Students designing their own circuit board

The goal of this workshop was to get familiar with the program EAGLE and replicate a known circuit with it. In a second step a stencil is used to apply solder paste and mount the beforehand replicated PCB with SMD components.

Eurocircuits GmbH is a specialist manufacturer of prototype and small batch PCB.  Also, they offer DRC and DFM online- tools. http://www.eurocircuits.com/

Autodesk, Inc. makes software for the architecture, engineering, construction, manufacturing, media, and entertainment industries. https://www.autodesk.com/

Designing the Board

For this workshop the program EAGLE is used to visualize the circuit diagrams. EAGLE consists of several components: layout editor, circuit diagram editor, autorouter and an extensible component database. (By the way: the full EAGLE license is free for students!). 

To get started we replicate an easy toggle switch to get familiar with the functions of EAGLE.

Most components can be plugged from the integrated libraries and placed within our chosen frame. But the first consideration should be whether SMD or wired components will be used. This can also be changed later on, but might mess with the already drawn design. When all components are placed, it is time to connect them via the net function. After that the visualization is done and can be checked for inconsistencies.

Now to the hard and interesting part: To fit the design on a chosen circuit board and route the board. When placing the parts on the board diameter there are a few considerations to be made. A component might have its own requirements in terms of spacing and placement and should not overlap with other components. Further, in general a smaller board is cheaper but makes the routing of the airwires harder. The intersecting of airwires can also make the routing process more difficult.

In the following routing process the airwires are turned into copper traces. They can be placed, like the components, on the top as well as the bottom layer of the board. Here, again the choice is between a rather smaller board and two soldering operations or a rather larger board and only one soldering operation. A possible way to start the routing process is to go with the “save” routes (the short, direct and horizontal or vertical routes).  Also, there should be either a horizontal or a vertical main direction to make the routing neater. To change from the bottom to the top layer there is a via option, where the drill hole size and other features can be adjusted. Make sure there is enough space between the routes and that they don’t cross each other on the same layer. It is best to stay within either a 90° or 45° bend for the routes and avoid acute angles. Moreover, take into account that the width of the copper traces has an impact on the durability (think bending the circuit board). 

This finished design can be directly uploaded to for example the Eurocircuits website to order the finished PCB and a stencil for the solder paste.

Mounting SMD components and soldering

We got a PCB already prepared with our toggle design to learn how to apply the solder paste and mount the components by hand and with a camera based manual pick and place system. To make life easier we use a stencil-mate to keep our board and stencil in place. To apply the solder paste it is laid out in a big blob just above the stencil. With slight pressure the scraper is pulled over the stencil in a 45° angle to get the paste into all the drill holes. The last step is to lift the stencil from the board and check whether the solder paste is properly applied. If not, just wipe the board clean and stencil again.

We are happy with our stenciling so with the help of a pair of tweezers and a magnifying glass we carefully place all the components on our board and try not to smudge the solder paste. Now our board is ready for the reflow oven. The temperature and time profile is set accordingly to the solder paste information and after a few minutes our board is ready. After cooling we can repeat the process for the other side of the board as we picked a design with a component placement on both layers.

And it is done! The LED flashes and we just successfully completed our first board.

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