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Recently the Microscan Applications team was tasked with meeting a barcode challenge for one of our automotive electronics customers. During a quality control step in the customer's manufacturing line, barcodes on PCBs are decoded before the PCBs are placed into an ICT fixture for testing. In this case, barcodes on both the top and bottom surfaces of the PCB needed to be read. However, once a PCB was placed in the ICT fixture, the barcode on the bottom of the PCB was no longer easily accessible. Because of this, operators were required to scan both sides of each PCB by hand before they were placed into the ICT fixture. This manual step increased the risk of handheld readers being dropped and damaged, and also increased the risk of the boards being damaged by excessive handling.

The clear solution was to minimize handling of the PCBs by integrating barcode readers directly into the ICT fixtures to read both top and bottom codes automatically. However, these fixtures did not afford us the space to integrate our readers below the PCBs once they were placed inside. Given the limited integration environment, the question was: How do we read all of the barcodes on the PCB by integrating readers only above the PCB test area?

To find our answer, we decided to look at the challenge from a different angle. That is, by using angled bounce mirrors, we developed a solution that enabled our readers to "see" codes on each side of the PCB, even when they weren't directly visible.


The Concept
An imager takes a picture of a barcode, decodes it, and then outputs the data as a string to a host system for storage or further processing. In order to read the barcode, the imager needs to "see" the entire code. However, sometimes the imager cannot be properly positioned directly in front of the barcode due to limited space in the integration environment. To work around this issue, single or multiple bounce mirrors can be implemented to shape the imager's " line of sight" to the barcode. This allows a much wider integration area in which the imager can be positioned and still read barcodes.

Mirrors
To create an indirect optical path for an imager, a first surface mirror is the best type of mirror to use. First surface mirrors have a reflective coating on the top surface of a material, usually glass. Traditional mirrors are second surface mirrors, where the second surface is a shiny substrate behind a piece of glass.

Second surface mirrors can cause undesired reflections that the imager can "see," which can cause an unreliable reading environment. Second surface mirror are not recommended as bounce mirrors, but can be used if necessary.

Implementation
In applications where only an indirect optical path is available, mirrors must be used to simulate a direct optical path from the imager to the barcode. There are a few things to keep in mind when using mirrors:

  • Keep track of the total optical path. The element size of the barcode limits the total distance of the optical path from imager to barcode.
  • Light is reduced each time it is bounced. Depending on the size of the mirrors used, the light that reaches the barcode and is returned to the imager will be reduced compared to light that returns to the imager in a direct optical path application. The smaller the mirror, the smaller the amount of light that reaches the barcode and is returned to the imager. Imager shutter speed and gain will need to be adjusted to make up for this lighting difference in indirect optical path applications.
  • The use of mirrors does not eliminate the need for the imager to be at a slight angle in relation to the barcode. You will still get glare and/or hotspots if using right-angle mirrors, as you would if the imager were at 90 ° to the barcode with a direct optical path.
  • Pins or other shiny-surfaced materials within the Field of View (FOV) of the imager through the optical path can cause " hot spots" that may cause decoding problems. Using the imager's Window of Interest (WOI) will help to eliminate those "hot spots" from the area surrounding the barcode in the FOV. This helps by focusing the decoding algorithms on the area in the FOV where the barcode is located and thereby speeds up decode times.
  • Any size, number, or shape of mirrors can be used, keeping in mind the guidelines above.


Getting back to our automotive electronics customer"¦ The solution was to mount two barcode imagers in the ICT fixture above the PCB test area and used bounce mirrors to see barcodes on both the top and bottom of the PCB once it was placed inside. The mirrors directed the optical path from the barcode to the imager through prefabricated holes in the fixture. With this solution in place, the operators in our customer's facility now need only to place the PCB into the fixtures and let auto ID do its work. The integrated imagers automatically read codes on both surfaces of the PCBs before testing, reducing the risks associated with handling the equipment.


Even more helpful technical articles are available on Microscan's Tech Support Self-Help portal.

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