Requirement
Working with a long-term customer, BVM were ask to produce a custom PCB carrier design housing & utlising twelve PCIe mini-cards in combination with a mid-range graphics card, itself requiring eight PCIe lanes. The PCIe mini- card slots needed to be capable of running up to Gen 3 speeds. At the heart of the system was to be a mini-ITX card utilising an iCore i7 series CPU.
The completed unit was to be built into a fanless enclosure, powered from 12 volts be operational within an environment from -40 to +70 degrees centigrade.
The challenges
On the surface, the design initially appeared a relatively simple PCB design to produce as each mini-PCIe card only required one lane of PCIe and power. However, upon investigation – the challenges of the design became more apparent.
Firstly, there were the physical constraints of locating the mini-card sockets in a logical orientation on the carrier card. Secondly, there was the requirement to split the PCIe x16 signal from the motherboard into (i) a 1×8-lane and (ii) 8 x 1-lane – which meant the design required the use of a large PCIe switch.
Secondly, due to the physical size of the PCIe mini-cards and the number required, track routing of the board was a major issue – as in order to obtain full Gen3 speeds, the PCB design was critical. All tracks had to be impedance matched and carefully routed to avoid ‘cross-talk’. This involved manual routing, taking care to measure the track lengths for each side of a PCIe pair and judicious use of ground plane and guard/’keep-out’ areas.
The whole module was to be embedded in a system enclosure which would provide conduction cooling.
The Design
The physical dimensions of the carrier were dictated by the client, 280x220mm. This was physically large enough to place the twelve mini-cards and a socket for the graphics card. Selection of the PCIe bridge chip was also a sizeable challenge. It had to be capable of meeting the functional requirements, available in ‘modest’ quantities (many PCIe switches are freely available – but only in larger volumes to OEM PC manufacturers, directly) and be suitable for use in an industrial environment regarding lifetime availability. The chosen chip was the PEX8764 – a 16 port Gen 3 PCIe switch in a 1,500 ball BGA format.
On-board power supplies were required as each mini-card slot had to take up to three amps – and also a set of indicator LEDs to provide visual feed-back of each mini-cards connectivity/activity.
The Solution
The finished carrier solution resulted in a multi-layer PCB measuring 280×220 mm. All twelve mini-card and the PCIe graphics cards were located on the top side of the card together with the power supply modules which facilitated cooling via heat pipes to the enclosure baseplate.
Each channel of the finished solution was tested and tuned in the registers of the PEX8764 to guarantee Gen 3 operation following which all channels were verified simultaneously. The finished system was then tested in BVM’s in house environmental chamber to ensure operation across the defined temperature range (-40 to +70 degrees centigrade.
The Result
After initial prototyping and a number of field trial quantities had been provided to the customer for their own internal and external field testing, a 4 year supply program was embarked on with our client – resulting in the shipping of around 800pcs of carrier PCB and supporting mini ITX solution, all required for supporting a military based surveillance operation in North America.
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