How to make PCB design - Black keyhole

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How to make PCB design

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To create the PCB layout, you can either draw the design by hand or use a CAD tool. Once you have the design, you have a number of choices for actually creating the board:

           ■ Using an etch-resistant pen

           ■ Printing the layout onto overhead projector (OHP) transparency film and using                                      photosensitiveboard

           ■Using laser printer toner transfer

           ■ Using CAD and a desktop router to cut out the pattern on the copper-clad board

           ■ Sending the Gerber design files away to a PCB shop

Each of these approaches has pros and cons. The last option has started to become cost-effective, with prices as low as a dollar a board for a run of ten boards, and will produce professional-quality results. However, you do need to wait, sometimes for a week or two. We will look at each of these approaches in turn.

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Etch-Resistant Pen

 Custom etching involves using graphic and chemical techniques to convert a copper-covered board into a custom-etched one. By doing your own custom etching, you can construct highly reliable, tightly compact circuits that require few jumper wires.

To design a custom-etched PCB, you first need an insulated board (usually 1⁄16 in.thick and made from a fire-resistant epoxy-bonded fiberglass) that is completely covered on one or both sides with a very thin copper coating. Next, you must transform your circuit schematic into a PCB layout. This involves rearranging components in such ways as to make all conductive pathways short and direct. The layout should also eliminate any wire crossing, if possible.

Once you feel that your hardwired sketch is complete, the next step is to transfer it onto the copper-coated board. Afterward, the trick is to etch out all the undesired copper-coated sections while leaving the conductive pathways intact. At this point, there are a number of different transferring/etching techniques from which you can choose.

To make your custom-etched board, you first transfer your hardwired sketch onto the surface of the board with, say, a pencil. Next, you drill in the appropriate holes where component leads are to go. Now, with the etch-resistant pen, you trace over the pencil sketch, making sure to encircle the drilled-out holes. After that, you place the board in a tub of etching solvent (typically ferric chloride) and wait until the copper dissolves away from the sections of the board that are not coated with magic-marker ink (the ink does not dissolve in the solvent; it acts to protect the underlying copper). After the board is removed from the solvent bath, it is washed off with water, and the magic-marker ink is then removed with a rag doused in rubbing alcohol. Using a PCB kit is great for simple, single-run productions. PCB kits are easy to use,inexpensive, and require practically no special equipment other than what is provided in the kit itself. However, one problem with these kits is that you can construct only one circuit board at a time. Another problem with these kits is the limited precision you get by using a magic marker to create conductive pathways. If you are interested in making multiple copies of a circuit, and if you are looking for greater line precision, a more sophisticated technique that involves photochemical processes is required.

Using a PCB Service

 It used to be that designing a PCB was a very specialized skill, and in some ways, it still is. For very complex projects heading toward quantity production, the services of a specialist PCB design company are worth considering. However, programs like EAGLE PCB will take a lot of the difficulty away, both by automating the track layout on the PCB and enforcing design rules such as minimum track widths, separations, and so on. This makes it entirely feasible to design your own PCBs and submit the finished design files to a PCB service. This service is likely to be almost entirely automated, so you will need to make sure your design is right before you send it, because these services will not check your design. 

A PCB service will produce a great-looking result. All of these services generally allow a two-layer board, which provides a top and bottom copper layer. What is great is that they also do the following:

        ■ Create the vias that link a track on one layer to one beneath it.

        ■ Provide a silk-screen layer that can label the components and mark their positions.

        ■ Provide a mask layer that covers all the copper not actually intended to be soldered.

The design files that you send to a PCB service are called Gerber files.Each of the files has a different extension that indicates its contents.

All that remains to do is to pay, create a zip file containing the files, and e-mail them to the PCB service. Your boards will come back in the post days or weeks later, depending on the service level you chose. When schematics get even a little complicated, you rapidly find that trying to route everything on one layer of copper becomes impossible, and a two-layer board becomes necessary. The top and bottom layers can either connect where a component lead naturally passes through both layers or using vias that are holes in the board just to allow a trace to jump layers.

A Note About Board Layout

 When arranging components on a circuit board, ICs and resistors should be placed in rows and should all be pointing in the same direction. Also, make sure to leave about a 2 mm border around the circuit board to allow room for card lifters, guides, and standoffs. Bring power supply leads or other I/O leads to the edge of the board, connecting them through an edge connector, D-connector, barrier-strip connector, or singlebinder posts fixed to the edge of the board. Avoid mounting heavy components on circuit boards to prevent damage in case of a fall. It is also a good idea to place polarity marking on the board next to devices such as diodes and electrolytic capacitors. Placing labels next to IC pins is also helpful.

Consider labeling test points, trimmer functions (such as zero adjustment), inputs

and outputs, indicator light functions, and power-supply terminals as well.

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