Via tenting and plugging

Brad Schick schrieb:

I want to put vias in the central pads of several QFN/MLF devices to

improve grounding and dissipate heat. To reduce costs, the vias are

0.3mm which is fairly large compared to a small QFN20 part's central

pad. To prevent solder paste from draining away during reflow, I believe

I need to plug those vias.

 

I'm considering plugging and tenting the vias from the bottom-side only

so that there is no top-side solder mask to interfere with good reflow

bonding to the chips. Another approach might be to only plug the vias

from the top (no tenting), but I'm not sure how accurately this can be

done in "prototype" runs.

 

Two questions:

 

1. Do either of those approaches seem practical?

I think plugging from the bottom is no good idea if you want to solder

on the top layer. Just think of air residues under the chip during

soldering. For a good soldering result, you need a good and planar

surface on the top layer. This can probably only be achieved by plugging

from the top side and then adding a fresh metal surface (by electroplating).

I would recommend to talk to your board house, they should know.

And I must confess that I didn't practically use this method yet.

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

Tilmann

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

 

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

 

Thanks that makes sense, particularly if I put the vias inside the

package itself. Two more (probably obvious questions). 1. How do you

draw a filled circle? Using the polygon tools works, but its sort of a

hassle. 2. I actually have not found a way to turn off stop masks on the

board itself. Only seems to work in packages. Is it possible on boards?

-Brad

Brad Schick wrote:

2. If I go with bottom-side tented vias, how can do this in Eagle? I

have only been able to turn off the stop mask entirely, not for just one

side. I was looking for a way to draw a polygon on the board that would

"subtract" any stop masks inside its bounds, but I haven't found a way

to do that.

 

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

 

 

Thanks that makes sense, particularly if I put the vias inside the

package itself. Two more (probably obvious questions). 1. How do you

draw a filled circle? Using the polygon tools works, but its sort of a

hassle. 2. I actually have not found a way to turn off stop masks on the

board itself. Only seems to work in packages. Is it possible on boards?

 

-Brad

1. Draw CIRCLEs with width = 0.

2. Pads and SMDs: CHANGE STOP ON | OFF in the library only

    Vias: See Design Rules, Masks tab, Limit

HTH

--

Mit freundlichen Gruessen / Best regards

Richard Hammerl

CadSoft Support -- hotline@cadsoft.de

FAQ: http://www.cadsoft.de/faq.htm

On 02/23/2008 12:27 AM, Tilmann Reh wrote:

 

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

 

Actually, putting the vias inside the package is a hassle. One of my

devices will be used more than once and I'll have to create new package

variant each time I want the via to be in a different location within

the central SMD pad. I'll also have to create a bogus pin on the symbol

to make the via grounded.

Richard, why is it not possible to turn off stop masks on the board

directly? It would be much simpler to just add the gnd via to the board,

turn off its automatic stop masks, then draw the stop mask I want.

Its rather strange that the board editors has a button for "change,

stop, off" that does nothing. Could making that work as expected be

considered for 5.0 still?

-Brad

Brad Schick schrieb:

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

 

Actually, putting the vias inside the package is a hassle. One of my

devices will be used more than once and I'll have to create new package

variant each time I want the via to be in a different location within

the central SMD pad. I'll also have to create a bogus pin on the symbol

to make the via grounded.

I didn't tell you to put vias into packages. Of course vias are always

placed directly in the board. That's why I explicitly mentioned that you

have to add your stop mask for each via separately, i.e. in the board.

Fortunately, you can use COPY for this.

Richard, why is it not possible to turn off stop masks on the board

directly? It would be much simpler to just add the gnd via to the board,

turn off its automatic stop masks, then draw the stop mask I want.

 

Its rather strange that the board editors has a button for "change,

stop, off" that does nothing. Could making that work as expected be

considered for 5.0 still?

