Case Study: Achieving Near-Perfect Reads of Data Matrix Codes in PCB Assemblies


Reading time ( words)

A major supply chain solutions company performs PCB assembly in a Mexican plant. The solder connections on the PCB are inspected with a vision solder inspection machine. A 2D Data Matrix code on the bottom of each PCB needs to be read at the time of inspection so the inspection results can be stored in a database.

Originally, the customer was not able to read the matrix codes because its AOI system wasn’t equipped with a camera on the bottom. The problem was solved with the installation of a vision-based reader that handles a wide range of degradations to the printed code. The result was a read rate over 99.9% and a significant improvement in the throughput of the assembly process.

2D Data Matrix Code Reading Challenges

The supply chain solutions company produces a wide range of PCBs for its customers and often inspects solder connections with AOI systems that offer inspection speeds up to 55cm2/sec for post-reflow and 65cm2/sec for pre-reflow.

The system integrator was tasked to provide the AOI system and make it work in this application. However, the code reader that is most commonly used in this type would be unable to provide high enough read rates due to degradation seen in the codes. Though Data Matrix codes have built-in error correction, 'no-reads' can still occur when codes are incomplete. So apart from a fast, compact reader capable of reading codes with varying amounts of degradation, the system needs the ability to emulate a USB keyboard in order to easily interface with the AOI system.

Cognex1.JPG

Figure 1: This picture shows the AOI's entrance and space where the reader needed to be installed. A support base was designed to be set up on the xyz axis as shown with the red lines.

PCBs can be marked with a number of different marking technologies such as dot peen or laser etching. 2D Data Matrix codes require as little as 20% contrast between the code and the marking surface, much less than linear barcodes. This allows the code to be marked on difficult surfaces of metal, non-metallic, and plastic components. Built-in error correction allows the encoded information to be captured even if the Data Matrix symbol is somewhat degraded.

But the range of 2D Data Matrix marking techniques and PCB materials means that the appearance of the marks can vary dramatically from one situation to the next. The result is often codes with low contrast, poor cell position, or inconsistent cell size. In addition, the surface being marked can be matted, cast, or highly reflective, and is seldom as clean and uniform as a white label.

In addition to the selected marking method, the parts come in different colors or shapes and can be made from different materials. Surfaces include smooth and shiny, furrowed, striped, streaked, or coarse granular. Reading such marks has been an enormous challenge until recently.

Space and Time Limitations

In this application, a conveyor feeds PCBs into the machine, but code reading needs to be performed outside the machine because of the potential for the machine to be moved. Only about 2in are available above the surface of the PCB to mount the reader. The high speed and continuous operation of the conveyor gives the reader little time to acquire and analyze the image.

Cognex2.JPG

Figure 2: This shows the limited space available to install the reader in the AOI.

The system integrator installed a Cognex DataMan 100 code reader, which met all of the requirements of this application. This image-based reader uses solid-state camera technology to capture full-frame images in a single snapshot, avoiding the need to cross a barcode with scan lines from end to end the way lasers do. When an imager captures a frame containing a barcode, it can locate that code anywhere within the frame. The code reader also incorporates a proprietary algorithm that can identify and decode even severely damaged or poorly marked Data Matrix codes.

Cognex3.JPG

Figure 3: Another view of the reader installed in the AOI.

Improved Throughput on Inspection Operation

The installation of the image-based ID reader enabled the process to produce a near 100% read rate. The supply chain solutions company has increased its production by reducing downtime required for opening the inspection machine and manually entering the labels. The result is substantial time savings and throughput improvements for its customers. The management is so pleased that it is planning to purchase additional devices for other assembly lines.

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