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Newbie to PCB design

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elearn

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Hi, Just wanted to how people generally make PCB through the design phase. I only designed few circuits on LTSpice and want to get them on PCB. I know a bit of eagle i.e know how to create schematics. The components already needed footprint types and it was hell lot of pain to sort out what kind of footprint i need. Autorouter also didn't help me much. Are there general guidelines for a newbie - to learn about footprints and routing ?
 
If you are planning to build it at home and not manufacture it outside, then take a look at free PCB designer called pcbartist. There is a recent threat on all the PCB softwares recommended. Do a search in forum!
 
Software-based pcb design is all about the libraries, netnames and part names. The schematic software probably will assign sequential netnames if you don't explicitly name things. I recommend taking the time to assign meaningful names. Same with the parts. I must have a dozen different 1 K resistors in my libraries. Think through a naming system. For example, 1K00-QW5 is a 1.00K, quarter watt, 5% resistor. You don't need the second decimal place for 5% parts, but you will for 1% parts so get it in there now for consistency. The decals for the part are RES-QW-D for the lay-down part, and RES-QW-U for an upright part.

If this sounds like a lot of work that is not circuit designing, it is at first. Know right up front - schematic and pcb software DO NOT save time. What they do is increase the quality and consistency of the output.

ak
 
Hi, Just wanted to how people generally make PCB through the design phase. I only designed few circuits on LTSpice and want to get them on PCB. I know a bit of eagle i.e know how to create schematics. The components already needed footprint types and it was hell lot of pain to sort out what kind of footprint i need. Autorouter also didn't help me much. Are there general guidelines for a newbie - to learn about footprints and routing ?
Most autorouters do a poor job, do the routing manually.
 
Spend a bunch of time thinking about the mechanical nature of the PCB. Fancier software that does 3D pix can help if you have trouble visualizing the finished product.

Clearance for mounting screws? Include the head and tool space requirements also. Are heavy parts (transformers usually) near the mounting holes to avoid board flexing? Connectors also: if far from a mounting hole, inserting and removing high-force connectors is difficult if the board is springy, and could stress or even crack a PCB trace. Access for fuses, trim pots and those type of parts need to be considered. Are ribbon cables routed to clear the components? It's hard to work on or probe a circuit if a ribbon cable is covering up the whole top. Consider the tools you use to work on the circuit board. Can you fit your soldering iron in between two very tall capacitors to change a 0803 resistor? Do you have places to touch a meter or scope probe if you need to debug? Some small SMT-t0-SMT circuit nodes may not even allow you access to the conductors somewhere unless you add a test point.
How about cooling air flow for higher power components? Try to avoid aligning inductive components, wire-wound resistors and such to avoid overlapping magnetic fields.

These are some of the things that I think are important to a well-designed PCB, but are generally seldom discussed in PCB design. Usually it's all about circuit traces and complicated inductance/capacitance issues, but sometimes the basic stuff can sometimes cause you just as much grief. If making many boards or multiple revisions - absolutely put the revision number and/or date on the silkscreen. If not using silkscreen, put it onto a bare section of copper.

I like to put in a few extra parts (especially if this is a prototype design) in case I need to make changes, such as adding a capacitor for unexpected filtering. Perhaps a 2nd resistor in parallel to tweak a resistance that I thought needed to be 1000Ω but really needs to be 763Ω. I also like to add two small grounded holes near the edge of the pcb. I solder in a short U-shape bare wire and that makes an excellent point to attach a ground clip for a scope or meter. I always provide an easy point to measure power voltages also. In some cases where a power supply is on the board, I split the power trace between the source and the load, and use a small wire jumper soldered in place. That allows me to cut the wire and measure the current draw exactly. When I'm done I solder it back in.

Another potentially big thing - mostly for prototypes - is to NOT connect logic inputs directly to ground. You might some day realize that grounded enable pin needs to be disabled somehow, and unless you have a DIP package, lifting a pin may be impossible or difficult at best. A short 2-3 mm length of trace before contacting the ground plane will not usually cause electrical issues, and will allow you a place to use a xacto knife or drill to isolate the pin from ground, then use a tack-soldered wire.
Some parts are sometimes hard to get. For a lot of larger capacitor thru-hole sizes, typical lead spacings of 2.5, 5, 7.5mm etc. may not always be available. I'll often overlay PC part patterns of several sizes at once, so no matter what size I end up finding, it will fit into the board with out a lot of funny lead bending. For transistors that sometimes have different pin outs: EBC or ECB, I use a "special" hole pattern with "ECBE" which allows different types to be inserted either forward or backwards without a lot of crazy lead twisting. You can't make one board that handles every possible part, but for some odd, unusual and difficult parts to find, putting flexibility in the PCB is sometimes a good time saver.

Concerning the electrical issues, ground is often a problem. Don't think of your circuit as just current flow going from power supply to components and then ground and that's all. Think of the ENTIRE CIRCUIT. Electricity 101: current flows in a loop. Think "Current flows through various components, eventually to a ground and then back to the power supply." If you think like that all the time you will more likely consider the ground currents that will be flowing into your PCB's ground plane. If you merely think "...all this relay current flows into the transistor and to ground..." and that is all, you'll assume all ground is perfect. Think also of the current path from the transistor back to the power input connection, and make sure the 'trace' is short and thick enough, and the supply at that point is adequately bypassed.

And one more thing: good luck!
 
Hi, elearn..

EAGLE has a good 72 page manual (I printed it and annotated the pins out of it in order to learn). It has well explained steps for the layout (ground pour, for example).

