A GUIDELINE ON PRINTED CIRCUIT BOARD DESIGN for YEAR 3 AND 4 ENGINEERING STUDENTS UNIVERSITY of AUCKLAND DEPARTMENT of ELECTRICAL AND ELECTRONIC ENGINEERING BY FRED NASSENSTEIN DEC 1992 UPDATED MAR 1995 UPDATED DEC 1996 BY SLAVEK PRZEPIORSKI Libray List Open Protel list THE COMPLETE PROCESS OF CIRCUIT DESIGN 1. SCHEMATIC DESIGN 2. PCB DESIGN 3. ARTWORK PLOT 4. MASK (NEGATIVE) 5. BOARD EXPOSURE - DEVELOPING CHEMICAL PROCESS - ETCHING - PHOTORESITS REMOVAL 6. TINNING 7. DRILLING 8. BOARD ASSEMBLING 9. TESTING Process 2 to 6 are briefly mentioned in this guideline . The work involved with process 4 to 6 is usually carried out by a technician in this department. CAD Contents: 1. - Introduction 2. - Chemical Process 3. - Computer Aided Design 4. - Using PROTEL-PCB 5. - Example 6. - Appendix 7. - Library Parts INTRODUCTION Having gone through the process and struggle to design an electronic circuit, which should result in the form of a circuit diagram, you have reached the stage of needing to create your circuit at a physical level. Printed circuit board design could be difficult as well, since you may include mistakes from your circuit diagram and possibly create some more with this procedure (unless you are very careful). But relax creating a PCB could (hopefully) be a nice expression of your technical capabilities as well as an expression of your artistic skills. A PCB is the most common, practical, reliable and low cost (in particular for larger quantities) solution to assemble a cluster of electronics components (see #1 other techniques). The aim of PCB design is to physically place and interconnect (using conductive tracks acting as wires) electronic components on an insulating material according to the plans in the circuit diagram (or schematic). CHEMICAL PROCESS In practice we do not actually print the physical circuit on a board, this is usually achieved by means of a photographic and chemical process (see #2 for other techniques). The board is usually made of a glass fibre strengthened epoxy based material (sheet) with good electrical and mechanical properties (ie.: good insulator and high mechanical strength). The epoxy board is covered with a very thin copper layer either on one (single sided boards) or on both sides (double sided boards) which we have to configure in such a way so that the copper can be used as individual conductors (see #3 for multi layers). This is usually achieved with the aid of an additional UV light sensitive photo resist (plastic film) attached to the surface of the copper layer (see figure 1). By exposing the photo resist with Ultra Violet light the resist will stabilise and remain unstabilized for the areas not exposed. With the use of a developing process (similar to normal film processing) we are then able to remove the unexposed photo resist areas leaving the exposed areas which will act as a protective film for the etching process. By placing this board into an etching solution (acid) we are able to etch (remove). Finally with the aid of a solvent we have to remove the photo resist so that the copper can be soldered on to. Usually the copper surface will be roller tinned or sprayed with a solder lacquer to improve the soldering and to avoid corrosion. It is obvious that with the above described process we are able to create any shape of a conductive structure and thus electrical connections on an insulating Fig.1. material. To obtain the desired structure necessary for our circuit design however we need to generate a MASK or NEGATIVE (black on transparent film) which we have to place between our film coated board and the UV light exposure unit. COMPUTER AIDED DESIGN (CAD) In the Electrical and Electronics Engineering Department there are several PCB software packages available on the network able to produce the artwork for such a mask. In this session however I will only outline the use of PROTEL- WINDOWS since it is easy to use and performs very well for most designs. At present we use the above software to create a PLOT ARTWORK FILE in Postcript format which can be send to an outside firm (ie.:BTL Technologies). This company is able to produce a high resolution negative film necessary for the final MASK and manufacturing process of the PCB (see #4 for other techniques). Before going further in to detail in how to use the PCB software, you should consider the following: 1- KEEP THE TRACK LENGTH SHORT AND WIDE The copper layer on the PCB is very thin (30 um) this means that a certain length of track between two points does not have zero resistance or zero inductance!! This is not necessarily a problem since most signals will carry low currents any way, however for very fast and high current signals, especially power supply tracks, this could cause some problems. Therefore keep the track length short (especially analogue input signals) and, in particular for all the power supply connections, as wide as possible (#5). 