6 Replies Latest reply on Jan 17, 2013 11:14 AM by wolferm

    Paste layer mods in decal

    Wayne

      Hi all,

      I've been attempting this off and on for quite some time now.  I think I know the answer to the question but am interested if there are some insights out there....

       

      In decal editor I create a part - say a QFN for this discussion - with a thermal pad in the center.  I add my vias through the different ways to do it.  Now I want to restrict the amount of solder that is placed during the stencil printing cycle so I want to add "X-outs" to the decal on the paste layer.

      How do I do that.  I've tried every which way I can think of and still have not found it.  The closest I've gotten is by placing an unassociated piece of copper on layer 1 as the thermal pad, placed the vias, added copper on the soldermask layer to prevent mask from being applied and made my paste apertures on the paste layer, again using copper shapes.  The issue is, however, that because layer 1 copper shape is unassociated with a pin it will be not be tied to ground - and so will cause an issue with the vias that are tied to ground.  (Unless I add it to ground in the layout process which I'm trying to avoid having to do.)

      Making the thermal pad connected to one of the ground vias seems to make that shape appear on all layers with no way to control/modify the aperture on the paste layer.

       

      Anyone have any insights into doing this?  So far I've let me stencil manufacturer know which apertures I need X'ed out or thermaled...not an ideal situation.

       

      I hope I've made the situation clear, if not, let me know.

       

      Thanks in advance for any advice.

       

      Wayne.

        • 1. Re: Paste layer mods in decal
          vladimir.stoyanov

          Dear Wayne,

           

          There are two things which doubts me. Please, see attached decal of one of my QFNs:

          1. First advice is to associate copper in L1 and L21(solder mask top).  After this calculate thermal pad area. Solder paste should be 50% of  this copper. Draw some squares in L23(paste top) and associate them to  thermal pad.

          2. Do not put vias in the decal - put them in the design between paste  squares. Usually I use via 0.3/0.55 and via grid 0.9 mm or 1 mm.

           

          I work in this way and I don't have a problem.

           

          Kind regards,

          • 2. Re: Paste layer mods in decal
            jduquette

            This issue goes back and forth on how to connect a thermal pad.   People have two techniques. 

             

            I prefer to keep all the connectivity in the schmatic.  I add an extra pin on the symbol in the schematlc and tie it to the thermal pad; I used to go as far as to add multiple pins and then make them through hole in the pad so the thermal vias are tied to the decal too.  That has routing limitations though if you ever need to shift a thermal via so now I only define the pad.  That will connect me to a top side ground plane and I'll add thermal vias as needed.

             

            Others like to define things in the layout.  They will add a copper shape for the thermal in layout, not the decal, and set it to the appropriate net (i.e. ground).  This puts all the responsibility for the thermal performance on the layout designer.  I like to classify that as post processing.

             

            I'm typically the engineer that selects the parts, sets up the library, defines the circuit on the schematic, and then does the layout.  That is why I like to tie it all together on the schematic, so post processing doesn't get lost during a revision change.

             

            You have the right techniques.  You need to pick which you prefer to use.

            • 3. Re: Paste layer mods in decal
              Wayne

              Thank you both.

              It kind of sums up what I've been pretty convinced of for some time...there is no easy, cover all the bases, do it all in decal editor way to do it.

              I suppose I'll continue with what I've been doing...

               

              Pads, seems like here is one area the product could be vastly improved.

               

              First: allow holes/vias to be added to a part without calling them "pins".  This would allow a person to add thermal vias without having to add pins to the schematic symbol.

              Second: allow modification of individual layers such that we can make grid squares or X-outs on the paste layer etc. as we need to without having to go through the gyrations we currently need to go through.

               

              Thanks again for the responses.

              Wayne.

              • 4. Re: Paste layer mods in decal
                wolferm

                I agree with PADS there is no all inclusive way in decal, but in all honesty if Mentor would get there act together there should be.

                Expedition does have the ability to do almost anything within a Cell that you can think of. But of course get ready to mortgage your life to own it.

                It's this kind of basic functionality in todays design environment that should be in ALL level CAD systems.

                OK don't let us autoroute adhearing to the rules or nto be able to use soem advanced feature etc unless you pony up the big bucks,

                but this kind of stuff is basic to being able to make land patterns correctly.

                Just my 2cents.

                • 5. Re: Paste layer mods in decal
                  Wayne

                  Amen to that!

                   

                  This is basic functionality at its...well...most basic.  I shouldn't have to cough up a few extra 0's on the right side of the number to get it.  I won't name names but "other" much cheaper CAD packages have it.

                   

                  And since we all know inflation is alive and well...that's my 3 cents!

                   

                   

                  • 6. Re: Paste layer mods in decal
                    wolferm

                    Thanks, I didn't want to say it but, but, agree very much YES other far less expensive CAD packages than PADS

                    do have much more basic & advanced level functionality than PADS for the buck in many areas.