Dave Jones has a nice tutorial. You can also try to design a board, post it here, and receive feedback from forum members. What has also helped gain information is teardowns. Looking at many boards gives familiarity. How other people do things, etc. It also makes you ask questions (for example, when I was a kid, I used to look at RAM sticks and wonder why the wiggly lines. I arrived at the conclusion they must be the same length. However, when I opened an LNB, it was way too much magic for me to handle).

The footprint choice only matters when it matters. For example, you shouldn't bother that much at this time about resistors' footprints because you can shape them and control their length, so you have flexibility.

What you should pay attention to, though:

Fab houses have different capabilities depending which country you're in. In my country, I wasn't able to make PCBs with very thin traces or small clearance, etc. I took that into account and made my traces thick enough.

One of the mistakes I've made was to use the default components' drill and pad sizes. These are for countries with good machines, etc. I had to make the pads bigger because the drill actually ate the pad.

All the best,
 
I agree with previous post to look at other PCB designs for inspiration and note what makes them fuctional and easy to use. (Pretty if you like symmetry)!
I like to pack my components in together and have built custom parts and pads as I needed them. One tip that you might find very useful in PCB layout:
I place all my components on the PCB without any connections first. Then I bring them as close together as possible. At the same time pull all terminals, post holes, as well as input/outputs to the edge of the board. Then connect parts and rotate them as needed to fit like a puzzle. Then set my power and ground lines to feed all the parts. Once done do a connection tracking and complete copper pours on top and bottom. Sometimes I go thru 6 to 7 versions before I am happy with the final product. Cheers, Rom
 
I also recommend to lookup manufacturing tolerances, specs, spaces, trace width, gap width by contacting a local PCB manufacturer. They usually have a checklist and will be glad to provide you with one to encourage your business. If you are planning to design PCB's for power control and switching (several amps not milli-amps), then I would also recommend to google "PCB trace vs amps calculator", as well as "wire gauge vs. amps table". I found these two items very handy and I always use them as a quick reference to set my minimum PCB trace width as well as wire gauge connecting to the associated terminals for input/output. PCB design is an art and getting savvy at it requires a hands on approach and practice.
 
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Whilst I am in no position to offer advice as all this is new to me also, but I can let you know how I have got on doing this, and some of the silly mistakes I have made.

I’m working on my first project and decided to use a modular approach to the pcbs, six small boards for the sixteen seven segment leds, and two other boards to hold the microprocessors and MAX7219 ics plus other components.

My first efforts went fairly easily using the default settings of the pcb software, that is until I came to make the boards, the traces were much too thin, some even etched away, and the pads way too small for me to comfortable hand solder.

So I increased the pad size to the largest I reasonably could, the smallest being for the ic sockets, I also increased the trace width as much as I could within reason, at the moment I am more willing to use jumper wires than use thin traces, I also set the copper pour to the ground net and try to get this to run completely around the board.

The next mistake was to use the wrong foot print for two components, the library did not have the part number I was using and components that looked to have the same pin layout were used, only to find they were not quite the same size and my components would not fit, two more boards scraped.

The auto routing does not optimise the layout, although you think it would, it seemed to go about it in a logical but kind of clockwork fashion, often creating impossible situations for itself as a result. A lot of manual juggling was needed to find a solution, despite moving components a bit and having it re-route several times.

It is much easier to manually route the traces.

Also don’t be mean on board size, there needs to be room for mounting holes, I tend to put these at the corners or on the edges, but allow extra board space for them.

If you are going to drill the boards by hand, the only way I have at the moment, I set the pad hole size to 0.6mm diameter as this gives a nice centre for the 0.8mm drill bit to locate in.

Anyway don’t make the same mistakes I have, and good luck.
 
Whilst I am in no position to offer advice as all this is new to me also, but I can let you know how I have got on doing this, and some of the silly mistakes I have made.

I’m working on my first project and decided to use a modular approach to the pcbs, six small boards for the sixteen seven segment leds, and two other boards to hold the microprocessors and MAX7219 ics plus other components.

My first efforts went fairly easily using the default settings of the pcb software, that is until I came to make the boards, the traces were much too thin, some even etched away, ....
I wonder how thin were your traces and what method you used to PCB transfer that most were dissolved!?
 
Probably the pcb was in the bath too long.
I had issues with thin traces before which I corrected.
1- when ran the PCB thru laminator, the paper was too thick and traces did not stick to PCB well.
2- same issue with photo sensitive coated copper when board was over exposed
I still love ironing method. I use thin magazine pages for paper which saturate with ink and desolved in water with no efforts. I am able to etch 1mil traces with ease even though I usually choose not to for oxidation reason and lifting during soldering.
 
I wonder how thin were your traces and what method you used to PCB transfer that most were dissolved!?

Sorry I am new to all this, I don't remember the default trace width or pad size, and yes it was in the bath too long. I am using pre-sensitized board and the hydrochloric acid and peroxide mix, and it was etching very slowly.
I am currently using trace widths of 0.8mm and pad diameters of 2.5mm with better success, but the etchant seems to be variable and I end up adding a bit more acid and/or a bit more peroxide to get it going each time I use it.
Unfortunately I don't have a laser printer but envisage the "iron on" film a better and cheaper way of doing the pcb layout.
 
If this sounds like a lot of work that is not circuit designing, it is at first. Know right up front - schematic and pcb software DO NOT save time. What they do is increase the quality and consistency of the output.

ak
When I build one offs for home projects, I do a hand layout and cut the board myself with a Dremel and a carbide blade. Then I tin the board and surface solder all the parts on. You will learn a lot doing that kind of layout work, optimizing placement and simplfying. It is essential IMHO to becoming a designer.
 
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