surface mount vs through hole

Discussion in 'General Electronics Chat' started by deefactorial, Jun 11, 2008.

  1. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    I have come to a cross of roads.

    Here is the background info; I have developed this system that uses a Pic micro controller, and communicates to several peripherals. The dsPIC33 microchip that I'm using has two UART ports, two SPI ports and two I2C ports. I've developed my system on the explorer 16 development test board. The peripherals are a hand held GPS device with a serial interface. A 3D magnetometer with either a serial interface or a SPI interface. A touch LCD screen with a Serial or Spi interface and two more Serial devices. I have established communication with all of the peripherals using the dev board. I will only need to communicate some of the devices at the same time, for example take the data from the magnetrometer and display it on the LCD screen.

    Here is my problem. I would like to make a PCB board with two serial ports and wire the LCD screen to the SPI port on the microchip. This will be a low quantity production. What are the pros and cons of going with surface mount or through hole? How do I go about modifying the Gerber files? Is there any standardized design guidelines when it comes to the layout of the IC's? How do I verify that I've done the layout properly?

    So lets say I've found this magical software to design a PCB board with all the IC's I need already in the application. I design the PCB board. I get it printed. I find all the resistors, capacitors, diodes... and get them soldered on. If it is surface mount, what is the easiest way to get this done? do you just send the board and the componets to a company that does this or do you have get special equipment to do it yourself?

    This is the first time I've ever done anything like this so please have patience with me.

    Thank you for any replies.
    Dominique
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    IMHO, it's a pain to mix thru-hole and SMD. You might simply see what components are available, though, and decide on that basis.

    Every component has a specified size and lead arrangement. Any CAD program should allow you to make custom pad layouts to suit any component. My very ancient DOS-based Cad program lets me do that. The manufacturer makes available a set of package outline specs for every device they carry.

    Gerber files are dependent on the board layout. You generate the photoplots once the board layout is done, so new Gerber files are just a matter of selecting them from the output menu.

    You don't seem to be working with RF, so design guidelines aren't so stringent. There may be material available that goes through layout guidelines. If not, there should be, as I picked it up in bits and pieces over a number of years.

    There may be some fab houses that will take your plots and BOM and send you finished boards. I don't think it will be cheap.

    I don't care to work with SMD in packages under 805, and you have to be a bug for neatness, as confusing parts is only too easy. On the other hand, going mostly SMD lets you place components on either side of the board, which can save space. But if you make your own PCB's and can't plate the vias, soldering top to bottom layer jumpers is a bit tedious.

    With the future in mind, think about learning some new skill sets, or prepare to part with some bucks for each design. Keep in mind that the fab house that does your boards checks for lead and pad clearance, but isn't at all concerned about fatal errors in the layout.
     
  3. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    most of the more advanced micro chips that I have been looking at are only offering them in the TQFP packages. the biggest PDIP package I could find was the dsPIC30F4013 the only problem with that chip is that I don't think it will have enough ram to run my application.

    So I think I will have to go with SMD.

    So what I understood from what beenthere has said is that the Gerber files are the output of the design of the layout. Use any CAD application to design the PCB board using the IC's manufacturer specs. Designing using SMD you can save space by placing components on either side of the board. Mixing through hole and SMD is no good. I could probably pay someone to do if for me but it will be expensive, whereas if I put the effort forth to learn the technology I could save money, especially in the long run.

    Thanks, been there.

    Is there any way that I can take the Gerber files from a previous project and derive a layout from it ? or am I better off to just start from scratch ?

    What did you mean by working with packages under 805 ?

    What does plate the vias mean ?

    Do you know of any resouces of information that I could research to get a higher level of understanding in terms of surface mount technology, designing PCB board layouts and prototyping SMD circits?

    I found this website with a list of tools to help me out.
    http://www.geda.seul.org/

    I intend to try out the pcb application.
     
  4. roddefig

    Active Member

    Apr 29, 2008
    149
    0
    I've seen programs that will take gerber files and produce a layout, but they look to be more trouble than they're worth.

    Passive components (resistors, capacitors, etc.) come in standard sizes, 0805 being one of those.

    Vias are holes in your board that connect one side to another. To be electrically conductive they have to be "plated," where a machine draws conductive material through them. If you don't have a plater then you have to solder wire in the holes which can be a pain.

    I have yet to find any good resources on the web about PCB layout, but maybe someone else knows of something.
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
  6. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    Thank you for the tutorial. There was a lot of info to digest in there.

    What are the leading manufacturers that are used to get PCB boards printed?

    Is there any schematic diagram and PCB combination software available to make my life easier?

    what are all the different standardized sizes of components ? do you have any resources I can research about this ?

