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Altium 0402 Resistor Footprint

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jnbrown View Drop Down
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    Posted: 13 Jan 2020 at 10:25am
I just started using Altium having previously used PADS.

When using the IPC Compliant footprint wizard (medium density) using the dimension from Vishay CRCW data sheet I get the following footprint dimensions:

a: 0.55     b: 0.65    I: 0.2

The recommended dimensions from the Vishay datasheet are:

a: 0.40     b: 0.60    I: 0.5

The dimension from our PADS library are:

a: 0.57     b: 0.62    I: 0.39

It seems that the "I" dimension (spacing between pads) is  a lot smaller using the Altium wizard compared to the Vishay datasheet and current PADS library.

I asked our contract assembler and they said to use the Altium footprint as it will result in less tombstoning.

Would appreciate any opinions on this.

Thanks

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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 12:05pm
When I use the Free Library Expert Pro "Nominal Density Level" I get these dimensions: 


The PADS library parts seems to be the closest match. However, the PADS b: dimension is slightly large because there is no metal on the side of a resistor terminal and the package Width Tolerance is only +/- 0.05 mm. 

The part is 1.00 mm long with a 0.50 mm gap (between two 0.25 mm lead widths). 

The Altium I: dimension of 0.20 mm is too close and the heel joint would be 0.15 mm. Way out of line and I don't know why the assembly shop would recommend that one. 

The Library Expert Pro heel joint is 0.05 mm to cover the terminal tolerance. 

Download the Free (no license required) Library Expert Pro and use the Altium interface to produce an accurate footprint and High Quality 3D STEP model - 

Library Expert Pro actually shows you the package and terminal outlines on the pad so you can visually see where the solder is going to flow. And the body and terminal are translated to Altium on a mechanical layer. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jnbrown Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 3:43pm
Tom,

Thanks for the great advice.

It is kind of disappointing that our assembler does not know what a proper footprint is. 

The wizard in Altium has many options, maybe if I select the right ones I would get a footprint that makes more sense. 

I will give Library Expert Pro a try.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jnbrown Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 4:19pm
When I run it with the numbers from the datasheet I get slightly different results:

a: 0.58     b: 0.60    I: 0.35

Not sure why it is different from your results but not a long way off.
Also are the rounded corners (set to 25% of pad width) preferred to square corners?

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 4:40pm
Are these the same package dimensions and tolerances? 

The tolerances are important. 




I get these results. If your results are different, we need to talk.  



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Post Options Post Options   Thanks (0) Thanks(0)   Quote jnbrown Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 5:22pm
Tom,

Using the numbers from the datasheet you posted the link to I do indeed get the same numbers as you.
Vishay has a lot of versions of the CRCW datasheet.
Here is the one I am using that matches the P/N I am using: http://www.vishay.com/docs/28773/crcwce3.pdf

The tolerances are different and it is dated 02-Nov-17.
The one you posted the link to is dated 18-Nov-10

Also we have Stackpole as a second source, fortunately their dimensions match Vishay except the tolerance on the termination width is 0.15mm instead of 0.1mm for Vishay.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 5:46pm
Yep, in the IPC world, the tolerances change the pattern when using the same Nominal Package Dimensions. 

They are part of the mathematical model. 

Read this:

For additional info on a popular lead form:

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jnbrown Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2020 at 6:11pm
I will look at those forum posts.

Thanks again!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jnbrown Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 2020 at 3:15pm
Originally posted by Tom H Tom H wrote:

Yep, in the IPC world, the tolerances change the pattern when using the same Nominal Package Dimensions. 

They are part of the mathematical model. 

Read this:

For additional info on a popular lead form:


Tom,

Those links do not seem to be working.
Would still like to read those.

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 2020 at 3:20pm
I updated the links. 
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