IC Packaging and SiP Blogs

Degassing, other times called outgassing, is a common operation in IC package and interposer designs. As the name implies, the technology allows for gases to escape during the manufacturing process and prevents bubbling. It has a second benefit – in a manner opposite to dummy metal fill, degassing can lower the density of metal in a region or across an entire layer of your design.

In large designs, or when a fine mesh pattern of degassing holes is used, there may be thousands to millions of holes per substrate layer. Holes are laid out in a consistent grid structure, with those holes in the pattern too close to surrounding elements/shape edges, being depopulated from the overall map.

Today, two primary methods are used to represent this pattern. If the shape of the mesh holes is square, then a cross-hatching pattern of fill lines can be used to create the negative hole pattern. Here, anywhere a complete degassing hole cannot be placed is filled with additional line fill. A more flexible, general-purpose pattern where any shape of degassing hole from rectangles to circles and hexagons is used, will often rely on individual discrete holes defined in the overall filled shape.

Both solutions can lead to a significant increase in database size and memory footprint. And, in turn, these decrease performance of key operations like on-screen rendering.

With the 17.4 release of Allegro® Package Designer Plus, Cadence has taken an exciting new direction for describing the degassing hole patterns in your design. To learn more about the impacts on your designs and flows, and why you need to consider moving up to take advantage of this technology, keep reading!

A New Data Model for Efficiency 

If you are using Allegro Package Designer Plus with the Silicon Layout option, you’ll find the Advanced Shape Degassing command in the Si Layout menu (on selecting the Silicon Layout option). This command uses the new data model. You will need to have access to this license in order to get the full benefits of the architectural changes.

To describe regular patterns of elements like these, it is normally beneficial to incorporate the description of the pattern itself into the modeling. Cadence has taken this approach and applied it to the database architecture for degassing hole patterns.

Does this make a significant difference in database size and performance? It certainly does! A shape with flat degassing holes using the 17.2 data model and having 250 million square holes would require an on-disk database footprint of approximately 4GB. Using the 17.4 data model for the same design, the shape and complete hole pattern needs only 6MB! When the time comes to render your design on screen, the same organization will see orders of magnitude faster draw times.

Below, you’ll see a side-by-side comparison of 17.2 (left) and 17.4 (right) as the shape is degassed and then the display moved around to view different areas of the resulting pattern:

Performance Gains

Degassing a large design with many solid fill shapes using a small mesh hole pattern could be quite time-consuming in the 17.2 release. The new architecture’s power can be harnessed not simply within the display and modeling of the holes, but also in their definition and placement calculations.

Going back to our example above, creating that 250-million-hole pattern in the database would take many hours of processing and significant amounts of accessible memory on your machine. It was a task better suited to a server application. That same request, done in 17.4, will take roughly 5-6 minutes. Memory usage will still be high during spacing and clearance calculations, of course, but even that is heavily reduced over the previous flow.  

Action such as streaming out GDSII data for your design see a similar improvement. However, you must realize that the GDSII format itself doesn’t have a concept of a hole element for a filled polygon. As a result, the final GDSII file must flatten these into a collection of positive polygon outlines which, taken as a whole, define the complete shape.

See Your Pattern Information in Show Element

Prior to 17.4, it was necessary for you, as the user, to remember your degassing parameters that had been applied to your shapes. If you had multiple passes with different sizes, shapes, or offsets, remembering the order they were applied could be… difficult.

For this very reason, in 17.4, bring up show element on a degassed shape to see the information about all the configured passes. Shown below is a very simple three-pass setup on a shape:

Not only can you see the different passes (listed in the order they will be applied), you also see a key data point: the status of the pattern! Since degassing hole patterns need to be refreshed when you modify the design and impact the shape’s outline areas, the status line – and the DEGAS_OUT_OF_DATE property behind it – make sure you know which shapes have been touched and need their pattern updated before you go to manufacturing.

You can use the find by query interface, or even the find by name tool in property mode, to find all those shapes that are out of date and refresh them at the click of a button any time you choose.

And Now, for the Bad News

You knew this was coming. What are the downsides to the new degassing architecture in 17.4?

Firstly, because this is a change to the structure of the database, you will not be able to use Advanced Shape Degassing in 17.2 Compatibility Mode. Likewise, if you need to send your design to a colleague who only has 17.2 available, you will need to defer your degassing hole patterns. In 17.2, these can be shown again. However, this will be done using the 17.2 data model. It is best, therefore, if you and your design chain can all leverage 17.4.  

Secondly, know that you cannot edit, move, or delete individual holes in the pattern. In the older data model, where each hole was a discrete element, you could modify them if you wanted. Now, you can no longer do that. There are more appropriate ways to manage this pattern manipulation, though. Use NO_DEGAS properties on key design elements to ensure holes are not placed above/below them or degas keepout regions to keep the pattern entirely out of areas of the design. The advantage of these techniques is that, as the design changes over time and you need to refresh the degassing pattern, those keepout areas will always be respected. Were you just to move or delete a hole, it would shift back to its on-pattern location with the next shape refresh.  

Thirdly, if you are updating an existing design from an earlier release and want to use the new data model, you will need to defer your existing (flat) hole pattern. Run the Advanced Shape Degassing tool, then, and press the “Update all degassed shapes” button. While the parameters will not have changed, the database model needs to update to the new structures. This will take care of that and recreate all your mesh patterns for you.