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Via structures—those reusable patterns of conductor clines
and vias designers rely on to maximize their productivity—have a
long-standing place in the robust escape routing feature set in the Cadence IC
Many of us use only the basic capabilities in the via
structures repertoire, though, to create structures in the current drawing and
replicate them across a pattern of pins.
Today, we'll look beyond those basics to see how a crafty package
designer can really save time and energy while reducing potential errors. Take
a step into the future and read on to learn more!
There are many reasons that you may wish to hold off on
using via structures until later in the flow. Most auto-routers, for instance,
do not support placement of via structures during routing. Similarly, you may
not know how many layers will be needed in the final substrate design until
you've finished doing some preliminary routing or received the final ECO of the
die pin layout and BGA ball assignments.
Whatever the reason, a handy flow to keep your design work
simple while still being able to quickly optimize to the final solution with
complex via structures between layers is to convert from seed vias into the
As an example, consider the desire to use the auto router
for the initial routing, with via structures for the inter-layer connectivity.
Start by selecting your via structure definitions. Based on these, create
simple, single via padstacks with pads the same size on each layer as the
extents of all the elements that will form the final structure (vias, clines,
etc.). The pad will ensure that adequate clearance is maintained between the
detailed via structure.
After adding these via pad definitions into the design and
into the appropriate via lists in constraint manager, use the auto router of
your choice—the brand new Advanced Package Router, available now in 16.6, for
instance—to do the actual detailed routing. If the router is not able to get
to 100% completion, or you need to make any minute adjustments, you can do so
with the rich set of interactive routing commands.
Once the routing is completed to your satisfaction, start up
the Route -> Via Structure -> Replace Via with Via Structure ("replace
via with via structure") command. If you
haven't used this command before, you can enable it from the
IC_Packaging/Early_Adopter folder in the User Preferences. From the command,
pick your placeholder via and the final via structure and do a global
replacement. Instantly, you've converted your design to the final via stacks.
And, best of all, you can always put them right back to the
temporary vias if you need to run the auto-router or interactive routing again
for any reason! How's that for failsafe?
Just as you might want to delay the injection of the final
via structures until later in the design, you might also need to make changes
to existing via structures because of a change in the substrate parameters.
Perhaps you receive a notice that the die needs an optical shrink applied, or
that the scribe dimensions weren't included in the definition. These can have a
dramatic impact on your routing, depending on how far along you are.
This time, let's assume that a 10% optical
shrink is to be applied to the die, which can be done quite easily by importing
the source die text file again and replacing the original placed die component.
And, with the die replace functionality, all the clines and vias connecting to the
pins will automatically move/stretch to the pin's new location.
Unfortunately, this can result in changes to the via
structure definition or require that you make changes to the locations of vias
and cline vertices in the structure to achieve DRC-free escape routing from
under the die. If you've already used via structures for your pin escapes, fear
not! You only need to modify a single instance. Change via
locations, cline widths, routing bends, and whatever else might be necessary to
escape that single pin properly. When you are finished, run the Route -> Via
Structure -> Redefine ("redefine via structure") command. Again, if you
haven't enabled this command, you can do so in the IC_Packaging/Early_Adopter
folder of your User Preferences.
With this command, pick the instance of the structure that
you've updated to compensate for the die shrink. The via structure definition
will be updated to match the changes made at the instance level, and you have
the option of automatically updating all the other instances to match these
changes. In a single step, you've updated all the escape routing for hundreds
of pins on your die all at once. What could be easier? Should you need to save
this modified definition to disk, you can always write out an XML file for the
new definition using the Route -> Via Structure -> Add.. ("add via
structure") command's export feature, too. So you can update all your other
designs just as easily with the batch replace via structure command.
It might be that you associate via structures with escape
routing from a complex flip-chip (or offset via patterns from your BGA pads up
to internal routing layers). But, don't be so quick to ignore your wire bond
designs. Via structures definitely have an application here, too!
The wire bond toolset allows you to define escape distances
and an optional via to be placed at the end of that stub. This is perfect for
simple escapes or for single-layer packages. But, if you need a more complicated
pattern that has multiple segments in the cline or continues the escape on
internal substrate layers, consider using via structures instead.
With a regular escape, as you push and shove your bond
fingers, only the first segment (and optionally the via at the end) will move
with the bond finger, stretching to maintain connectivity with the rest of the
route. By using a via structure instead, the entire structure will move along
with the finger as it moves. This gives you control of the where stretches will occur in the routing to compensate for bond finger adjustments.
We've covered three advanced concepts here. But there are
as many ways to leverage via structures in your design flow as there are
package designers. Do you use these design elements in a way you want others to
know about? Or, do you have an idea for an enhancement that would make them
more useful to you and the rest of the design community? Drop us a line and let
us know! We'll be happy to help get your suggestions heard—whether it's by
your fellow designers or the Cadence IC Packaging engineering team.
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