Hi,I am trying to set up my via through hole. I have a drill of 10 mils and I am curious on how small I can have my top, internal, bottom pads. I am wondering if my internal pads need to be larger than my top and bottom pads too.Also, I have read that my thermal and anti pad need to be about 14 mils larger than my drill. Does this sound correct?Thank you,Louis
Louis,Everyone has their on spin on what the pad sizes for vias should be. The best place to start is the fabricator vendors that you normally deal with to fabricate your boards.Normally, the inner pad sizes are larger that the top and bottom to ensure you don't have drill breakout if the inner layers shifts.As far as the thermal and anti-pad sizes, 14 mils over drill sizes seems really small and you risk drilling a via thru a plane that it is not connected to due to the plane layer shifts. The fabrication vendor will normally drill the via hole larger than the size specified on your fabrication drawing and in the ncdrill data so he can achieve the specified hole size after plating so that is another thing to consider when constructing via padstack.As I said previously the first place to start would be the fabrication vendors you are currently using to fabricate your PCB boards and they should be able to provide you all the information you need.Hope this helps,Michael CatramboneUTStarcom, Inc.
There's two distinct ways I know of to develop your own padstacks, providing your company doesn't already have a design guideline for this:1) Develop the padstacks with the help of your board fabricator. They will give you their 'preferred' sizes and will provide you additional guideance when dealing with having to minimize sizes occasionally. They are a 'true' direct source.2) IPC-2221, Generic Standard on PCB Design. GET IT. look up www.ipc.org and order it. Look at Section 9: General requirements of Land/Holes. As I said: "GET IT!" Don't guess at these things. It will only cost you in the end. All the information is available. You can even search through the internet for PCB DESIGN, but order the standards (IPC) first, then do searches.Hope this helps. Good day.Mitch