DRC clearance error

On 05/11/2019 17:41, Nav Khan wrote:

I am using a library for MCP73871 battery controller. I tried two different librarys but I get the same clearance error as shown in the picture. This is qfn package where the centre is the exposed pad and is grounded.

Changing DRC clearance does not get rid of the error. Is it a faulty library or am i doing something wrong ... MANY THANKS

 

There are two separate issues here.

First, the four thermal vias in the centre pad are misaligned such that

one of them is practically touching pin 13. You need to re-centre them.

They're also not being recognised as ground vias, by the look of it,

although the airwires suggest they are correctly named. Which version of

Eagle are you using? I think the rules changed at some point.

Second, you've got pad-to-pad clearance settings in your DRC rules that

don't allow for this footprint. I'm not familiar with the device in

question so I can't say whether the centre pad really needs to be as

large as it is, or whether you need a board house that can manage very

small clearances.

In practice, even if those clearances are OK for manufacture, you may

well need to adjust (or remove and hand-craft) the automatic solder mask

on all the pads of a device like that.

Hi Thanks for the response.

The vias are non issue as I just added them to check if that would help in the clearance. So the clearance issue is still there even without the vias. I am new to this and this is my first design, I was really under the impression that the librarys available online are accurate. The manufacturing view of the device shows the central pad so large that it is touching the other surrounding pads. I should get down to designing my library...Thanks again

On 05/11/2019 18:58, Nav Khan wrote:

I am new to this and this is my first design, I was really under the impression that the librarys available online are accurate. The manufacturing view of the device shows the central pad so large that it is touching the other surrounding pads. I should get down to designing my library...Thanks again

 

So, first lesson... never trust anybody else's library! This is a rule

that those of us who use Eagle a lot (or, indeed, use any other PCB CAD

software in a professional environment) find ourselves repeating on the

user forum. You cannot trust what you find on the web, to the point that

it's usually quicker just to create the part yourself than search,

download, and verify.

If you're experiencing differences between the library and the board,

this is something of an FAQ. It's down to DRC rules. However, this is

mostly an issue for through-hole components, where the "annular ring"

setting plays havoc with newbies' expectations. In essence, the library

defines the minimum sizes needed for the component, while the DRC rules

define the capabilities of the board house for that project. If the

board house can't achieve the ultra-thin copper ring that the library

defines for a PTH pad, the board gets a fatter one. I wouldn't expect

that to be your problem as it doesn't normally affect SMD pads.

The manufacturing preview may not always be fully accurate. Autodesk

generally say that, where there is a discrepancy, the board editor is

the one to trust. Or you can generate Gerber files and open them in

something like gerbv, which should be definitive.

BTW, the Element14 forum, and the old CadSoft NNTP server it's linked

to, are not the best place for Eagle advice any more. You'd get a better

spread of response (including official support) from the Autodesk forum

at https://forums.autodesk.com/t5/eagle-forum/bd-p/3500

Am 05.11.2019 um 21:07 schrieb Rob Pearce:

On 05/11/2019 18:58, Nav Khan wrote:

I am new to this and this is my first design, I was really under the

impression that the librarys available online are accurate. The

manufacturing view of the device shows the central pad so large that

it is touching the other surrounding pads. I should get down to

designing my library...Thanks again

 

So, first lesson... never trust anybody else's library! This is a rule

that those of us who use Eagle a lot (or, indeed, use any other PCB CAD

software in a professional environment) find ourselves repeating on the

user forum. You cannot trust what you find on the web, to the point that

it's usually quicker just to create the part yourself than search,

download, and verify.

 

If you're experiencing differences between the library and the board,

this is something of an FAQ. It's down to DRC rules. However, this is

mostly an issue for through-hole components, where the "annular ring"

setting plays havoc with newbies' expectations. In essence, the library

defines the minimum sizes needed for the component, while the DRC rules

define the capabilities of the board house for that project. If the

board house can't achieve the ultra-thin copper ring that the library

defines for a PTH pad, the board gets a fatter one. I wouldn't expect

that to be your problem as it doesn't normally affect SMD pads.

 

The manufacturing preview may not always be fully accurate. Autodesk

generally say that, where there is a discrepancy, the board editor is

the one to trust. Or you can generate Gerber files and open them in

something like gerbv, which should be definitive.

 

BTW, the Element14 forum, and the old CadSoft NNTP server it's linked

to, are not the best place for Eagle advice any more. You'd get a better

spread of response (including official support) from the Autodesk forum

at https://forums.autodesk.com/t5/eagle-forum/bd-p/3500

According the data sheet the exposed pad measures 2.5x2.5 mm.

You can check/correct it and maybe the problem is solved.

So, second lesson... never trust anybody else's library! as Rob already

said.

--

Mit freundlichen Grüßen / With best regards

Joern Paschedag

Joern, the minimum is actually 2.6x2.6mm and I did exactly what you said and it got rid of the clearance error.

Thanks

--

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Joern, the minimum is actually 2.6x2.6mm and I did exactly what you said and it got rid of the clearance error.

Thanks