2- USE DECOUPLING CAPACITORS ON EACH IC SUPPLY CONNECTION Because of the above reasons it is near to impossible to supply each individual IC with an ideal (zero impedance) voltage supply especially at high frequencies. This can cause problems since switching currents will induce noise in the supply voltage and disturb the function of the circuit. It is therefore recommended to use decoupling capacitors between the supply pins (as near as possible to the pins and for every individual IC (#6) this will reduce the impedance at high frequencies. Include these decoupling capacitors in the circuit diagram before starting with the pcb design 3- KEEP THE TRACKS SHORT AND SUFFICIENT SPACED FROM OTHER TRACKS The impedance (caused by the capacitance and leakage resistance) between tracks is not zero, this is often the cause of unwanted oscillations in particular when using fast circuits or high gain amplifier circuits. In some cases it could be necessary to use a guard ring around sensitive signal tracks. Use sufficient spacing to other tracks in particular the input signals of amplifier circuits (exceptions can be made for data signals) and when using high voltages (#7). 4- TRY TO USE SINGLE SIDED BOARDS ONLY Use single sided boards (copper layer on the bottom side) only, unless you are using so many components and you have no other choice than to use the second layer as well (#8 and #9). Single sided boards are cheaper and easier to manufacture than double sided boards ! 5- LIST THE PHYSICAL DIMENSIONS OF COMPONENTS Before using the computer, make a list with all dimensions, lead spacing and lead diameter of the components you intend to use. This is also important when you intend to use Protel Schematic! Example of an axial component: Note that the lead space is larger than the component length and that the component is soldered on the bottom. Bottom side of the PCB only. 6- LIST THE PIN NUMBERS In a circuit diagram you normally use symbols for the individual components where every input or output (of an IC) and any connection to other components corresponds with a physical pin number (see the Data Books for physical dimensions and pin configurations). Specify the pin numbers in your circuit diagram since you will need this information in the PCB design (unless you have created a netlist with a schematic program such as Protel Schematic). USING PROTEL - PCB ADVANCED 1- READ THE MANUAL Like any other software it is recommended to read the manual first to get some basic understanding of this package! In some cases there is necessary to read Help File available from Menu Bar. 2- USING A NETLIST It is possible, for example after you have created a circuit diagram with PROTEL-SCHEMATIC and the generation of a so called NETLIST, to “load your schematic (as a netlist)” directly in to PCB (see # 10). It is however very important that all the pins of the components are assigned as well as all the packages (ie.: dipl4, axial 0.3, etc.) and make sure that all physical packages you are using exists in the PCB Library! Advantages of using a Netlist are; -mistakes are eliminated to a large extend -automatic placement of all the components from the schematic diagram (you could otherwise forget some components!) -using the DRC option allows you to compare your PCB file with the actual schematic diagram (and also check your spacings) -all logical connections are visible with so called rubber bands allowing optimum placements of components and tracks Disadvantages of using a Netlist are; As mentioned above you must specify everything correctly in your Circuit Diagram otherwise, and especially when you are not that familiar with Protel Schematic, you could spend quite some time solving problems with netlist transfers which are always likely to occur. Unless you have enough time to spend to learn this exercise, it is not recommended to use a netlist when you are an unexperienced user. 3- STARTING PROTEL-PCB Protel-PCB will ask you what file name it should load, however since you have not yet created a name and directory yet (when you use PCB for the first time), you should now press the ESC button, in case your pcb file already exists type the name of your directory and your design file name. 4- USE IMPERIAL DIMENSIONS From the Data Books you might have noticed that the semiconductor industry is not using the metric system as a standard, all dimensions are in inches!!! The dimensions of passive components on he other hand are often specified in millimetres! Keeping the above in mind and to avoid too much confusion it is suggested to select the Imperial sizes in PCB (Options), all dimensions will then be in 0.001 (mils) of an inch (or multiples). 1000 mils = 1 inch = 25.4 mm = 2.54 cm 10 mils = .254 mm 1 mm = .039 inch = 39 mils 5- VISIBLE GRID and SNAP GRID PCB is using a visible grid (will not appear in the artwork) which will help you to place components and tracks in a symmetrical order (you don’t want your board to look like a mess!). Snap grid acts like an “invisible grid” where when placing components or tracks they will snap or lock on to even when the cursor is not placed on this grid. 6- Set the VISIBLE GRID to 1000 and the SNAP GRID to 100 mils !! 