Two Layer PCB....which layer is the top

Discussion in 'The Projects Forum' started by jerseyguy1996, Apr 9, 2011.

  1. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
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    I am attempting to use the toner transfer method to etch a two layer pcb. I have practicing different ways to make sure the two sides are lined up before applying the iron to it. I suddenly realized that things weren't looking right and I think I may have the wrong layer photocopied as a mirror image. When working with a 2 layer pcb, which layer is typically the top layer (the one with the components sticking up) and which layer is the bottom layer (the one with the component leads sticking up which would also be the side that you solder the component leads to the board)?
     
  2. retched

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    The bottom is where the soldering is done on PDIP (through-hole) components.

    The TOP is where through-hole components typically rest.

    The TOP usually contains the silk screening that says what designation the components are. e.g. this Resistor is 'R4' this capacitor is 'C53'

    Hope that helps.
     
  3. Wendy

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    For the toner transfer method both images should be mirrored, as they will invert during the transfer.

    How I make PCBs

    I screwed that up making this PCB. After I made a board I realized I had it backwards, and had to throw it away.
     
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  4. retched

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    Come to think of it, why do you care which side is top?


    Bill is right. You can consider both sides "top" as both images need to be reversed for transfer. The act of flipping the board "reverses" the layout, so you just treat both sides as "tops" and be sure to measure, measure, measure.

    Choose the same point for both sides.

    I cut my boards BEFORE etching, as square as I can, and use registration marks or fiducial marks to line up both sides.
     
  5. jpanhalt

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    I am not so sure of that, depending on how one displays them on the screen. If shown as a transparency, e.g., blue (bottom) and red (top) as if the board were completely clear, one (bottom) is already mirrored relative to looking at it from its face (i.e., exposed to air) side.

    I suggest the OP simply print a test pattern, like an E in the upper left corner and see what works.

    John
     
  6. Wendy

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    It may differ somewhat depending on the software you use. The image on the program will show the layers, one will be inverted, and one won't. However, when it comes time to make the transparency the software will likely show the image as it is needed. Problem is, for photo transparencies you need it non mirrored, for toner transfer it must be mirrored.

    One way to help with this is put some simple text on the image, so you can read it and see which which way you need to print the template.

    I wound up using Gimp (and it's hires mode) to mirror my images.
     
  7. jpanhalt

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    That why when I make a transparency, I always include some symbol, like an E, in a waste area. The more dyslexic one is, the harder it is to find something something that works. I am bad enough, that an N won't work for me. I also write "INK" sometimes.

    John
     
  8. jerseyguy1996

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    These are all great replies everyone. So in terms of the print out of the traces. One is labeled "Layer 1" and one is labeled "Layer 2". After I figure out which one needs to be mirrored, transfer the toner, and etch the board.....which side is the top? Layer 1 or layer 2?
     
  9. jpanhalt

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    Just make your board like this: [IMG]http://en.wikipedia.org/wiki/File:M%C3%B6bius_strip.jpg[/IMG]
    Then it won't matter. Actually, whichever you want to call the top is the top. It's your board. Retched gives a standard rule; however, there are many boards with SMD components on both sides, and no TH components. I would call the side with the most components, test points, or headers on it the top, but I suspect there is no hard and fast rule to apply.

    John
     
  10. jerseyguy1996

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    I'm sorry I am not asking the question correctly. I am guessing (although I could be completely off base here) that the designer of the pcb intended for the components to be assembled on a specific side of the board and I am wondering what a pcb designer would typically consider to be the top, Layer 1 or Layer 2 (with the assumption that through hole components would be inserted into the holes from the top). I have included a zipped file with some xps images of the two pcb layers if that helps.
     
  11. jpanhalt

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    That question was answered in post #2.

    If you replace the words top and bottom with accessible and less accessible, respectively, it may make more sense. The side you need access most easily, say for adjusting trimmer pots or adjusting jumpers, is often, but not always, the top side. In reality, the accessible side may be mounted on the bottom, side, back, or top of whatever enclosure is used. As I alluded to, designing is usually done from the component side looking down, i.e., components on "top." That is because in modern day electronics, pin numbering is based on looking at the top side. In the days of vacuum tubes and point to point wiring, the convention was different, but that is a different story.

    I couldn't open the xps images. If there is something that would change the assignment discussed above, please re-post with pdf, jpg, png, of gif images.

    We are now up to eleven posts, so let me ask, why is it important to you?

    John
     
  12. retched

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    Are you talking about mirroring on SCREEN? or mirroring a printout for etching?

    It seems you are talking about on screen so the vias match up.

    If on screen, the software should automatically set that up.

    If for etching, the software should already set that up too.

    What PCB software are you using?
     
  13. jerseyguy1996

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    I apologize for the confusion as I am having trouble explaining my problem. I understand that the components are typically inserted into the top and then soldered onto the bottom of the pcb. I am confused about the labeling convention of the layers that make up a pcb. So if the pcb designer is designing a two layer pcb then there will be copper etchings on the top and bottom layer of the pcb. He will draw those layers separately on a PCB design software which will show the pads for each component as well as the tracings that connect the components together. So now I view and print each of those layers separately. The designer has labeled one of the layers "Layer 1" and one of the Layers "Layer 2". I would suspect that the designer has intended for "Layer 1" to be either the top or the bottom layer of the PCB and "Layer 2" would be the opposite. I would also suspect that the designer intends for the components to be placed on the top side of the board and soldered on the bottom. So my question is, when I make the board I will have "Layer 1" on one side of the board and "Layer 2" on the other side but how do I know which layer the designer intends to be the top and which layer does the designer intend to be the bottom? It seems like it would make a difference as to which side of the board I place the components and which side I solder the components.

    I guess another way to ask it would be to say, if I have "Layer 1" on one side of the board and "Layer 2" on the other, which layer would the silk screen go on if I was a pcb manufacturer?
     
  14. Wendy

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    Top layer. The PCB drawing software I use is ExpressPCB. There, the silkscreen is top, as is red layer. The green layer is the bottom. I can select which is displayed using icons.

    Different software is different. I remember someone asking what you are using, I don't remember seeing your answer.
     
  15. retched

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    or Both.

    If you are doing 2 sided...then you get silk on both.

    The silk shows what part goes where.. since there are parts on both sides.. both get silk
     
  16. Wendy

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    OK, I agree. I don't design myself that way, but the technology is changing.
     
  17. jerseyguy1996

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    I downloaded a free version of ViewMate which is what I used to view the gerber files sent to me from TI. I believe the attached may help to explain the issue. These are the gerber files that I got from TI. Again forgive my confusion. Hopefully you will be able to view the circuit with these files. You will see that there is a separate file for Layer 1 and Layer 2 and I don't see that it is specified which layer the designer intends to be the top and which layer would be the bottom. Unfortunately I can't print to PDF and I can't figure out how to export the image to software that can convert to jpeg.
     
  18. jpanhalt

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    Can you put the image on your monitor, print screen, then post that as a jpg or whatever? Do you have PhotoShop or similar?

    John
     
  19. jerseyguy1996

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    I think I figured it out. One of the gerber files included notes for the manufacturer. It has a heading titled "Silkscreen" and Layer 1 checked off. I am going to assume that Layer 1 is the top etchings since the silkscreen would go on that side.
     
  20. retched

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    That would be a valid assumption. ;)
     
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