Vision Completes 2D Data Matrix Inspection System

July 13, 2015
An image-based inspection system accelerates delivery of high volumes of printed circuit boards into supply distribution channels.

High-volume production in manufacturing or assembly operations requires efficiencies throughout production and distribution. An international company that designs, fabricates, tests and assembles printed circuit boards (PCBs), among other products, recently updated its optical inspection system to accelerate supply chain distribution at its Mexico plant.

The electronics supplier has grown exponentially since the 1960s, serving the aerospace, automotive, industrial, medical, telecom, enterprise computing and storage computing industries. Large volumes of PCBs make up the backbone of its business. The Mexico assembly plant uses a ViTrox automated optical inspection (AOI) system to examine PCB assembly in its many stages: bare boards, solder paste quality, pre-reflow and post-reflow. This optical system offers inspection speeds up to 55 cm2/sec for post-reflow and 65 cm2/sec for pre-reflow.

However, a vital component for high-volume assembly is removing quality control bottlenecks so finished goods can enter the supply chain. The original optical inspection system wasn’t equipped with a vision system on the bottom, and inspection results were manually entered via labels. A faster solution was needed.

The PCB supplier turned to Cognex’s DataMan 100 code reader and Jalisco, Mexico-based InterLatin, a manufacturing solutions integrator, to design and implement a more efficient solution. InterLatin installed the 2D data matrix vision-based system to minimize inspection reruns due to the large volume of PCBs and the challenges of reading a 2D matrix code at high speeds.

“We were concerned that the code reader most commonly used in this type of application would be unable to provide high read rates due to degradation seen in the 2D matrix codes,” says Dacil Amelia Cruz Vargas of InterLatin. The read-rate metric is the total number of barcodes recorded divided by the number of attempted, and the goal was a 99.9 percent or better read rate.

2D matrix codes are used for PCBs because of the low contrast requirement between the code and marking surface, around 20 percent. With this application, the 2D matrix is marked on the underside of the PCB. This printing method is suited for difficult surfaces, such as metal, non-metallic and plastic components. The 2D matrix codes also include built-in data error correction that allows the encoded information to be captured even if the data matrix symbol is somewhat degraded.

Other inspection variables can be the marking formats, either dot peen or laser etching.

In this application, a conveyor feeds PCBs into the machine from another room and the 2D codes are read outside the optical inspection system because of the need for machine mobility. The small vision system is located only about 2 inches above the incoming boards’ surface. The 2D camera can identify up to 45 reads per second from this continuous conveyor line.

The camera technology is a variable-focus, liquid lens technology. It gathers full-frame images in a single snapshot, and the system’s algorithms decode the 2D matrix within the captured form. The liquid lens uses two iso-density liquids, as oil is an insulator while water is a conductor. The variation of voltage leads to a change of curvature of the liquid-liquid interface, and it leads to a change of the focal length of the lens.

Practically speaking, this variable-focus technology eliminates the need to manually control the lens and reduces maintenance time in the process. In combination with the decoding software, the single-screen image allows a 2D code to be read once and then can be retrieved for post-inspection analysis or training.

The camera and housing footprint for this application is quite small (23 x 42 x 64 mm), and also comes with targeted lighting. The camera’s lighting provides much needed illumination for the PCB 2D matrix codes. A conveyor, originating from the assembly area, moves circuit boards through a small rectangular cutout in the wall to the receiving inspection system. As soon as the PCB leaves the assembly area, the vision system captures the 2D matrix code.

However, the decoding software literally fills in the gaps when lighting and data is missing from the 2D code. The software uses pattern-matching technology to handle a range of degradations, such as missing data or exposure issues.

The PCB supplier enters its inspection results for every PCB into a database and then examines any parameter deviations before releasing them. The vision system emulates a USB keyboard and interfaces with the optical inspection system. When connected to a PC over USB, the barcode reader appears as either a COM port or a standard USB keyboard and a user can control the connection type by scanning the desired connection code.

So far, the image-based system is producing near a 100 percent read rate. “Our customer has been pleased to have such high read rates,” Vargas says.

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