7 DO NOT CHANGE TOO MANY OF THE DEFAULT SETTINGS 8- DEFINING THE BOARD DIMENSIONS You could in fact now start to define the outline dimensions of your board, however in many cases it is better to do this at a later stage since the size depends not only on the number of components but also how you place them (unless of course you are restricted by a certain size you could start now). When using a Netlist please refer to #11 ! 9- PLACE THE COMPONENTS BY PHYSICAL SHAPE You might have noticed from the Protel-PCB Manual that you can not place components by just calling a part from the library with the usual part numbers (say a LM741), you can only load a physical equivalent such as: DIP8 - LM741 AXIAL0.4 - RESISTOR (.33 W) CD0.2/1 - DECOUPLING CAPACITOR (lOOnF) 10-PLACE COMPONENTS FROM THE TOP VIEW When placing components, place them in such a way as if you are looking on top of the board since the physical layout or pin configuration of components in the Data Books are usually shown from the top. 11-KEEP THE COMPONENTS CLOSE TOGETHER Large spacings between components will cause undesired long tracks and will increase your PCB dimensions and thus cost. 12-KEEP ANALOGUE AND DIGITAL CIRCUITS SEPARATED In case your application involves a combination of analog and digital circuits it is recommended to keep these circuits physically separated on the PCB (to minimise noise). Try to avoid common supplies and grounds, in case you have no other alternative, keep the supplies and grounds separate and connect them at one point only. 13-PLACE COMPONENTS IN ONE DIRECTION AND ORIENTATION Start placing components either all vertical (i.e.: all with the reference pin1 at left top corner with IC’s) or horizontal (when this is more practical) and in such a way so that it will be the best allocation to make the interconnections (tracks) as in the schematic diagram. This is more important for larger PCB’s and where a “tidy” appearance is required. 14-LABEL AND COMMENT THE COMPONENTS When placing components Protel-PCB will ask you for a Component Name (physical type) following a component designator and comment (value or part number). Example: NAME=AXIAL 0.4 (for a standard resistor) PCM5/2.5 (for a standard capacitor) DIP 8 (for an OPAMP) DESIGNATOR=R1 (for a resistor, R2 for the next and so on) C1 (for a capacitor) IC1 (for an integrated circuit ie.: OPAMP) COMMENT=10 Kohm (for a particular resistor) 1 nF (for a particular capacitor) LM741 (for an OPAMP) NOTE: After placing many components the text might disturb the visibility when placing tracks, suggestion; turn the comments off (edit component). 15-USE LARGE PADS Since you have to solder each component connection or PIN to a particular track the Autotrax software will create PADS (or donuts) whose size and shape are determined by a default setting. Try to keep the size of the pads large enough (in some cases you might have to alter the default settings to a larger size ie.: Edit Pad).Also adjust the size of the pad in relation to the size of the holes to be drilled later on, so watch the lead sizes of your components (#12) ! Do not forget to specify the hole sizes, a not defined hole size (ie.:0) will cause no pad holes in the artwork plot! RECOMMENDED MINIMUM PAD SIZES: 60 mils for Via’ s (double sided boards only drill size 0.6mm) 70 mils for resistors, capacitors etc. (drill size 0.8mm) 60x100 mils rounded rectangle for IC pins (drill size 0.6mm) Use larger pad sizes when increasing the hole or drill sizes!! 16-PLACING THE TRACKS The best (other people might disagree) way is to start laying tracks for all signals first and the (wider) tracks for the power supplies last ! Change the Visible Grid to 100 and Snap Grid to 25. Always consider your currents especially with power supply tracks! When using single sided boards place the tracks as if you are looking through the board (imagine the circuit board is transparent) and place the tracks on the solder or bottom side (default: blue layer) only! For double sided boards you place the tracks on the solder side the same way as single sided, but note the tracks on the component or top side (default: red layer) are also viewed from the same perspective. See Figure 2. Avoid the number of top layers (component side) as much as possible since soldering (and especially desoldering) components on this side is often difficult and could, when you are not very careful, damage your tracks ! RECOMMENDED MINIMUM TRACK SIZES: 16 mils for signal tracks (thin tracks are difficult to etch) 100 mils for power supply tracks, see the trax width graph in the appendix for other sizes. 17-OUTLINE THE DIMENSIONS OF THE PCB BY PLACING TRACKS ON EACH LAYER USING THE BOARD LAYER. When you have finished placing all components and tracks, you should define the outline of the PCB by placing tracks of 16 mils on the board layer (turn it on in Options!) around the designed board, this will determine the final dimensions on each layer of your board! 18-GIVE YOU BOARD A NAME BY PLACING A TEXT (STRING) Do this on each layer used in the design, but do not forget to mirror the text on the solder side (you look through the board remember!). This can be done with the EDIT-CHANGE-MIRROR. 