It does work - however differently than you expect. I agree it can be

somewhat confusing. The "stop" property of a via really works - as long

as the via has not an automatically generated stop mask by the DRC setting.

So, in your case, simply set the DRC/masks/limit value to a drill

diameter greater than that of your vias, then you can change their stop

mask property directly. (The limit "rule" overrides the individual

settings of the vias.)

Tilmann

Brad Schick schrieb:

On 02/23/2008 12:27 AM, Tilmann Reh wrote:

That's easy: Turn the stop mask completely off, and then manually draw

whatever mask you want into the appropriate layer. This must be done for

each via separately, but hopefully there are not too many of these

special ones...

 

 

Actually, putting the vias inside the package is a hassle. One of my

devices will be used more than once and I'll have to create new package

variant each time I want the via to be in a different location within

the central SMD pad. I'll also have to create a bogus pin on the symbol

to make the via grounded.

 

Richard, why is it not possible to turn off stop masks on the board

directly? It would be much simpler to just add the gnd via to the board,

turn off its automatic stop masks, then draw the stop mask I want.

 

Its rather strange that the board editors has a button for "change,

stop, off" that does nothing. Could making that work as expected be

considered for 5.0 still?

 

 

-Brad

It works for vias in the Layout:

  ==> http://www.cadsoft.de/faq.htm.en#06062601

--

Mit freundlichen Gruessen / Best regards

Richard Hammerl

CadSoft Support -- hotline@cadsoft.de

FAQ: http://www.cadsoft.de/faq.htm

On 02/26/2008 04:35 AM, Richard Hammerl wrote:

 

Richard, why is it not possible to turn off stop masks on the board

directly? It would be much simpler to just add the gnd via to the board,

turn off its automatic stop masks, then draw the stop mask I want.

 

Its rather strange that the board editors has a button for "change,

stop, off" that does nothing. Could making that work as expected be

considered for 5.0 still?

 

 

It works for vias in the Layout:

  ==> http://www.cadsoft.de/faq.htm.en#06062601

 

Thank you Tilmann and Richard for the pointers. I should be able to do

what I wanted now.

But I'd still says Eagle's implementation is rather odd. It seems like a

much simpler (and more useful) behavior would be for the "change, stop,

off" command to always work for all components on a board.

Eagle could auto-generate mask openings for vias based on the DRC but

still let me turn them off individually (irrespective of the limit

setting). Likewise, I should be able to turn of a device's stop mask on

a board and draw my own. It seems odd to force the creation of a new

package just to change stop mask in what might be a one-off situation.

Particularly when the "change, stop" command already exists at the board

level.

I'm not sure how common tented vias are, but another nice feature would

be to let users change the stop mask setting to the following 4 values:

"off, on, top only, bottom only". Then I wouldn't have to draw it

manually and make sure it gets moved together with the via (which is

currently one advantage of putting in into the package).

Thanks again,

-Brad

Brad Schick schrieb:

But I'd still says Eagle's implementation is rather odd. It seems like a

much simpler (and more useful) behavior would be for the "change, stop,

off" command to always work for all components on a board.

I told you it can be somewhat confusing.

I think this has historical reasons. Earlier versions of EAGLE didn't

support the "stop" property for vias, and their solder stop mask was

globally defined by the masks/limit value in the DRC settings.

Later, the stop on/off property was added, but the masks/limits

parameter is still working the same way for compatibility.

I'm not sure how common tented vias are, but another nice feature would

be to let users change the stop mask setting to the following 4 values:

"off, on, top only, bottom only". Then I wouldn't have to draw it

manually and make sure it gets moved together with the via (which is

currently one advantage of putting in into the package).

For unplugged vias, it is a very bad idea to add solder stop laque only

at one side. Normally, liquid laques are used, so it will also stain the

other side. Tented (or partially tented) vias are a specialty that

always has to be communicated with the board house. I think providing

such options would be somewhat "dangerous" for all those around who

don't even know about plugging and tenting.

Tilmann