L-3 Communications finds that Expedition Enterprise Grows as their Needs Grow

L-3 Communications was able to use their existing Expedition Enterprise product to design an HDI-technology printed circuit board, despite having never done so.

“Using the Expedition Enterprise flow, we were able to analyze what needed to be done with high speed constraints, understand the final product and what it needed to look like, and apply the tools to deliver a reliable board.”

It's no surprise that military contractors often work with the very leading edge of technology, pushing previous boundaries, and moving into the unknown. When L-3 Communications recently took on a very complex design in a limited-size footprint, they indeed pushed boundaries into the unknown, but printed circuit board success was ensured by L-3's use of Mentor Graphics Expedition Enterprise and its capability to grow as the customer's needs grow.

Fast Signals, Dense Routing, and Small Form Factor: A Big Challenge

Working together, electrical engineer Michael Talbot and printed circuit board designer Jayson Harames at L-3 Communication's office in Salt Lake City, Utah, have used Expedition Enterprise for a number of designs. They have established a standard workflow using Expedition and add-on products.

Talbot and Harames were given a project that would push the designers' knowledge and skills, but Expedition would be there with the tools they needed. They knew they were in for a challenging design when they were presented the schematic and the requirement that it be contained on a 3U-size board. The small form factor was one thing, but heaped on top of this was the fact that the onboard processor would be running at 1+ GHz, there was 512 MB of DDR memory, a large FPGA, and a serial RapidIO switch.

They had been using Expedition Enterprise for some time to produce a number of challenging layouts, but this would require technology that they had not used extensively and education about the technology as well.

High Density Interconnect (HDI) printed circuit board technology had been introduced to L-3 shortly before this project. However, this team had only limited experience with HDI. While they had designed other boards with perhaps a couple hundred microvias, this would be their first with multiple-thousands.

Entering Constraints

With knowledge in hand and a mentor from Mentor, the L-3 team moved into constraint entry. The design called for several serial RapidIO busses as well as PCI Express busses…all operating at multi-gigabit-per-second rates. With the Constraint Editor System (CES), the extremely tight tolerances require to match differential pairs were straight forward.

And, CES is very intelligent: "Constraint Editor helped because it looks at the entire net; it doesn't differentiate between pieces of the net. It also allowed us to pre-skew signals at a large connector to account for the differential skewing the connector produced," said Talbot.

"The most useful feature on CES was entering the complex equations for our DDR2 constraints. It made it a lot simpler, and made it much easier to route the board successfully…we met the real DDR2 constraints as opposed to the rule of thumb ‘match everything.'"

Pre-Layout Simulation

Another challenge of the board was the proliferation of very-high-speed nets…multi-gigabit-per-second class signals. With switching speeds in this range, signal integrity (SI), as well as power integrity (PI) become critical factors to ensuring a properly working PC board.

"We used HyperLynx to establish the basic ground ruled for routing high-density lines," said Talbot. "A lot of time was spent simulating to establish a set of rules to route our multi-gigabit lines. We used parallelism to make them easier to route."

With HyperLynx simulation, routing can be simulated and tolerances reduced based on the actual signals present in the traces, rather than rules of thumb.

Layout and Routing

If it seems that constraint entry and simulation are treated with extreme care at L-3, there is a reason: L-3 does not produce prototype boards. So, as Harames observes "getting it right the first time is critical to what we do!"

Of course, that carries through into layout and routing as well. They found that Expedition handled the HDI technology as smoothly and accurately as their previous designs. Mentor tools have a reputation of being easy to learn and quickly apply to new problems. "Handling HDI within Expedition was relatively straight forward," said Harames. "I was able to use the tool the way I was accustomed to."

Project Debriefing

Both Talbot and Harames agreed that they were very pleased at how well Expedition Enterprise handled moving into technologies with which they had little previous experience, as well as Mentor's consulting support to ensure their success. Despite the fact that they had to educate themselves in a new technology, their schedules were met. In fact, the layout schedule originally called for 12 weeks of time, but had to be reduced to just six; nevertheless the new schedule was achieved.

There are also forward-looking benefits to this particular project and the educational process that accompanied it. "We're going to benefit from the success of this project," said Talbot. "We have already started to incorporate HDI interconnect in more of our boards."

“We’re going to benefit from the success of this project. We have already started to incorporate HDI interconnect in more of our boards.”