MIT gets the lead out

A printed-circuit board (PCB) is made of several layers, with a solder mask acting as the top layer to protect the underlying copper etchings that provide interconnects between components from solder shorts.

A printed-circuit board (PCB) is made of several layers, with a solder mask acting as the top layer to protect the underlying copper etchings that provide interconnects between components from solder shorts. Electronic components are placed over holes in the solder mask, and liquid solder is pushed into the hole in the mask. As the solder hardens, it provides a solid connection between the underlying copper and the component. Solder hardens quickly, but increasing the speed of the soldering process would significantly increase the productivity of electronics-manufacturing lines.

Researchers at the Massachusetts Institute of Technology (MIT) Laboratory for Manufacturing and Productivity (Cambridge, MA, USA; web.mit.edu/lmp/www/) are developing a faster, cheaper alternative that precisely shoots liquid solder from a gun onto the circuit board, potentially eliminating the need for masks and significantly speeding the process. The initial MIT work on the solder gun has demonstrated solder 'shot' rates of 4500 drops/s, according to research-assistant Wayne Hsiao.

"A basic understanding of the physics of deposition is important to forming a good interconnect," explains Hsiao. However, evaluating the solder gun posed several challenges, from the size of the solder ball (~300 µm), to the featureless solder "droplet," to the speed of the droplet (3 to 4 m/s). "The whole impact process is complete in about 1.6 ms or so," Hsiao says.

Hsiao turned to Photron (San Diego, CA, USA; www.photron.com) to provide a high-speed acquisition system. The Ultima SE is a self-contained unit that can acquire up to 4500 images of 256 × 256 pixels with an 8-bit gray-scale depth. Hsiao uses it at its maximum speed, 40,500 frames/s, at a partial frame size of 64 × 64 pixels. A 500-W Lowel (Brooklyn, NY, USA; www.lowel.com) halogen lamp illuminates the simulated circuit-board target during acquisition.

The Ultima SE and solder gun are controlled by a standard Gateway (Poway, CA, USA; www.gateway.com) Solo 9550 mid-range laptop connected to both instruments through a 1394 FireWire connection. Hsiao triggers the solder gun, and the imaging system is automatically triggered 0.5 s later to allow the solder droplets to reach the target surface. The system acquires 3.4 s of video, or approximately 130,000 frames. Out of the captured frames, only 100 frames (2.5 ms) show the moment when the solder droplet strikes the circuit board. Hsiao uploads the stored video into Xcitex (Cambridge, MA, USA; www.xcitex.com) MiDAS (Motion & Integrated Data Analysis System) Professional Analyst software. Hsiao strips away the empty frames, focusing on the approximately 100 frames that show the droplet impact.

Hsiao uses MiDAS software to automatically track and calculate the droplet size as a function of time. He uses the 1-D line-tracking feature, which allows him to draw a line just above the surface of the PCB. The software then tracks all the changes to the image along that line using algorithms based on derivative changes above configured thresholds. Using the resulting frame-by-frame droplet-size data, he can determine the appropriate shot parameters, then annotate, store, and report his data.

Explains Hsiao, "Using MiDAS, I can draw a line across the field, set a threshold value, and track the changes to the solder. Afterward I can download it all to Microsoft Excel. We do 50 droplet-impact sequences per run, and at 100 images that's a lot of information. This is the only commercial software I have found that can do all this."

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