AIR AS THE DIELECTRIC
BACKPLANE CONNECTORS HELP DETERMINE HIGH-FREQUENCY SIGNAL QUALITY, BUT THE MUST ALSO BE AFFORDABLE. USING AIR AS THE DIELECTRIC, TOGETHER WITH A SPECIAL GEOMETRIC ARRANGEMENT, IS JUST THE ANSWER TO ACCOMPLISHING THIS BALANCING ACT.
When transmitting signals in the 10GBits/s range, board-to-board connectors frequently reintroduce the losses and interference that signal conditioning seeks to overcome. The connector system is thus of paramount importance in assuring that the highest possible level of signal integrity is maintained in high-speed datacom and telecom applications. It must not in any way degrade the optimum working of the signal conditioning applied i.e., the losses and crosstalk introduced by the connector system must be as low as is possible.
Parallel to this requirement is an overwhelming demand to increase connector density, and to reduce connector height and linear board length. At the same time, the need to reduce the connector's applied cost and physical weight makes the design challenge for backplane connector manufacturers all the more difficult.
The design requirements by their very nature appear diametrically opposed to the achievement of an ideal low-loss, low-crosstalk backplane connector system. Thus, a new approach to connector design is unavoidable. By harnessing air, FCI believes it has found just the right path.
At the heart of the AirMax VS (virtual shielding) connector is what is referred to as the insert molded leadframe assembly (IMLA). A look at image 2 shows that this basic building block supports both differential pair and single ended signal lines, or a mix thereof, which helps to keep parts inventory to a minimum.
The AirMax VS inverse connector system comprises a vertical receptacle for the backplane and a right-angle header for the daughter card. A 150-pin connector consists of 10 IMLA columns, each with 15 pins. The design of the basic IMLA building block is radically different than that of competing connector products in one key area: The AIRMAX VS solution uses air as the dielectric material, both between signal pairs within an IMLA column and between adjacent IMLAs.
THE BEST DIELECTRIC
Traditionally, such leadframe structures have been totally encapsulated in plastic and isolated from one another by interleaving grounding shields, two design aspects which serve to add both cost and weight to the connector. Harnessing air the most efficient dielectric there is for use in a backplane connector is in no way simple or arbitrary. Connector system performance depends on the detailed development of optimized geometries for the IMLA and connector system as a whole to successfully minimize insertion loss.
FCI recognized early that a shift in mindset was required here. Rather than thinking of the IMLA column as a simple electromechanical device, it needs to be considered as a precision-engineered circuit. Field coupling between signal lines within a column and those of an adjacent IMLA are thus governed by empirical research that is designed to identify the optimum combination of contact and air gap widths.
Image 3 shows the nominal outline for a 150-pin connector receptacle. Centerline spacing between IMLAs is 2mm, and each column carries ten signal lines and five ground lines in a differential pair configuration, with differential signal pairs being separated by a ground line. There is a 1.4mm vertical spacing between the lines of adjacent IMLAs. In this example, the connector supports a 25mm daughter card spacing and yields 25 differential pairs or 38 single ended connections per linear cm of PCB. The tight tolerances adhered to in the connector manufacturing process ensure perfect consistency in IMLA geometry, a crucial factor for minimizing any drift in impedance, insertion loss and cross talk across the connector when subject to fast rise times.
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A fundamental point not to be overlooked here is that, due to design constraints, connector systems using interleaved grounding shields either do not include shields on the end columns, or, if they do, these can be subject to a certain amount of misalignment. Either way, this creates an impedance mismatch on the end columns that is difficult to manage.
Image 3 also shows two more crucial design factors used to achieve low cross talk. Two adjacent IMLAs are in fact arranged in "antiphase" (opposite directions), which means, for example, that the first IMLA column begins with signal lines, the second with ground lines, and the third again with signal lines, and so forth. Most important, however, is the vertical offset of 0.1mm between adjacent columns. And this value has not been selected at random. The relationship between these dimensions the vertical spacing within a column and the distance between IMLA centerlines has been carefully engineered by FCI, in order to optimize the effect on signal coupling and measured crosstalk.
Signal Integrity up to 6.25GBits/s
In reference designs, the AirMax VS connector has demonstrated excellent signal integrity at speeds from 2.5 to 6.25GBits/s, allowing users to scale systems to greater than 12GBits/s without requiring a basic platform redesign.
Configured for differential pairs (edge-coupled; five pairs per IMLA; 63 pairs per inch), the connector displays a maximum insertion loss of 0.7dB at speeds up to 6.25GBits/s (2.0dB max. at up to 20GBits/s), and an impedance range of 100(+/-8)Ohms. Near-end multi-active crosstalk is a maximum of 2.5%, while far-end multi-active crosstalk is 3% max. These differential parameters are characterized for a rise time of 55ps (20% to 80% range). In the single-ended signal configuration (95 lines per inch), the connector features a maximum insertion loss of 2.0dB at speeds up to 6.25GBits/s with an average nominal impedance of approximately 60Ohms. Near-end multi-active crosstalk is a maximum of 9.0% and far-end multi-active crosstalk is a maximum of 3%. The single ended parameters are characterized for a rise time of 150ps (20% to 80% range).
The performance figures given above relate to an AirMax VS connector using IMLA columns spaced at 2.0mm. The modular nature of the basic AirMax VS product design has subsequently resulted in a version with 3.0mm IMLA column spacing. Naturally, crosstalk for this variant is correspondingly lower. Initial results show that the differential connector's near-end crosstalk drops to less than 1.75% and its far-end crosstalk to less than 2.25%.
VARYING THE AIR GAP
For designs where connector density is a less critical factor, there are other very significant benefits to be gained from increasing the IMLA spacing. By expanding the column spacing, system designers can reduce the number of layers in their high-speed backpanels and daughter cards in this instance by as much as 50% while simultaneously improving the connector's electrical performance. Increasing the connector's IMLA column spacing from 2 to 3mm enables system designers to route high-speed signals using only two signal layers instead of four. The corresponding reduction in manufacturing complexity and cost is evident.
Also worth noting is that the production of an AirMax VS connector with a different IMLA spacing requires only the front header housing, its back retaining piece and the receptacle housing to be retooled. The architecture provides system designers with almost full custom control over connector density (and thus electrical performance), PCB layer count and signal routing.
Adaptations of the basic AirMax VS connector design have already begun to appear. The first, a standard 120-position connector, uses IMLAs providing four differential pair columns each, thereby supporting a reduced daughter card pitch of 20mm. Under future consideration are a right-angle receptacle for coplanar applications and a BGA receptacle version.
I would be pleased to address any additional questions!
Gerhard Strobl, EXT 37
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