Take Tighter Control Over Your Shape Degassing Patterns with Cadence 16.6 Allegro Package Designer and SiP Layout

With metal density and balancing requirements getting stricter with every year that passes, how you perforate the plane shapes of your designs needs to adapt.

Whether it is a new hole shape that allows for a more consistent pattern fill across the layer, multiple passes with progressively smaller holes to achieve that optimal balancing of metal, or perhaps a complex interplay of holes on neighboring layers in your cross section, the Cadence IC package layout tools continue to evolve to make sure your needs are covered.

To learn more about the new degassing options available with the most recent 16.6 software ISR releases, and to understand what is still to come in the 17.2 release that will soon be available, read on!

New Degassing Hole Shapes

Typically, a degassing pattern uses regular holes that are either square or octagon in shape. These might be rotated for more of a diamond pattern look, but they are the most common holes in use today. There are other options available to you; however:

• A circular degassing hole will result in a pattern with cut outs of similar size and shape to the voids around your circular via and pin pads on the same layer, allowing for consistency of pattern
• A rectangular hole will allow for more directional control of the pattern, with the holes facing in the north-south or east-west direction (or even at a 45 pattern to the corners of the substrate) to feed escaping gases along the desired paths
• Oblong holes, on the other hand, have the same benefits as rectangular voids do – but without any sharp, 90-degree angles in the outline of the holes. And, with the rounded corners, they can get just that small amount closer to surrounding geometries.
• Hexagon holes, the newest addition, can be placed in a staggered like pattern, allowing for straight paths of metal between the holes, optimal paths for current flow in power nets

So, before you go straight to your standard hole setup, ask yourself this question: does one of the alternative hole shapes offer advantages over the shape I am using today?

Inter-Layer Degassing Hole Array Interaction

When laying out your degassing patterns, you likely want to make sure that they don’t overlap with holes on the layers above and below. Not just for manufacturing reasons, this time, but also for signal integrity and interference.

How do you best accomplish this? There are a few options, depending on your desired interplay of holes:

• Partial overlap – Look at using a custom origin point for your degassing pattern, rather than one of the corners of the shape. Using the same origin point for all the shapes in your design allows you to set an offset from that location that is unique for each layer. Stagger the offset by half a void pitch on each layer for optimal offsets.
• No overlap – Set the adjacent layer void clearance to no overlap, and the tool will automatically suppress any degassing holes which would overlap a hole in a shape on the layer above or below. When using this setting, we recommend starting from one layer in the design and working either up or down to establish your degassing patterns based on the specific layer-layer void clearance rules you want.
• Void to shape overlap – If you only want to create degassing holes when they will be fully inside a shape on an adjacent metal layer, you can accomplish this as well. Set the adjacent layer void clearance to the inside shape option. With this, holes are suppressed if they are not inside a shape on the layer above/below, meaning that degassing holes will never overlap high speed traces or vias, for instance.

Multiple Degassing Hole Passes in a Single Shape

So, you’ve degassed your shape. But, your metal density is still too high for your needs. What do you do? Why, you run additional passes with smaller degassing holes! These will fit smaller holes into areas where previous passes could not squeeze in a hole meeting your clearance rules. Referring back to the picture in the last section, you’ll see the “Clear existing voids” option near the top of the form. Turn this off, and the configuration on the form will be applied without first clearing the existing degassing holes. To see this option on the form in your tool, make sure you are on the latest 16.6 ISR release, and that you have turned on the “degas_multi_pass_beta” option enabled in your user preferences. This is a standard feature in 17.2, but for customers needing to access this right away for their substrates, we’ve given you early access in 16.6. Give us your feedback if you choose to try it out!

Below is a simple example showing a two-pass degassing pattern. Because the metal density was still slightly too high with just the large degassing holes, the second pass smaller holes were added, bringing the layer density into spec.

A note, however. Only the first, or primary, degassing array settings are stored on the shape. These form the core degassing holes, with additional passes being performed to meet strict metal balancing requirements.

When to Refresh Your Degassing Holes

While you can degas your shapes at any point during your design flow, when you do it, and when you update the shapes, can save you significant time. Degassing a shape can add hundreds to thousands of holes to the shape. This means an increase in the database size, more complex shape outlines to consider when doing DRC updates, and a need to refresh the pattern when you make changes such as adding via connections to the shape or voiding around another net as you route through the shape.

Consider degassing your shapes as late in the flow as possible. This will maximize the database performance. Before you start doing metal balancing and density analysis, or before you begin SI and PI characterization, is an ideal time.

And, when you DO make changes to a shape, or just before you go to manufacturing, always make sure to update the degassing holes on all your shapes. A button on the degassing form will do this for you with a single press, remembering the custom settings for each of the shapes you have degassed.

Still to Come…

There are more exciting improvements on their way in the 17.2 release.

A new “Advanced WLP” option for SiP Layout provides tools specifically aimed at these substrates, with their very thin metal layers and strict metal density and balancing requirements. A metal density scan tool will allow you to see areas of the design where there is too much, or too little, coverage. From here, you can decide whether you need to more aggressively degas large metal areas or, perhaps, you need to add metal with thieving patterns, cline fills, or others strategies. If you have a particular go-to solution for bringing your metal density into spec when it is too high or too low, let us know – we might be able to automate that for you in an upcoming release of the tool!

Do you have other ideas? Talk to your Cadence customer support representative or get in touch with us by commenting on this blog or contacting the authors. We’d love to hear from you on this topic or, indeed, on any challenges you face when designing your package substrates!

Bill Acito Jr.