    So from the information that I gathered so far my process is to make a schematic diagram of the circuit. Get a parts list of all the components. Get the manufacturer specs for the components. Decide what style of PCB board to make based on the printing manufactures specs. Take the completed schematic diagram and convert it to a PCB layout using software. Create the Tracks. Do the Design Rule Checking. Get someone else to check it too. Once the PCB board is designed, get the board printed. Once I have the board; I can get all the components. Get the components soldered onto the board. then test it out.

    I should also find out what method of soldering I should use before I design the PCB board.

    Does anyone know of any companies that provide SMD soldering services ? or is this one of those things that with a little bit of effort it should be easy enough to do myself ?

    Thank you all for your input, very informative and in the right direction.
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If you can justify the investment in tools I would go with SMD.
    The parts are less expensive and easier to store. To make easy to assemble boards
    use 0805 (or larger) for resistors and capacitors. Use components that have a lead pitch 0.5mm or larger. Try not to use leadless parts or parts with ground pads if possible.

    My tool recommendations are at ---

    http://www.luciani.org/getting-it-done/assembly-hints/assembly-hints-index.html#smd-hints

    Details of the open source EDA software I use and my footprint library are at

    http://www.luciani.org/geda/geda-index.html

    I use PCB Express for PCB manufacturer. I assemble prototypes myself.
    For some of the boards I have done checkout ---

    http://www.luciani.org/works-in-progress/works-in-progress-index.html

    (* jcl *)
     
  8. roddefig

    Active Member

    Apr 29, 2008
    149
    0
    nanovate: That looks like a good tutorial! I'll have to sit down and read through it sometime.

    I forgot to mention an excellent book called "The Circuit Designer's Companion," by Tim Williams. Amazon link It goes over a lot of important but hardly mentioned topics like proper grounding techniques and so forth. It's well worth the money.

    There are "standard" sizes for components, but most manufacturers modify them slightly. I always create custom footprints for anything that isn't a resistor, capacitor, or inductor. Every manufacturer includes a mechanical drawing of the package in their datasheet that you can work off of.

    gEDA includes both a schematic editor and layout program. The process of going between the two is a little convoluted and documentation is rather sparse, but with a little hunting you can figure it out. There is a tutorial on their website that gives a general overview of taking your schematic to PCB. Google is your friend!

    People have also written utilities for gEDA that will generate schematic symbols and footprints for you. I use these religiously, as I hate having to draw things out by hand. They save quite a bit of time (especially with footprints).

    I believe having a good pair of tweezers for SMT work is invaluable. Digikey carries some for $3.00. Go for the ultra-fine tips and get a pair of straight and curved. A good soldering iron that is easy to handle and has a good tip is important as well. Fine solder and solder wick is a necessity. Finally, I'd pick up an Xacto knife, they come in very handy.

    My personal favorite for PCB manufacturers is Advanced Circuits. They have great deals for students and I like their customer service.

    SMT soldering is not hard, knowing the right techniques is half the battle. If you have never done any soldering before I would suggest you go trash-picking for some electronic junk to practice on first. You can easily solder 0603 and larger packages with your naked eyes and ICs are easy once you know what to do. I wrote a guide for a workshop that I gave at my university a while back, I'll upload it when I get home later today.

    Edit: attached.
     
    Last edited: Jun 13, 2008
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're going to delve into SMT, I suggest a few pairs of 51SA tweezers. I really like the Excelta brand; made by the Swiss. Theirs are antimagnetic and antiacid. You can use those to pick a splinter off a gnats' rear end if you have enough magnification (well, for me anyway - my vision isn't what it used to be.)

    I like to have a few pairs of them around. The tips are easily damaged if dropped, as they are extremely fine-tipped. However, with care they can be re-dressed to rounded or sharp points as you desire, using an emery board (nail file) or other suitable abrasive.

    Soldering is fairly easy if you're using Sn63/Pb37 solder and spotlessly clean components. If you've handled the components, the natural oils your skin contact deposited upon them will resist the soldering attempt; use the purest concentration of isopropyl alchohol you can obtain to cleanse board and components immediately prior to soldering. Pb Free solder is the rage nowadays; but it's much more difficult to obtain a decent solder joint with it. You have to use higher temperatures on the iron, and hold the components absolutely still when the solder is in the "plastic" state (between liquid and solid). True Sn63/Pb37 solder is "eutectic" - that basically means that while it's cooling, it goes directly from a liquid to a solid state with no "plastic" state in between. It's movement in the "plastic" state that will cause bad solder joints, among other ills.
     
    Tabur likes this.
  10. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    I'll second that! It is a very good book with lots of nuggets of useful knowledge.
     
  11. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    Thank you for all the excellent info.

