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PCB design

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 2, 2006.

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  1. 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
     
  2. My PCB design tutorial might be useful for you:
    http://www.alternatezone.com/electronics/pcbdesign.htm

    Dave :)
     
  3. 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
     
  4. Bob Masta

    Bob Masta Guest

    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:
    www.daqarta.com/lptxh.htm
    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
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  5. Noway2

    Noway2 Guest

    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.
     
  6. 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
     
  7. 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?

    http://www.artmondo.net/printworks/articles/ferric.htm

    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
     
  8. 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
     
  9. Bob Masta

    Bob Masta Guest

  10. Noway2

    Noway2 Guest

    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. http://www.sigcon.com/pubsIndex.htm#bypass capacitors

    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.
     
  11. Deefoo

    Deefoo Guest

    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
     
  12. Noway2

    Noway2 Guest

    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.
     
  13. Noway2

    Noway2 Guest

    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.
     
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