making oval SMD Pads in V6

Question

Hi Everyone,

I want to make oval pads for a QFN package. The recommended approach I have seen is to create an official SMD INSIDE a polygon on TOP layer. I have done this but there are 2 noticeable issues.

1) The resulting pad does not look smooth in gerber viewer in the arc region

2) I get many Eagle DRC and PCB mfg errors because I used a width of 0 for the polygon. If I use bigger widths then it will look even more choppy.

Do I have the methodology correct or is there another way? Why doesn't Eagle allow oval shaped or arbitrary shaped pads to start with?

Best regards,

autodeskguest · Accepted Answer

On 2/06/2014 10:52 a.m., ezrcer wrote:

Thanks All for your responses.

I knew that the solution for this is to use a SMD pad and a polygon.
HOWEVER, the main issue with this as I mentioned in my original post is
that it generates a lot of DRC errors if I use a thin wire or a thin
width polygon (below DRC setting) to have a clean small pad. I was
trying to make a 0.28x0.85mm oval pad with the arc only on one side so
any thickness on the polygon or wire was not going to result in a
clean/accurate rounded pad. The other side was to be square.

I was able to make a clean pad by using the "roundness" property of an
SMD pad but the arc is on BOTH sides and I have really no control on the
arc radius like I would with a wire etc.

Best,
Sadry

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I see you ignored my pointer and continued to try to use a polygon. With

the scale (small sizes_) you are working with, that will not work well

as you have found out. So you have to use the wire method to create an

arbitrary pad shape.

Lets use your pad size of 0.28x0.85mm for this walk through:

Place an SMD of size 0.28 x0.28 (yes it's a square one and has no rounding)

From the center of the SMD drag a wire with a width of 0.28mm.

(orthogonal) that is 0.57mm long (0.85 - 0.28 = 0.57)

That gives you your pad shape. Square at one end, round at the other.

0.28mm wide and 0.85mm long.

Because the wire is about 11mil wide and that is greater than the

minimum wire sizes we usually specify in the Design rules there is no

DRC error because of that wire.

The wire is considered part of the pad because it has one end located on

the SMD centre.

But there is more to do.

We need to address the solder stop and cream masks as these are not

generated automatically for the wire, only for the SMD.

As this component is small pitched I would consider defining it all here

and letting the DRC setting take care of the rest of the board.

The first thing to do is turn off the stopmask generation for the SMD.

Uncheck that in the properties of the SMD.

So for the solder mask: Assuming a 0.07mm stop mask distance.

On the tStop layer draw a wire between the same two points as the

previous wire. Make its width 0.42mm

Next draw a rectangle (tStop layer) over the SMD. You can do this by eye

being guided by the stopmask of the wire.

Cream is done in a similar way on layer tCream but the sizes are less

than the size of the wire and SMD.

This is only one pad and you need to handle this group of objects as one

while in the library. Placing that first SMD in the correct place makes

copying the pad (group) along the remainder of that row easy, by setting

the grid size to the pitch of the pads.

All the best

Warren

Former Member · Answer

OK, found out that you can change the roundness % in SMD properties to create oval pads by setting it to 100%. I made the following QFN part. However, still do not know how to make a part that has ONLY one side of the pad round. Any suggestions 
 Attachments:

autodeskguest · Answer

James Littlefield wrote on Fri, 30 May 2014 20:15

Its sort of a messy solution but what I do is create two pads for each
actual pad.   One pad is rectangular and of the proper dimension.    I
then overlay one end of the pad with a circular pad of the right diameter
and offset from the inside edge by one radius.    This gives you an
overall shape that has right angle corners on one end and rounded on the
other end.    In eagle 6.x you can assign multiple pads to the same pin
on the symbol.

Or use one pad and a polygon of any shape to get what you are looking for.

The centre of the SMD must be within the polygon. And the width of the

polygon should be something that won't give you a DRC error--that can be

tricky when the pads get small.

I think this is the best way as it won't give you and DRC errors.

James.

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