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Author Topic: I need help designing solder pads for an Eagle library part - TLC5947  (Read 1112 times)
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This is my first time designing an SMD part in Eagle and I'm pulling my hair out.

The Sparkfun tutorial has so far been easy enough to follow:
http://www.sparkfun.com/tutorials/110

But when I got to the part where I had to design the footprint, everything went all pear shaped.  I can place the pads and size them fine. Positioning them is a pain in the ass since I can't see a grid smaller than 1mm so I just have to go by eye with the alt grid set to 0.025, bit I seem to have the hang of that.

The problem I'm having though is figuring out what dimensions to use.

Here's the datasheet.  It tells me what the spacing between the pins is, but the overall width and height of the package seem to list two dimensions in MM seperaed by a line and I don't know if that means I'm supposed to average them cause they're min and max or what:
http://www.ti.com/lit/ds/symlink/tlc5947.pdf

But that's not really very helpful anyway. I'm supposed to look at this datasheet instead for the actual pad sizes:
http://www.ti.com/lit/an/slma002g/slma002g.pdf

And I started to design the landing pattern based on that but after getting all the pins right (or so I thought) I went to place the pad for the heat sink and it was huge in comparison.

Btw, I'm building the htssop-32 here, which is a DAP package.  So the heat sink pad should be 11 x 5.2.

The problem it seems is the example I based my pad landing sizes on was the 24 pin PWP package.  Fair enough.  But it says below the example that I should refer to the parts datasheet to get the landing pad sizes.  So now I'm going in circles with one document saying I should look at the other and vice versa. 

So I guess what I'm asking is how do I determine where to place the landing pads for the pins, and what size they should be?  Even if I placed the center of the pads according to the measurements from the datasheet, there's no information on how large to make them or exactly where the center of the pad should be. 

I also found this document which helps some with making the heat sink, but right now I just need to make sure I get those pins right:
http://www.ti.com/lit/an/slma004b/slma004b.pdf
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Leighton Buzzard, UK
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after you select a pad, you can select its size
or just type in a size 11 x 5.2 for example
I usually set it to the largest size - so I have more room to place the chip

you should be able to see much smaller grids just zoom in
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The easiest method when doing a part in eagle, especially the footprint, is just to use an existing parts footprint. Then just copy it and change the pin assignments and you're done
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pad size is ideally dependent on soldering technique used - surface tension effects for instance.  It may not be too critical so long as you allow for dimension variability - long pads make hand-soldering easier note.  the two values separated by line are max and min.
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Mmcp:
I am working with a small pitch part here, so tolerances need to be tight.  And 11x5.2 is almost the size of the entire chip.  I need tiny pads for leads which are only .65mm apart.

MarkT:
I am not going to be hand soldering these, they're going to be assembled by the PCB manufacturer.  I don't know anything about how surface tension will affect things.  I just wanted to go with the part maker's recommendations as that would be safest.

Nadav:
I don't have a preexisting part I can use.  Well, I have one I think is similar but I can't be sure since I don't know what size the pads.  And besides I'd really like to learn how to do this right.  The other part doesn't have vias for heat dissipation and I'll probably have to design a couple more parts which aren't in the library.

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Well I've laid out the pads for the pins.  I didn't need to change the spacing between them, just how far apart the two sides were.  I then added the heat sink pad in the middle.  Took forever to get everything centered and positioned properly.  I rounded a few of those measurements that were between two values to the closest whole milimeter.  So I made the package body 11mm long and 6mm wide.

But the heat sink pad, while not wide enough to touch any pin pads, seems awfully long.  It touches both ends of the IC.  None of the pics in the datasheets show a heat sink that large.  he just have a small area in the middle of the chip.  What's up with that?
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Well I just figured out why I was having so much difficulty finding the informaton I was looking for.

TI has an outdated / incomplete  datasheet on their site!

Right after the bit in the datasheet where it shows the outline of the pad and says that the thermal pad information is on a seperate sheet, there are several missing pages.  When I went to the main page for the chip and clicked on the link to the datasheet there while looking for the thermal information, I found a more complete version of the datasheet with the missing pages:

http://www.ti.com/lit/ds/sbvs114a/sbvs114a.pdf

So that solves that problem.

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Quote
I can't see a grid smaller than 1mm so I just have to go by eye with the alt grid set to 0.025, bit I seem to have the hang of that.

What version of Eagle are you using? I'm using the hobbyist/freeware version, and I can set the major grid to 0.1mm just fine, and it will draw those lines and snap to that size.
When setting the grid to 0.05mm, it doesn't draw the lines, though -- but if I set multiple to 2, then it draws every other line, which is still pretty good for placing things correctly.
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Leighton Buzzard, UK
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Eagle light  6 if you zoom in you will see the grid
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Yeah, I can see it now, I guess I didn't zoom in far enough or something when I had it at 0.025.
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