My footprint library - rounded pads

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d1wang
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Joined: 13 Nov 2018, 02:19

My footprint library - rounded pads

#1 Post by d1wang » 04 Jan 2019, 04:59

I put my custom library on Google Drive. There is a spreadsheet listing the manufacturer documents that I used.

All the SMD pads have "rounded" edges.

The library is work-in-progress. The footprint do not conform to IPC-7351C specification, so please use at your own risk.

Here is a preview:
Patterns.png
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mtripoli
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Re: My footprint library - rounded pads

#2 Post by mtripoli » 07 Jan 2019, 17:49

Hi d1wang,

I appreciate that you took the time to generate these footprints, however, I would suggest you look up some white papers on how pad shape defines the surface tension of the resulting melted solder shape. A rectangular pad will result in surface tension of the solder to "pull" the pad to the center of the pad, centering the device being soldered. The solder actually makes a sort of trapezoid shape with a "center line" to it. When you round of the corners you create a more rounded ball of solder that can pull the device of center. As I said, there are many white papers from TI, Analog, Linear Tech, Vishay: most of the big companies have extended info on soldering SMD components and why one shapes the pad in which they do. If you look at the IPC spec (which in my opinion spells out worst case and their pad designs with the space around them (courtyards, etc.) is complete overkill and flies in the face of a good analog board design, but I digress).

Again, I congratulate you helping others out by posting footprints, however, if one does this, they should be done according to something like IPC so that manufacturability of the PCB will be successful.

B'rgds,

Mike Tripoli

d1wang
Posts: 31
Joined: 13 Nov 2018, 02:19

Re: My footprint library - rounded pads

#3 Post by d1wang » 08 Jan 2019, 05:49

mtripoli,

Thanks for the feedback. I started re-doing the patterns because the default chip resistor patterns have pads that are too big and too close (I believe the default patterns are for wave soldering), thus making it difficult to use resistors as jumpers.

I am afraid I'm too lazy to read through IPC-7351. I just follow manufacturers' land pattern and calculate the radius using the equation:

Code: Select all

 r = MIN(0.25,min(PAD_WIDTH,PAD_HEIGHT)*0.25)
If there are better equations, I am all ears.

I have updated the library to include three QFN-28. The E-pad vias are based on TI recommendation. The location of the vias are documented in the spreadsheet.
qfn-28.png
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d1wang
Posts: 31
Joined: 13 Nov 2018, 02:19

Re: My footprint library - rounded pads

#4 Post by d1wang » 14 Jan 2019, 05:16

I was doing all calculation on Google Sheets and then enter the coordinates manually. But it takes too much time. So I made a Web tool for automating the e-pad mask design: https://jsfiddle.net/d1wang/5t6urzwo/

Currently it only calculates all the polygon vertex and then visually render the result. It doesn't export the points to DipTrace readable format yet.

I don't think the paste coverage calculation is correct. I have posted a question at StackExchange: https://electronics.stackexchange.com/q ... dr-version

I'd appreciate answer to my question or general suggestion for the tool.

d1wang
Posts: 31
Joined: 13 Nov 2018, 02:19

Re: My footprint library - rounded pads

#5 Post by d1wang » 31 Jan 2019, 05:20

Lots of changes in the last two weeks. https://jsfiddle.net/d1wang/5t6urzwo/show
epad.png
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KevinA
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Re: My footprint library - rounded pads

#6 Post by KevinA » 31 Jan 2019, 18:50

Download from PCB Libraries - PCB Library Expert SMD Reference Calculator
https://www.pcblibraries.com/account/us ... nloads.asp?

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