    I've installed the gEDA applications, I'm working with gschem to get a schematic diagram of the circuit I wish to design. I am having difficulty finding a symbol file for the dsPIC33 100 TQFP chip, or a generic 100 TQFP chip that I can modify. So from the research that I've done so far the symbol files are the schematic diagram files and the footprint files are used in the PCB layout of the actual circuit. Is there a resource where I can find a database of schematic and footprint files. specifically for SMD 805 ICs , 100 TQFP chip and maxum chips. I have downloaded John Luciani's footprint files. Does anyone know where I could get some good schematic diagrams of common circuits like UART, and SPI interfaces? I have found the schematic diagrams for the explorer 16 development board, so If I recreate these in gschem, I should be able to use them to recreate a PCB board.

    I've looked into tweezers, they range anywhere from $3 to $50 I will get a set or two next time I make an order; along with a very fine tip soldering iron with a variable temperature dial. I have never seen soldering wick before, It looks like an interesting concept and I am anxious to try it out. In terms of getting small quantities of SMD's like resistors, capacitors, and transistors. Is there a product that I can get for prototyping, like a development kit with a sample of 805 size SMD devices, or am I better off creating the circuit diagram, figuring out exactly what I need and then ordering that?

    Thanks for all the input, I feel like I'm getting a grasp of this process.
     
  12. roddefig

    Active Member

    Apr 29, 2008
    149
    0
    I use gmk_sym to create symbols files and then tweak them as needed in gSchem. Take a look at http://archives.seul.org/geda/user/Apr-2005/msg00089.html for documentation on gmk_sym. You can also read the gEDA symbol creation guide, located here, but it's a bit out of date and gives some bad advice IMHO.

    To make footprints use footgen: http://dlharmon.com/geda/footgen.html. I tried to draw a footprint graphically in PCB and never want to have to do it again. Footgen is really well done and quite handy.

    As far as solder wick goes: you can't do SMT soldering without it. The solder suckers are too big and often end up sucking your component off the board as well as the pads that it attaches to. I wouldn't even bother using them.

    Digikey sells assortments of components. Go here and scroll down to "Kits." Personally, I don't keep any components on hand and end up ordering whatever I need for a project, but I can't think of a good reason to go one way or the other.

    I don't know off-hand of any site that has schematics done up in gSchem. Google is your friend!
     
  13. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    All right, I have gone to a place where I am now stuck. I have created my schematic file with all the components needed for my circuit board. I created all the nets. I can convert the schematic into a .pcb file for the PCB application. I have footprints for nearly all the components I will need. The problem I am running into is that I need a footprint for a RJ11 right angle connector, I can't generate one with footgen application. I have the auto cad drawing of the specific footprint I would need from the digikey pdf document of the part. I have searched google for applications to create footprints with, I came accross Kicad and OrCad but those seem like a whole other system for circuit board design. I just need one footprint, and I think it would be a good exercise in creating/modifying my own footprints. Does anyone know what application I would use to create footprints in? Is there any design documents or tutorials on creating footprints with open source tools?

    When I was looking at components to purchase on the digikey website, I found that there was over three hundred different types 0805 0.1uF Capacitors, How do I choose the best one for my application ? What material is best suited? What are the options "series", "temperature Coefficient", "tolerance" and "Voltage-Rated" ? I am a little confused at how to make the best choice.

    I also created my diagram with polarized capacitors but I am unsure if this is a requirement for my circuit. Is there any way for me to find out?

    Thank you all for your replies I am so grateful for all the support I have received thus far.
     
  14. m4yh3m

    Senior Member

    Apr 28, 2004
    186
    42
    additional question in regards to polarized capacitors:

    would the circuit simulator software react properly with polarized capacitors and let you know if it was backwards? i'm trying to find the one i purchased many years ago, but info like this is good to know
     
  15. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Check out my Perl module PCB_9 in the gEDA section at www.luciani.org.
    There are basic commands like element_add_pin, element_add_lines, etc.
    You may be able to modify one of the example programs.

    The dielectric and voltage rating depend on your application. In general
    for low volume work I would use X7R for values > 1000pF and NPO for values <=1000pF. For ceramic capacitors you do not need to be as conservative with
    the voltage rating as you do for Tantalum and Electrolytic capacitors.

    For a 0.1uF capacitor in an 0805 package I use
    Kemet C0805C104M5RACTU 0.02(100). For an 0603 packagae I use
    Kemet C0603C104K5RACTU 0.03(100). Both of these are 50V 10%.

    There are less expensive parts but paying a little more for a 50V X7R part
    means I only have to stock one part number.

    After you purchase all of your parts you may want to check out my component
    storage ideas at http://tinyurl.com/57vfkm ;-)

    (* jcl *)

    http://www.wiblocks.com
     
Loading...