|
||||||
Перевести эту страницу на русский язык
Making PCBs at home: my method
Making electronic circuits using PCB boards is possibly the most efficient method, because the results are reliable, hard and repeatable constructions. Unfortunately, this method requires rather many efforts, but i suppose, that these efforts cost themselves. As to me, i sometimes prefer a PCB to any other technique, even for a 'sample' schematics and just trial experiments. Especially it is important in HF/VHF equipment, where the layout of component, and not only their connectivity, plays extremely big role in success or fault. Here I would like to describe the process of PCB's making, which I use during many years, since 1988, when I was 14 years old (I remember that time quite well - Gorbachev, hopes to the life improvement, serious political changes in Soviet Union and many other things, which are in the history already).
Stages of the PCB making process
Before the schematics is converted into the PCB, the following steps have to be made:
Creating a component and track layout for the PCB, based on the schematics, component sizes and other factors.
Transferring the layout scheme onto PCB material. Track's places are protected by the paint.
Chemical removal of all copper, which does not belong to tracks and contact pads.
Drilling the holes.
Soldering components into fresh PCB
Result of the first step is so called layout scheme, which represents the layout of components onto the PCB. This step is one of the most complicated, but it determines, how good the board will be. The next step gives the 'painted' board, where future track and pads are drawn with the water- and acid-resistive paint. After the third and fourth step, you will have the board, which is ready to insert components and solder them into.
Creating layout scheme
Typically, I do it by the following way: I draw the complete schematics, which include all the components which are to be placed onto the board. To simplify the viewing, avoid using 'grounding' symbols, draw all wires. Also for convenience ground network should be at the bottom of schematics, and the power line - at the top.
Then sizes of particular components should be determined according to what you have. Typically, there are a lot of similar components (resistor, capacitors, low-power-transistors etc.), and a small number of unique, usually big ones (transformers, high-power transistors and diodes etc.). The bigger is the ratio between smallest and largest component, the difficult is layout creation.
After all of the above the creation process itself starts. First of all, it should be decided, whether you need double- or single- sided PCB. Most analog circuits can be made using one-sided PCBs, but for digital boards double-sided PCB is usually required.
There are many words said and written about the strategy and rules of placing components and routing, but i would like to outline the following topics, which help to make analog PCBs with reasonable quality using minimal amount of time and efforts.
Try to keep ground track near the bottom of PCB (looking from track side), keep the power track at the top
Try to make layout similar to the schematics, do not try to make complicated 'rearrangements' without strong necessity.
It is better to create the layout in order, how the signal passes the stages: In an amplifier, for example, first stage should be processed first.
At the beginning, set the approximate width of the board and then try to fill it, moving from right to the left.
Do not forget to make the pads for external connections. If some net has more than one external connection, make the pad for each one.
If the board has some large components, think about their placement first.
In RF boards, follow the necessity of proximity of some components, as well as the largest possible distance between others (this is derived from schematics)
If the schematics is not straightforward and has many feedbacks and other difficulties, do not process these wires last - you probably will not have place for it.
There is nothing bad, if your final board will have some insulated wires at the component side, which serve instead of unroutable tracks, but I still do not like it
In double-sided PCB, try to minimize the number of tracks on the component side - they are more difficult to solder. Also, try to avoid using of excessive number of vias.
Do not make track's layout to dense - minimal width should be at least 0.7-1 mm. Thin tracks are very difficult to paint, and sometimes they are removed with the rest of copper during chemical dissolving.
For digital boards, another technique is used: according to the proximity in the principal circuit you place ICs in the regular grid-order, making several rows and columns. The nearest interconnections should be kept in mind - for example, if you have a counter and decoder connected to each other by 4-bit bus, chips should be placed to provide this connection by the easiest way. After rather simple placing, you route the board using double-side technique.
Layout scheme drawing is performed on gridded paper by a erasable pencil. If you feel, that the routing goes by wrong way, you may erase the wrong part and start again. Holes are presented as points, and tracks - as lines. The scheme may not reflect the actual shape of tracks, it must represent just a order of connectivity.
