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Double-Sided Boards (Advanced)
For more complex projects, you may find that your layout has more jumpers than you can tolerate. This is particularly likely with digital circuits, since there are no resistors that act as natural jumpers for other traces to pass beneath.
When you reach your personal jumper limit, the next step is to use a double-sided circuit board. Layout is similar to a single-sided board, except you should use a different color (like blue) to distinguish top traces from bottom traces. For CAD designs, the top traces will be on a different plane of the drawing.
You should still try to keep the top (component side) traces to a minimum, simply to aid in soldering later. The top and bottom traces will be connected where component leads pass through. For places where top and bottom traces must connect but no component is present, just put a hole. This will become a pass-through or "via" by soldering a stub of wire there later.
Mark the holes and drill and sand as before. You will find that you need to sand both sides now, since the copper will acquire a little rounded "volcano" around each punch-mark. Then scour and clean both sides of the board.
The top side must be protected while you work on the bottom side. Although you can use a broad-tip marker to cover the whole surface, this coating is quite fragile and will suffer little scratches and pinholes. Apply a better protective layer over it in the form of vinyl contact paper, wide masking tape, or wide cellophane tape. Whatever you use, be sure to burnish it down well along any seams and in areas where there are drill holes. This will keep etchant from leaking in while the bottom is being etched. Since capillary action will try to wick the etchant under the tape around the holes and along the edges, it is a good idea to use both marker AND tape to be safe.
Now draw the bottom side and etch as usual. Rinse, remove the tape, and rinse again to get rid of etchant that seeped under.
Now scour the marker from the top side. Leave the marker on the finished bottom side for now, and touch up any areas that have been damaged during etching, before applying protective tape as you did earlier for the top. Don't do any tinning yet... wait until both sides are done.
Now draw the top side and etch and rinse as before. Scour the marker from both sides and you are ready for tinning or final assembly.
Note that unlike commercial double-sided boards this method does not have "plated-through holes", so you will need to solder both sides of any component that connects to top and bottom traces. This can be particularly tricky around IC chips, since you don't have access under the package itself.
If the trace connects to a pin on the top side and continues under the chip while remaining on the top side, you need to insure that there is a good connection to the trace covered by the chip. The drill hole must be smaller than the trace, so that an adequate skirt of copper surrounds the hole.
Note that you can't use ordinary sockets with this method, since they don't leave any top surface of the pins exposed for soldering. If you are installing an expensive part and feel you really must have a socket, you will need one of the more exotic socketing methods like separate pin sockets or all-metal breakaway socket strips.
See also Printed Circuit Construction
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