IC Packagers: An Introduction to Off-Grid Degassing

All of you doing advanced node package or silicon interposer substrate design in Allegro^® Package Designer know what degassing is. And, while we talked last summer about massive performance improvements (plus additional information in show element about pattern up to date status and hole size information), this doesn’t tell a complete picture.

What happens when you apply your patterns of holes but, due to the traces, vias, pins, and other elements across your layers, some areas still need additional holes to pass the manufacturing rule checks for local density and maximum contiguous metal?

Enter the off-grid degassing tool, the topic of our discussion today. This tool is designed to run after you’ve added all the hole patterns recommended by your OSAT or foundry and you still find yourself with problem areas. Keep reading to learn how this tool can save you hours of time by automating what would otherwise be a heavily manual process.

First: The Problem 

To give a clear example of the problem, let’s look at a simple example. Below, I have a mostly empty plane that I’ve added a square of vias with routing to. This is a contrived example, I realize, but we want to keep things easy to understand.

There are no voids in the plane here at all, yet. 100% metal coverage is going to exceed the maximum allowed. We need to drop that density!

Second: Degassing to the Rescue!

Looking up our technology requirements, we can see that the recommendation is for a larger set of circular degassing holes with a smaller set of squares where the circles can’t fit.  The degassing interface makes this very simple to define as two separate passes. A quick cross-reference to the technology data to fill in the values and we are all set.

Degassing hole patterns (like metal fill or thieving) is done on a grid pattern. Done at an angle above, they still have a very consistent, regimented pitch and spacing. While density around our square of vias and traces looks good, inside the square is another matter entirely. That just won’t do. We need to get holes in between those traces.

Third: Finding Problems

How many holes, and what exact area they need to go in, can be determined by running the sign-off rule deck from your manufacturing partner. These rules can (and will) vary in complexity and stringency depending on the node you’re at, the vendor, and a host of other rules.

For that very reason, Allegro Package Designer doesn’t provide a complex UI to describe how to detect where extra holes are needed in a layer’s metal. Instead, it integrates tightly with the Cadence^® PVS and Pegasus physical sign-off tools. Running the sign-off deck against your final metal layer will load DRC region outlines into the MCM layout anywhere the maximum density or contiguous metal width rule is exceeded.

In our above image, as you might guess, that marker is going to show up where we’re getting all the vias and traces:

That’s right! As expected, we need to add some holes in there. You have a few strategies you could attempt. First, you could add more degassing passes to this shape with different offsets, angles, and parameters. Each new pass might only fix a handful of violations – and, normally, you are apt to have more than the one region as we have here. Next, it is possible to add holes manually for yourself. Certainly, you’ll be able to place enough in the pattern to be successful. The trouble is, if you have one hundred of these areas, that might take all day. And, you’d still need to make sure that you don’t add degassing holes that don’t overlap illegally with holes in the layers above and below this one.

The Grand Finale: Fine Tune with Off-Grid Degassing Holes

The remaining (and, in my opinion, best) option is to use the SI Layout menu’s Off-Grid Degassing Holes command. This tool, interface below, lets you indicate what manufacturing layer it can find the rule markers highlighting the areas in need of correction.

After specifying the layer and giving the hole size/shape to use when placing off-grid holes, the tool will search each DRC region marked for the layer to add as many more holes as it is able. These will be done in keeping with the spacing constraints and other considerations.

This is much quicker than adding holes manually, but it is preferable to describe dozens of additional degassing passes, too. While the passes WILL eventually result in enough holes being placed, as soon as you make changes to the layer for the next ECO, you may find that they no longer are enough. Back to the drawing board? Hardly! Allow the off-grid degassing tool to manage things on your behalf, and you’ll be done in no time.

As shown above, we can see the four extra square holes that have been placed. My manufacturing rules are all met, now, and I can continue to the next step in the design flow.

That’s a Wrap!

That’s all there is to it. If you have access to the Silicon Layout license option, within minutes, you can be free of density DRCs on all your layers. Should a future ECO lead to new density errors – or a change that requires you to delete some of these holes – you’ll be well-positioned to make those changes in the blink of an eye!

Have you used off-grid degassing before? Have you any comments for our engineering staff? Reach out and let us know if that’s the case. These are some of the most complex substrates being designed in the Allegro Package Designer tool today, and we would love to help make you successful!