19-OPTIMISE YOUR LAYOUT Increase the width of the power supply tracks (in particular ground) where possible, reduce the layers on the component side by swapping them to the solder side and minimise the number of Via’s! 20-DO NOT FORGET TO SAVE YOUR WORK!!! Although Protel-PCB will create an automatic back up (usually to harddisk) it is important that you make a back up to a floppy disk as well. Do this during the time you are working on your PCB design as well (say every l0 min) to secure your previous work. PLOTTING As mentioned in the beginning (page 4) the actual plot for the final Artwork should be created in Postcript Format (using the 1270DPI-Linotronix) and processed by an outside company. Supply your Technician with your PCB File when you reached this stage! However plotting or printing (to a standard laser printer) your Artwork on paper could still be useful for the following; • Checking all connections and holes. • Final layout of components. • Final layout of strings(TEXT) [...]... Circuit Technical Manual 107, Bishop Graphics 1982 5- Protel Autotrax Manual 1988 6- Protel Schematic Manual 1990 7- Printed Circuit Workbook Series, Volume 1, Clyde F.Coombs 1988 - LIBRARY PARTS The following attached list shows the physical dimensions of components available in the listed Libraries below Use this list also to define package outlines in Protel Schematic The are 3 Libraries are located... generate a report file, select YES and read this file and correct errors when they occur and restart the above! Corrections need to be made inside netlist file or diagram made by Protel- Schematic Particulary consider consistency between Protel- Schematic and PCB in: names for component patterns(footlayers), names of pins, number of pins Select Auto, Autoplacement Comonents automaticaly co into Box of Keep Out... physical packages you are using exists in the Library! In case you have created your own parts library you probably have to merge components from the Protel- PCB Library in to your library for all the components you are using in your design! Loading a Netlist in Protel- PCB At first you must draw a Box using the Keep Out Layer (turn this on with Setup Toggle Layers) from X,Y=0,0 (say track width = 15 mils)... placing multiple tracks), try to keep most of the tracks on the solder(bottom) side since soldering components on the top of the board is often a problem #10 To generate a Netlist File (your schem.S01) with Protel- Schematic, exit from Schematic and Execute POST in the Schematic Directory, do not specify the extension (S01) especially when you have multiple sheets of the same file name (ie.: fileX.S01 fileX.S02...PRINTOUT YOUR JOB Printing with Protel- PCB can be done with a submenue of File/Print/Final Artwork Before you do so make sure that the bottom left corner of your PCB file is at the xy coordinates 0,0 (when not move with the block commands)... 2-Name new component 3-Draw new component using pads and tracks on appropriate layer (yellow for layout andmultilayer for pads) 4-Name pads for your component.(Each name should be related to the name at Protel- Schematic- more convenient way for transferring netlist) 5-Save your work under new name inside your current library You can create new component using existing from library and changing just size... from netlist are connected with ratnets (rubberband) They indicate which pin should be linked to which one Happy with the way ‘ Auto Placement’ placed your components?? I doubt it! The Auto Placement in Protel- PCB does not work very satisfactory, you will do a better and faster job by placing the components manually ! The best way to continue from here is to manually place all the components to a better... You could in fact now automatically route the board, however this is not recommended since you are able (at least I hope so) to do a much better job, by using the manual router, than the Autorouter from Protel- PCB (which is usually placing far to many Via’ s) Manual placing tracks: Select: Current, Layer and select the Bottom Layer Select: Current, Track Width and select the desired track width Select:... 20mil-0.Slmm 100V l5OV 150V 300V 300V 500V above 500V 25mil-0.64mm 50mil-1.27mm 100mil-2.54mm 0.2mil/volt 30mil-0.76mm 30mil-0.76mm 60mil-1.52mm 0.12mil/volt Do not forget to check the following spacings in Protel- PCB and on your plot: #8 For some applications such as very fast circuits (>50MHz) you might need the second layer (usually the top layer) as a so called GROUND PLANE (all connections to this type... are available such as: Eurocards 100x160 mm (ie.:euro-a.pcb) and IBM-PC cards (ie.:atboard.pcb) NOTES: • To reduce the number of parts and depending on what technology is required (ie CMOS, LINEAR etc) Protel Schematic is using many different libraries which can be loaded individually by the program • The Schematic Libraries contains symbols of components whereas the PCB Libraries contains the physical . (unless you have created a netlist with a schematic program such as Protel Schematic). USING PROTEL - PCB ADVANCED 1- READ THE MANUAL Like any other software. not recommended to use a netlist when you are an unexperienced user. 3- STARTING PROTEL- PCB Protel- PCB will ask you what file name it should load, however since you