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Printed Circuit Boards – Part II of Creating our Masterpiece.

A Recent Built Electronic Kit.

A Recent Built Electronic Kit.

Last night I discussed the basics of laying out a circuit board. Tonight I will discuss some about how boards are constructed.  The picture to the right is a fairly simple printed circuit board, PCB, that came as part of an electronic kit I recently bought.  It will be helpful to look at that picture as I describe how a board is created.  Should you decide to try to make a circuit board yourself, a simple circuit such as that or even better some simple projects that can be found on line would be a good choice for your first attempt.

A PCB consists of a substrate material forming the board and one or more conductive layers on the board.  Simple circuits such as the one shown in the kit only have a conductive layer on one side.  More complicated circuits have a conductive layer on both sides of the board.  Some boards such as the mother board in your PC have multiple layers.  Often these inner layers are primarily almost a solid copper layer to form a shield and a “ground plane” to keep electrical noise at a minimum in high frequency circuits.  The process for making these kinds of boards are beyond the reach of us hobbyists.

The substrate material is normally fiberglass, (the green boards) or a phenolic material (the brown boards).  Fiberglass is stronger and can handle temperature and humidity extremes better, but is also more expensive.   If, as is the case of the power supply kit, heavy components are mounted to the board, fiberglass is the best choice.

Usually boards are purchased with copper already applied on the board.  Some sort of masking material is placed on the board and then the board is placed into an etching chemical bath and the non-masked part of the board is chemically removed by the etching chemical.  Getting the mask on the board is the first problem.   For very simple boards some electronics suppliers provide PCB etching kits that contain the chemical and a permanent marking pen, a Sharpie, to draw the traces on the board.  I have even tried fingernail polish to do a similar thing.  I ended up with a very crude looking board.

The next step up is some stick-on masking material.  Again this must be hand applied and applied accurately.  It is not a process I want to try.  I have seen many videos and articles about using a laser printer and transferring the laser printed image from slick paper to the board using a hot iron.  There are also ways of treating the board with a photosensitive material and placing a clear material with the traces on top of the photosensitive coating and then exposing the coating to some light.  Finally for more production runs the mask is actually printed on the board.

A final way to create the board is to mask the areas to not have the conductive traces and electroplate the traces on the board.   To get high quality boards the concentration and temperature of the etchant needs to be controlled as well as the etching time.  Not enough time and conductive material is left at places where you do not want it to remain, and excessively aggressive etching can cause material to be removed under the mask.  This is called “under cutting”.

Once the traces are created, holes must be drilled in the board.   If a two sided board is created and the top side traces are to be connected to the bottom side traces an additional process can be done called plated through holes.  This is a plating of metal inside the holes to connect the bottom traces to the top ones.

Like everything, I recommend  baby-stepping through the process and starting out with a simple single sided board.   Once you develop confidence and consistency you can move up to 2 sided boards with more complicated designs.

Should you decide to actually fabricate a PCB, LibreCAD is probably not the way you would want to design it. It would be necessary to make wider traces than the simple lines I drew and it would be necessary to hatch in all of the circles I drew for “lands”, the circles around the pins and wires going through the board.   This would be tedious and it is probably a smarter investment in time to learn special purpose software.

There are at least two PCB companies on the web that have free PCB cad software available for Windows machines.  These are ExpressPCB and 4Pcb.  For Linux and Mac users (as well as Windows users). There is a free version of Eagle PCB software.  There are also several other PCB design programs available for Linux, none of which I have tried.   The ExpressPCB site also has some hints that will be helpful in designing your board.  For example, how wide to make your traces.

Other methods of building your circuit can be a very old method called wire wrapping.   This uses special connectors and a special tool to wrap the wire tightly around the special pins to make an electrical connection.   I have seen this method used in older industrial equipment, but it is seldom used today.

The method I intend to use is to mount the components on a board and run small wires between each component.   I may be very sorry for trying this method.  We will see.

In addition to what I have written and the links provided above you can find many links and videos about PCB fabrication on the web.  A good site to check out is Instructables.  A link to that site is provided on my Resources Page.   The following video is a pretty good one showing the laser printer transfer method.

As always, I hope this has been worth your time and effort.

Gary.
Keep creating and making… it beats the heck out of watching TV!

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