1. iPromise

    Thread Starter New Member

    Aug 11, 2013
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    I took apart a cheap calculator I had and I didn't expect to see this. I thought i'd be looking at a solderable circuit board with electronic components such as resistors, logic gates and a microcontroller all soldered/screwed in.

    I'm a beginner with electronic/computer engineering and i'd love to have somebody shed some light. What type of circuit am I looking at and how far in terms of years for electronic/computer engineering must I pass to have the capability of creating such a circuit?

    Thanks,

    http://www.projectrain.net/images/20130819_204208.jpg
     
    Last edited: Aug 19, 2013
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What does the back of that circuit board look like?

    5 years with a co-op work experience.
     
  3. iPromise

    Thread Starter New Member

    Aug 11, 2013
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  4. LDC3

    Active Member

    Apr 27, 2013
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    What I see is a rubber keypad, a LCD display and contact pads for the keypad. When you press a key, the pressure makes the wires you see in the contact area touch wires below them (which you can't see). On the back of the board should be a large chip, which would be your MPU.
     
  5. LDC3

    Active Member

    Apr 27, 2013
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  6. iPromise

    Thread Starter New Member

    Aug 11, 2013
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    So what type of circuit would this be? A printed circuit board?

    Are printed circuit boards just layouts for how the circuit should be assembled? Whats the difference between a solderable circuit board and a printed circuit board?
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    A printed circuit board has traces (usually copper paths) printed (bonded) to a non-conductive epoxy resin fiber board. In many cases, the board is coated in a non-conductive lacquer (sometimes tinted green) to protect the traces on the board. Several areas of the traces are left exposed so electrical components can be soldered to the traces.
    I cannot think of why anyone would want a circuit board with out being able to solder components on the board. That would mean that a printed circuit board and a solderable circuit board describes the same thing.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,446
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    The heart of the calculator is under that black blob on the backside of the printed circuit board.
    It is a single chip that contains everything, the complete calculator on a chip. No resistors, capacitors, logic gates, MPU etc required. Everything is in one chip soldered directly to the PCB and covered in one blob of plastic. That is how they can make the calculator at such low cost.
     
  9. #12

    Expert

    Nov 30, 2010
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    Good guess at the word, "glob". I have seen this method called, "glob top". It's a cheap way to not bother putting the integrated chip in a protective package. They just pour something that turns solid on top of the IC.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    The technical term is COB - Chip on Board. But "glob" works in most uses.

    As the others have indicated, the circuit board you are looking at defines how things are interconnected. The glob of epoxy on the back contains the overwhelming majority of the electronics on a single silicon die. There are a handful of components that, for one reason or another, are mounted on the board itself. In this case, there are a couple of capacitors mounted off-chip, because it is cheaper to put large caps off-chip and endure the increased parts count and manufacturing cost than chew up expensive silicon real estate.

    As for how long to learn how to do it -- that depends on what "it" is. If you want to learn how to layout the circuit board, that wouldn't take too long at all. For boards of this ilk, you could probably be functional in a week or two and fairly proficient and efficient in a few months. For other circuit boards, you could spend years getting reasonably proficient and a lifetime getting really good. For the circuitry on the board -- well, there isn't that much there, is there? Here it is a case of understanding how to choose parts -- primarily the chip and the LCD, in this case -- and how to incorporate them into a design cheaply and reliably. In most cases (this may or may not be an example as a product like this is probably using a canned chip intended to make a basic calculator and drive that particular LCD with little or know felixibility given to the product designer) you would also need to understand how to design the software (firmware) that is burned into the chip and that would be the bulk of what you would spend your time learning, doing, and getting good at. Again, functional in a few weeks, but likely a few years to get solidly proficient.

    Designing the chp that goes under the glob is a whole different matter and opens up several more layers of effort -- are you designing the logic using a high-level hardware description language such as VHDL or Verilog? Are you designing the gates at the transistor level that will implemenent the logic? Are you designing the transistors themselves? Are you designing the pads with their I/O andprotection structures (that one is all but a black art and most chip designers just use the pads from the foundry-supplied pad library)? Are you designing the process that actually makes the silicon die? That last, BTW, is more chemistry and solid state physics than electronic engineering.
     
    iPromise likes this.
  11. Metalmann

    Active Member

    Dec 8, 2012
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