On double-sided PCB, three color pencils are to be used - two for each side and the last for vias. After you are done, you can determine physical sizes of PCB (DO NOT try to save material on 'cutting edges' - 3-5 mm from each side of PCB should be left blank) and cut your material to this size.
Transferring layout to the PCB
After you have a layout scheme, the process of PCB production becomes quite straightforward. Transferring layout means only transferring of holes positions, because the tracks you will paint, looking on the free space and according to your art taste.
Holes positions can be transferred easily: you take something like a hard needle, turn the paper with layout over the cut PCB material, aligning physical board margins with the scheme. Then you puncture the paper in places of holes with some force (a small hummer may be used). The result is, that positions of holes are transferred onto board as needle traces, which help with subsequent drilling.
If it is double-sided PCB, you have to drill the holes first, otherwise you can wait with it. Then, board should cleaned mechanically and chemically (acetone, alcohol etc.) to provide clean and fat-free surface (otherwise, paint will not have a good adhesion to the foil). After it, you take your favorite paint and your favorite drawing instrument and draw pads and tracks. I recommend to draw all the pads with a hole (or future hole) in the center first, making them as large as possible, but leaving enough space for tracks. Then you can connect all the pads according to the layout scheme by the tracks. After everything is done, board have to be checked in order to eliminate unnecessary paint leaks. Also, it is the good practice to compare the board with original schematics, because it is the last time you may introduce some corrections. After the paint is dry, you may go to the next step.
Chemical removal of unused copper foil
Classical and widely used method for dissolving of unprotected (by the paint) copper foil is using of ferric chloride (FeCl3) solution. The reaction goes fast, allowing to complete the whole process in less than one hour. But ferric chloride is rather expensive and sometimes hard to get, so my favorite method is using another solution. I use mixture of copper sulfate pentahydrate (CuSO4x5H2O) with ordinary salt (NaCl) in proportion 1:2. When dissolved in minimal amount of water, this mixture forms a deep-green solution, which is capable to dissolve metallic copper.Copper sulfate can be bought in an agriculture store for a quite reasonable price, because it is widely used for some purposes in plant growing (at least here, in Russia).
Unfortunately, the process goes not so fast, and typically 8-20 hours is required to process a PCB at room temperature. At 60-80 C, reaction goes much faster, but i still prefer to set it up over the night and do not worry about overheating and problem caused by it. At the end, reaction may slow down, and some heating with probably addition of reagents will not interfere.
After all unprotected copper is removed, you take the board off, wash it rigorously to remove chemicals, remove the paint with appropriate solvent, clean the copper surface again. If holes have not been drilled before, now it's right time to do it.
After all procedures described, your PCB is ready to be filled by components. All equipment i describe on my pages is made using such PCB's. The total procedure (except chemical treatment) for typical PCB can be made in several hours, and after the night your board is ready to solder!
P. S. Nowadays, the most modern and efficient method of PCB design is using special CAD computer software. Advanced system of this kind allow to draw your schematics in a special editor, assign a footprint to each component, place components to a board in WYSIWYG mode (sometimes even intellectual AutoPlace feature exists, which places the components automatically according to their connectivity), and then route connections on the board automatically (AutoRoute feature). The software automatically maintains the connection between your schematics and your PCB, do not allowing to place wrong tracks and enables the control over the completion of the boards, telling how many connections are routed, how many unrouted etc. After it, you may just print layout scheme on the printer and go on with it!
I started to use CAD systems about one half year ago, and now I stopped to draw the layouts on the paper by hands - I do everything on the computer's screen with a great help of the software. After achieveing of the optimum in palcing and routing, I print the layout out and perform all subsequent steps. The addtional great advantage of using CADs is that your constructions become fully documented 'automatically' without additional efforts, including full schematics with reference designators (inique numbers of components, like R1, C3, DA2, DD1.2 etc), and full PCB information with a complete parts and tracks layout. If desired, you can compile the list of components used and do a lot of thing by several mouse clicks!
The first construction, which was created using CAD software, is my dipolar power supply. Its description has just been finished.
73!, Valentin Gvozdev , RU3AEP.
| Overview of trial balance example and explanation. |
