The FireBrick FB2700 is one of the current models in the FireBrick family of enterprise-grade internet routers (see the FireBrick website).
The hardware was designed by Kevin Hones of Watchfront and Ben Mack. It uses an Intel network processor, along with DDR2 RAM, Flash, a Gigabit Ethernet Controller, and a Gigabit Switch Chip. Good power management was crucial, using switching regulators to generate 5 different voltage rails. Whilst this is a fairly conventional design, great care was taken to optimise performance and minimise noise.
Ben designed the PCB using a 6-layer controlled impedance stack. The remit here was to keep the design as clean and simple as possible, ensuring ease of fabrication and assembly, thus keeping costs low and reliability high. We route all PCBs manually rather than using an auto-router, as the results are neater with better noise performance. This is a relatively dense design, with BGA packages of up to 460 pins, yet the layout is clean and tidy. Good thermal design was also crucial, using power planes to conduct heat away from the ICs.
There are several high-speed interfaces in this design (DDR2 RAM, PCI, MII, USB and gigabit ethernet), which are always fun! These require controlled impedance microstrip and stripline tracking, which is achieved by running fixed-width tracks, at a fixed distance from ground planes, and with guard spaces around each track – effectively creating lots of individual coax cables. Some signals, such as the DDR2 clocks, are run as differential pairs to minimise noise and jitter – there are over 30 differential pairs in this design.
Track lengths also need controlling, so that propagation delays for a group of signals are matched (in some cases to within 10 picoseconds). For example, all 32 DDR2 data lines are exactly the same length, including the line lengths inside the ICs and to the external termination resistors – we like a challenge!
We liaised closely with the PCB fabricators, choosing track and gap sizes and a laminate stack that gave us the precisely controlled impedances needed, while sticking to their standard materials and processes in order to keep cost down. No blind or buried vias allowed here.
We are proud of the results – the board worked first time, with low noise levels ensuring optimum performance, and it passed EMC testing with flying colours. A number of people have commented on how good looking the board is – we believe aesthetics matter. And of course, being a FireBrick, the PCB had to be red!
The case was designed by David Edgerley over at ISIS Product Design. We wanted a design that stood out from the crowd. David has a good eye for clean elegant designs, and the result is a slab of steel and aluminium that reflects the FireBrick’s ethos of being solid, reliable, and ermmm brick-like 😉
The FB2700 is selling well, and is proving a very reliable design. We are currently working on the next generation.
Ben Mack, January 2013