PCB design

Very soon I will try to make my first PCB(I've got all the hardware ready and waiting) and I'm wondering what are the major pitfalls that I should know about? I think I got the etching part down as it seems pretty simple(I might screw up a few boards though until I get it right but thats the fun in learning)... I'm mainly worried about designing the actual paths and stuff... Like, say, ground loops and such(but maybe there are bigger pitfalls than this since most of my circuits will be simple for now(and one sided)).

Thanks, Jon

Reply to
Abstract Dissonance
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My PCB design tutorial might be useful for you:

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Dave :)

Reply to
David L. Jones

For very small stuff I used the press on stuff from radio shack, outline the board, the press down various objects from their stickers and be sure they are firmly down, let them sit in the chemical about 20 minutes, mine is not heated but have little pump blowing air into it

Reply to
James Douglas

Sounds like you are more interested in layout tips than those regarding fabrication, but you might want to take a look at my PCB page anyway:

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This is mostly about making boards with the Sharpie marker method, but has some general tips everyone needs to know about. 2 important tips: Don't bother to buy carbide drill bits to use in your Dremel tool... they will probably snap on the first hole, unless you use a "drill press" stand and have a steady hand. Much better to use ball-tip dental burs, which are much easier, can tolerate huge side loads, and will never break. (My set is over 20 years old.)

Second tip: Make sure your ferric chloride is not too concentrated... it will hardly etch at all! New etchant is sometimes sold too concentrated, and you have to dilute it to get it to work. This is absolutely mystifying when you first encounter it, since you expect "stronger = faster". Not true here!

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

Think about where the current will flow. Remember that everything must have a return current path (think about the flow again) and consider including a ground plane. If a ground plane is not feasible in your design, then consider alternatives such as a grid layout that aovids loops. Also be sure to include proper bypass capacitors close to the power pins of the devices.

PCB layout is a lot more art than science. Don't expect perfection on your first attempts.

Reply to
Noway2

Pretty thorough tutorial...

Thanks,

Jon

Reply to
Abstract Dissonance

I've got all the etchant stuff(200$ worth of stuff to do it using pressing method and UV)... I'm more intersted in actually making the paths and stuff. Like making the "design" or whatever its called. Obviously its just connecting wires around and stuff but I'm sure theres an art to it and some basic ideas to get me along.

Jon

Reply to
Abstract Dissonance

yeah. I'm mainly interested in design at this point cause thats the first thing to do. I think everything else will go smoothly except potentially drilling the holes. (I do have a large press but not sure if I will want to use it). I was thinking of actually making an electroncally controlled drill press to automate the process but I think thats to large of a project at this point(its probably pretty simple but just a lot of little things and will take some time that I probably should spend on other things).

oh?

Strange?

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heh, maybe not. It requires water it seems for the reaction. Theres probably an optimal ratio... it says 2:1 on that page.

I'll have to keep this in mind when I start etching.

Thanks, Jon

Reply to
Abstract Dissonance

What are these bypass caps? To reduce HF noise? Is this a standard thing to do(as I've not ever seen this mentioned or on basic circuits and schematics(I think))?

heh, yeah...

Thanks, Jon

Reply to
Abstract Dissonance

The way to do this is by specific gravity. 1.30 is optimal. (See

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for details.)

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

According to Dr. Howard Johnson's book, which describes it more susinctly than I could, there are three "rules" of what a power supply system must accomplish in digital logic for proper operation.

Rule 1: You must have a low impedance ground connection between the gates in order to prevent common path noise due to the return currents

Rule 2: The impedance between the power pins on any two gates should be just as low as the impedance of the ground pins to provide a stable voltage to the devices.

Rule 3: There must be a low impedance path between the power and ground.

Basically in order for the logic devices switch, from high - low or low

- high, electrical charge must be moved, ie current must flow. The current that flows can have a significant magnitude and a fairly high frequency content. In order to prevent these switch currents from disturbing the rest of the system, bypass capacitors are used to source the necessary current.

Here is a link to the Howard Johnson's website that discusses bypass capacitors.

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I too had never heard of them or even really considered their use until I started to get bit by noise issues in a field released product line. The capacitors should also be of a material that has a low ESR, or equivalent series resistance which is basically a parasitic resistance that can be modeled as being in series with the capacitor, limiting its effectiveness. Typical values for bypass capacitors are about .1uF, though use of .01uF and 1uF are both common depending on the speed of the device. Note that speed is relative to the signal rise and fall time, not the clock speed.

In addition to the bypass capacitors, a ground plane is one of the few structures that will provide a low impedance path for the return currents. WIth a proper ground plane and use of bypass capacitors, the power routing can be pretty much arbitrary.

If a proper ground / power system is not implemented, the power system will bounce as devices switch. The devices, which internally can be viewed as a comparitor can't tell the difference between this power bounce and a desired signal and will react if the bounce is serious enough.

Another arguement you may encounter is people saying that I am only using simple logic gates, or older stuff, etc, nothing new and high speed. Again, the important (but difficult point to grasp) is that the speed is not related to the operational or clock speed. It is a function of the IC design. Even 74ls devices, manufacturerd on more modern equipment can exihibit rise and fall times much faster than in years past.

Reply to
Noway2

to

I don't know what you're making, but start with a proper circuit diagram. As long as you are not doing high current/voltage/frequency/precision/... stuff you can get away with a lot on a pcb. Just route as you see fit while keeping an eye on the practical side, i.e. where do the connectors go, make all polarised components point in the same direction, use the largest traces (for easy etching) and the biggest pads (for easy drilling) possible and don't forget some mounting holes. You might consider using SMT so that you don't have to drill so much. Keep a lot of copper (on non-populated parts of the board) to reduce etching time, don't keep too much copper (in between traces f.i.) or you will get problems when soldering.

As they say at Nike: just do it.

And if the board won't work because of a pcb problem, just redo the board better.

--DF

Reply to
Deefoo

You can also use the unused copper area, tied to ground, as your ground plane. A solid surface, even one of odd shape and full of holes is still orders of magnitude better than serpentine traces.

Deefoo is correct though. Make sure that you leave enough space (clearance) around everything. Professional PCB houses can often times do down to a few thousands, but no need to push it when you don't have to.

Reply to
Noway2

You can also use the unused copper area, tied to ground, as your ground plane. A solid surface, even one of odd shape and full of holes is still orders of magnitude better than serpentine traces.

Deefoo is correct though. Make sure that you leave enough space (clearance) around everything. Professional PCB houses can often times do down to a few thousands, but no need to push it when you don't have to.

Reply to
Noway2

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