Tom
10-06-2002, 08:38 PM
I've been working out placement and routing grids for various pitch QFP's and here is the findings so far.
I have discovered two major features and one flaw:
1. That dividing the millimeter in halves is the best way to go.
Placement of parts using 1mm achieves the best fanout results for QFP's
Using a 0.125mm trace/space is superior and can be routed on the following grids:
0.25mm
0.125mm
0.05mm
The optimal Via size is:
Pad: 0.65mm
Drill: 0.3mm
Anti-pad: 0.8mm
Using a 0.25mm routing grid is easiest to work with, especially with 0.5mm pitch TSOP's and QFP's. The 0.8mm Anti-pad provides a clean return path for signals and allows for manufacturing tolerances. The 0.3mm Via hole size provides plenty of current and the 0.65mm Via pad provides plenty of annular ring.
2. I have also discovered that 0.1mm is the second best and 0.05mm the third best usable grid because the millimeter can be evenly divided with these numbers. This is the only grid technology to achieve dual track routing between vias placed on a 1mm grid system.
The optimal Via size for 0.1mm trace/space is:
Pad: 0.5mm
Drill: 0.25mm
Anti-pad: 0.7mm
3. The flaw is in the 0.15mm trace/space technology. It's usable, but cannot be totally optimized. I.E.: metric grids, via placement/fanout, trace routing channel optimization is limited.
My next study is going to be with various pitch BGA's and then various pitch SOP's. Download the Power Point file called "Metric Via Fanout QFP.ppt" from the Site Index for 20 routing illustrations using a combination of various QFP pin pitches, trace/space sizes and optimal Via sizes.
E-mail me if you can't find the presentation file and I'll send it to you. Here is a sample 0.5mm QFP using a 0.125 Trace/Space with a 0.25mm routing grid. The dots on the image below are 0.25mm apart and when you move a via out one grid, it allows room for another routing channel. The Via pad is Dark Green, the hole is bright green and the Anti-pad is yellow. The anti-pad shown is 0.85mm in size. It would be best to reduce that anti-pad to 0.8mm to allow for manufacturing tolerances. You want to avoid traces runs over via anti-pads because when they do the trace loses it's plane return path and causes signal integrity problems with that net.
I have discovered two major features and one flaw:
1. That dividing the millimeter in halves is the best way to go.
Placement of parts using 1mm achieves the best fanout results for QFP's
Using a 0.125mm trace/space is superior and can be routed on the following grids:
0.25mm
0.125mm
0.05mm
The optimal Via size is:
Pad: 0.65mm
Drill: 0.3mm
Anti-pad: 0.8mm
Using a 0.25mm routing grid is easiest to work with, especially with 0.5mm pitch TSOP's and QFP's. The 0.8mm Anti-pad provides a clean return path for signals and allows for manufacturing tolerances. The 0.3mm Via hole size provides plenty of current and the 0.65mm Via pad provides plenty of annular ring.
2. I have also discovered that 0.1mm is the second best and 0.05mm the third best usable grid because the millimeter can be evenly divided with these numbers. This is the only grid technology to achieve dual track routing between vias placed on a 1mm grid system.
The optimal Via size for 0.1mm trace/space is:
Pad: 0.5mm
Drill: 0.25mm
Anti-pad: 0.7mm
3. The flaw is in the 0.15mm trace/space technology. It's usable, but cannot be totally optimized. I.E.: metric grids, via placement/fanout, trace routing channel optimization is limited.
My next study is going to be with various pitch BGA's and then various pitch SOP's. Download the Power Point file called "Metric Via Fanout QFP.ppt" from the Site Index for 20 routing illustrations using a combination of various QFP pin pitches, trace/space sizes and optimal Via sizes.
E-mail me if you can't find the presentation file and I'll send it to you. Here is a sample 0.5mm QFP using a 0.125 Trace/Space with a 0.25mm routing grid. The dots on the image below are 0.25mm apart and when you move a via out one grid, it allows room for another routing channel. The Via pad is Dark Green, the hole is bright green and the Anti-pad is yellow. The anti-pad shown is 0.85mm in size. It would be best to reduce that anti-pad to 0.8mm to allow for manufacturing tolerances. You want to avoid traces runs over via anti-pads because when they do the trace loses it's plane return path and causes signal integrity problems with that net.