How to separate a short in the plane layers between 2 vias?

[kender]

Folks,

This is a rework question. I’ve made a mistake in a layout of a 4-layer board. I’ve placed 2 vias too close (or chose a wrong padstack for the vias). As a result, the nets became shorted in the plane layers (see the attached snapshot from the Gerber viewer). Is there a way to separate them (aside from drilling out one of the vias or reordering the board)?

Thanks!
Nick

 

[House_Cat]

Re: How to separate a short in the plane layers between 2 vi

You're going to need to cut the shorted region with a router, laser, or by hand.

I have corrected errors like this before by patiently cutting down through the board with a #11 blade in an Exacto Knife. Once the short was cut free, I used the pointed end of a toothpick to work a small amount of fast curing epoxy into the small slot that I had cut. The trick is to work slowly and carefully with the point of the blade.

I've also cleared shorts like this using an automated board router with a small bit. Once again, I backfilled the slot with a little epoxy when I was done.

 

[Rame]

Re: How to separate a short in the plane layers between 2 vi

Hi Kender,

House cat has good handi work beside designing Pcb,kudos buddy.If you try to attempt that you will be in soup,its not an easy task,it requires lots of experience to do that and moreover you will mess up the board.

My suggestion is to reduce the via & drill size for both the vias and generate the gerbers.


Regards

Ramesh

 

[House_Cat]

Re: How to separate a short in the plane layers between 2 vi

Ramesh,

I might point out that Kender has nothing to lose. He has a worthless board now. He wanted a way to try and fix the problem without drilling out a via, or reordering the board.

The only other choice is the one I suggested, or some variation of it. If he had a precision jeweler's drill press, he could take a 10mil drill carbide bit and drill a series of holes to cut out the short - but I suspect he doesn't have that sort of equipment.

If you're patient, and careful, you can do it with an Exacto Knife. If you screw it up, you're no worse off than when you started - you still have a bad board that you need to redo.

If the via hole is big enough, and only connects top-to-bottom, you can drill out the barrel of the via, thread a piece of wire-wrap wire through the via, and connect the top and bottom by carefully soldering the wire to the top and bottom via annulus. I've also used this technique a couple of times in the past. If the via connects to an inner layer, you obviously can't use this method.

I've been working with circuit boards for many years. After a while, you learn several ways to salvage bad boards.

House_Cat

 

[echo47]

I've used the x-acto knife technique many times to cut traces in inner layers. Install a long thin new blade, hold the knife with three fingers just like a jeweler's screwdriver, and spin it to slowly drill into the board. This provides good control. You can tilt the knife while drilling to elongate the hole. You'll probably be working under a magnifying glass or microscope, but remember eye protection for your curious onlookers because the knife tip breaks easily and flies away at high velocity.

 

[kender]

The general approach suggested by House Cat an echo47 – cut through the fiberglass – worked. I did it with a Dremel and a 0.5mm endmill (oddly enough, I didn’t break the latter) instead of a knife. I used the highest RPM and very gentle hand motion. It took 30 seconds to fix each short with 100% yield.

 

[Rame]

Re: How to separate a short in the plane layers between 2 vi

Hi House cat,

I agree with you,if the boards are worthless and he needs something out of it,then your method will be fine,but then my question is how effectively one can go through with the process?

i have gone through most of the answers posted by you in this forum,you seems to be very well versed in designing,signal integrity and fabrication too.

I request you kindly send across some article/documents,which explains about the split plane and what precaution one need to take,if you could send me step by step process(i do appreciate if it is in Pads) for the same....

Please also cc to...arkramesh@yahoo.com


Regards

Ramesh

 

[House_Cat]

Re: How to separate a short in the plane layers between 2 vi

I'm not sure exactly what you are looking for with regard to split planes. Precautions when using split planes are the same in all EDA software.

Planes usually provide the return side of signal loops. If your signal is a sensitive signal, when the trace on a signal layer passes over the gap of a split plane, there can be an impedance discontinuity that distorts the signal. For that reason, such signals are normally not routed over a split. If routing over a split can't be avoided, there are several techniques that can be used to minimize problems - such as use of bypass capacitors, placing an adjacent return trace, etc.

There is a lot of information available on the Internet regarding split planes. A few references are:

http://www.hottconsultants.com/techtips/pcb-stack-up-6.html
**broken link removed**
http://www.sigcon.com/Pubs/edn/diffuturn.htm
http://www.sigcon.com/Pubs/news/9_04.htm

If your question is how to create split planes in PADS, the help documentation that comes with PADS has the answers. If you are using PADS2007, there are at least a dozen pages in the help files that explain how to do it.

 

[Rame]

Re: How to separate a short in the plane layers between 2 vi

Hi Housecat,

Actually i am looking for document(split plane), which explains the segregation of components for mixed signal and how do one decide the layer stack up,placement of the component,routing etc....


I use pads 2005 spac2 and the split plane tutorial is not to the expectation.

I need split plane article on knowledge based rather then tool based.


Regards

Ramesh

 

[House_Cat]

Re: How to separate a short in the plane layers between 2 vi

I don't think I've ever seen one document that covers all of what you are looking for. There are several references on the internet that discuss plane segregation for mixed signal boards. Most of them are related to ADC/DAC placement and grounding on PCBs - this is one of the more difficult things to do properly on a mixed signal board.

There is an article from National Instruments at:
**broken link removed**

A note by Dr. Howard Johnson (author of High Speed Design book):
http://www.sigcon.com/Pubs/news/2_26.htm

An article from PC Design Magazine at:
**broken link removed**

Another PC Design Magazine article at:
http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

An application note from Analog Devices at:
http://www.analog.com/UploadedFiles/Application_Notes/294542582256114777959693992461771205AN280.pdf

And some good ideas on page 7 of:
http://www.analog.com/UploadedFiles/Application_Notes/495266810AN-404.pdf

You have to apply your general knowledge of electronic design to the problem. The best general reference books to learn the details of PCB layout and design are still the ones from Dr. Howard Johnson ( the two books listed on http://www.sigcon.com/ ).

 

[Rame]

Re: How to separate a short in the plane layers between 2 vi

Hi House cat,

Collected few pdf's from the link you provided and some where in my database already.

I know,the information iam looking for won't be found in single document.
Now,i wil make my own flow chart with the help of the documents collected from the link using my general knowledge on Pcb designing

Do appreciate for time taken posting all the links ....

Regards

Ramesh