of more than 20 different materials, and the level of blending of these materials greatly influences the performance of the brake, including force, heat stability and sound. Thoroughly inspecting the quality of a brake pad requires a microscope with high resolution and numerical aperture. In addition to this, easy switching between observation modes helps in quickly generating images that show potential imperfections in the greatest level of detail.

Images taken at different illumination modes show how different details of the surface structure become clear when changing to a different mode. By using fast switching between modes an inspector can quickly select the best way of showing all the relevant details.

Find every flaw

Industrial inspection is a highly multifaceted area where versatile equipment that requires little training to use is a valuable asset. The examples highlighted here clearly demonstrate how modern digital microscopes stand up to these inspection challenges with features that improve measurement, probability of detection and user-friendliness.