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Circuit board inspection
Circuit board inspection

Fast and Reliable Inspection of Printed Circuit Boards with Digital Microscopy

Digital microscopy has been used more and more for inspection, quality control and assurance (QC/QA), failure analysis (FA), and research and development (R&D) in the microelectronics industry, especially for printed circuit boards (PCBs). Digital microscopes are practical to use and allow an efficient workflow for inspection. Here the advantages of the following digital microscope features are discussed: i) rapid, easy tilting and rotation to view the sample from different perspectives, ii) integrated illumination for versatile contrast, iii) high dynamic range (HDR) imaging, and iv) large mosaic overview of the sample.

Digital microscopes have no eyepieces and the image is observed directly on a monitor. With the Leica DVM6 digital microscope, for example, users can quickly record high quality, reliable image data and easily make measurements and analysis during inspection of microelectronic parts, such as PCBs. Efficient inspection, QC/QA, FA, and R&D of microelectronics is achieved.

Circuit board inspection

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How Does a Digital Microscope Make Inspection Workflow Efficient?

The Leica DVM6 makes the workflow of inspection, QC/QA, FA, and R&D efficient thanks to multiple important functions:

  • Fast and easy microscope head tilting and sample rotation;
  • Mosaic overviews of sample, e.g. XY & XYZ stitching;
  • Integrated LED (light-emitting diode) ring light and coaxial illumination and backlight accessory providing versatile contrast to enhance sample features;
  • High dynamic range (HDR) capability for imaging [1, 2];
  • Intuitive software for microscope operation and data analysis [3];
  • Efficient, simple way to change magnification over the entire range (12:1 to 2350:1) [3];
  • Encoding (automated tracking and storing) of important parameters, e.g., stage, optics, illumination, and camera settings, for rapid recall at any time [3]; and
  • High performance digital camera with fast live image and 10 megapixel (MP) resolution [1, 4].

In this report it is explained how the first 4 points above contribute to a fast, reliable workflow for microelectronics. The other features of the Leica DVM6 have already been discussed in a previous report [3].

Setup and Start Inspection

The Leica DVM6 digital microscope is simple to set up and operate. Plug in its power cable, connect a USB cable to a computer running the Leica Application Suite X (LAS X) software, put in an objective lens, and the Leica DVM6 is ready for inspection.

Below is a photo of the Leica DVM6 digital microscope with a PCB (printed circuit board) placed onto its stage.

Fig. 2

Viewing PCB Components from Different Perspectives: Microscope Head Tilting and Stage Rotation

A fast and reliable inspection workflow for PCBs often requires that their complex, 3D components, such as amplifiers, voltage regulators, transistors, diodes, capacitors, assemblies, etc., can be viewed from various perspectives. The Leica DVM6 digital microscope has both a tilting head (–60° to +60°) and rotating sample stage (–180° and +180°) [5].

Examples of PCB images recorded at different viewing perspectives are shown below. The different viewing perspectives can be important when inspecting components; for example, the soldered leads of the voltage regulator circled in red. Changing the viewing angle allows different parts of the leads and components to be seen which can be “hidden” with a fixed, overhead view (tilting of 0°).

Mosaic Overview of Sample

Often when inspecting PCBs, it is quite convenient to have a low-scale overview and then zoom in on a point of interest at higher magnification.

The LAS X software allows different modes for a large XY (2D) scan, such as “mark & find,” “tile scan,” and “spiral scan.” As the stage scans along the X and Y directions, multiple images of the sample over the area of interest are acquired, and then LAS X stitches them together to form a mosaic picture [5].

An example of a large mosaic overview image of a PCB is shown below.

Fig. 7: Leica DVM6 mosaic overview of a large area of a PCB (printed circuit board).

Versatile Illumination Offering Multiple Contrast Methods and High Dynamic Range (HDR) Imaging

When doing inspection, QC, FA, and R&D of PCBs, versatile illumination and contrast methods help to make specific details or features visible more easily. The Leica DVM6 is equipped with an integrated LED ring light and coaxial illumination and, additionally, illumination accessories, such as a backlight, light diffusor, and light polarizer are available.

Ring Light and Backlight

The images below show the same area of the PCB imaged using ring light plus backlight or just backlight illumination. The backlighting causes light to be seen coming through holes or transparent or translucent parts of the PCB. Most of the holes in the PCB are vias [6]. There are also translucent regions of the PCB’s fiber glass epoxy substrate which are found around some of the vias.

Light Diffusor and Polarizer

The images below show the same area of the PCB imaged using ring light illumination with or without an accessory, i.e. backlight, light diffusor, or polarizer. The polarizer completely eliminates glare from highly reflective points, such as soldered leads. When imaging with a tilted microscope head, the images show that the diffusor eliminates glare from the varnish of the PCB fiberglass epoxy substrate. Tilting with backlight also allows details on the internal walls of vias (holes) in the PCB to be seen.

High Dynamic Range (HDR)

To avoid areas that are too dark or too light in an acquired image, high dynamic range (HDR) imaging [1, 2] can be exploited. The HDR imaging method captures a series of images of the sample with varying exposure intensities. The LAS X software then calculates a final composite image using different algorithms. The method can require more time to record a final image as multiple images of the sample with different illumination are taken, but in general more details on the sample are visible.

Images of a PCB area, showing a voltage regulator, taken with and without HDR are shown below.

Summary and Conclusions

Reliable and efficient inspection of printed circuit boards (PCBs) for quality control and assurance (QA/QC), failure analysis (FA), and research and development (R&D) is accomplished using the Leica DVM6 digital microscope. This report explained the following benefits:

  1. better PCB component inspection by changing the viewing perspective with simple microscope head tilting and sample rotation;
  2. obtaining a fast large-area overview of the sample using mosaic image building, i.e. stitching, to efficiently search for potential problem defects;
  3. optimal contrast to enhance features on the PCB using the integrated LED (light-emitting diode) ring light and coaxial illumination and accessories, such as the backlight, diffusor, and polarizer; and
  4. high dynamic range (HDR) imaging [1, 2].

These functions allow Leica DVM6 users to make fast and reliable data acquisition and analysis for better inspection, QC, FA, and R&D workflow efficiency.