Lessons Learned from Power Interconnect Design

The MicroTCA architecture gaining momentum in the telecom and commercial industries was perhaps a turning point in electronics design for the telecom industry. Previously, many telecom designs were highly customized and protected by proprietary rights. In contrast, the interconnect applications within MicroTCA are standardized, which may be a sign that open architectures — akin to those prevalent in computer industry — are on the rise.

Although a standardized connector design may seem basic, the lessons learned from the MicroTCA power-connector initiative are relevant to designers of power supplies and other industrial and commercial power electronics systems. The MicroTCA connector demanded high power density, hot plug capability, high reliability and low cost. The design process undertaken in addressing these requirements through contact and housing design provides a template for how to approach connector design in other applications.

MicroTCA's Origins

The PICMG organization originally developed the MicroTCA standard to meet the needs of the telecommunications industry. However, along the way, the standard caught the attention the military, aerospace, industrial, process control and medical industries. The attraction of this standard is that it enables the use of the latest processors, high-speed chips and interfaces, while minimizing space and power consumption to deliver high availability and maintainability. Because this is an industry standard, customers are able to leverage off-the-shelf solutions to build their value proposition at lower cost and shorter time to market.

MicroTCA power interconnect applications basically started out as a power-supply module and backplane configuration, and eventually embodied the Advanced Mezzanine Card (AdvancedMC) and backplane arrangement that is being designed into the market today. This method is an offshoot of the larger Advanced Telecommunication Computing Architecture (ATCA).

More or less, the application specified out as a board-to-board power application. The eventual winning design was a two-piece board-to-board power connector designed to meet the high reliability needs found in telecom, aerospace, medical and others areas. Some of the specific requirements of the new connector included the typical cost and economy issues, but the interconnect also had to meet the density requirement of 24 hot-pluggable 12-A power contacts and 72 signal contacts within 63.5 mm.

The MicroTCA standard proposed using AdvancedMC, an existing standard for ATCA mezzanine cards, as the main blades (or boards) on their own compact backplane and chassis (Fig. 1). The standard needed to develop a power entry module (power supply) to deliver power to the backplane, along with the necessary control lines to communicate status between the blades and the power supply. The standards body surveyed the industry for an integrated power and signal connector that could meet the power and contact densities and determined that a solution did not exist.

In conventional applications of delivering more power to a backplane, the length of the power connector is increased to add more contacts. Fig. 2 shows a comparison of a conventional power and signal connector (foreground) versus the size of the MicroTCA pct board (approximately 74 mm). Obviously, this connector can't fit in this application.

The power connector in Fig. 2 shows the size necessary to deliver the required MicroTCA power contacts, but it only has 24 signal contacts versus the 72 signal contacts required for MicroTCA. This shows the inability of a conventional connector to meet the size and power/signal density of MicroTCA.

In the MicroTCA application the power/signal connector must be approximately 63.5 mm to fit within this space. The initial power-delivery requirement for MicroTCA was to have capacity in excess of 1000 W, delivering 32 discrete power outputs, at various voltage and current levels, along with the already-mentioned control/status lines. Additional requirements for this connector were high reliability (a must in telecom), hot plugging and unplugging, and low cost.

With the requirements not fully articulated and an initial evaluation of existing power interconnects, the industry group determined that a new power interconnect design was necessary. In calling out the design objectives, it was determined that the new connector should:

• Use an industry accepted contact technology with established reliability

• Use low-cost manufacturing processes and materials

• Offer compliance with all existing and anticipated global environmental requirements, such as RoHS, for example.

• Be compatible with customers' various manufacturing processes (solder and press fit).

Connector Overview

The power connector used in MicroTCA is a two-piece connector designed to interconnect the MicroTCA power-supply daughter card (right-angle connector) to the backplane (vertical connector) (Fig. 3). The connector incorporates 24 power contacts, each capable of handling 14.5 A located in two rows. There are 72 signal pins for low-current power supply, as well as communicating status between the power module and the line cards. In addition, the connector housing has integrated alignment features to ensure proper alignment prior to power or signal contact engagement.

The complete integrated solution of power, signal and alignment post functions has been built in a single piece housing occupying only approximately 60 mm × 19 mm of pc-board space for the right angle plug and approximately 61.1 mm × 27.2 mm of pc-board space for the vertical receptacle. An additional requirement is to provide four levels of sequential mating of the connector.

The stamped and formed power contacts are based on the industry-recognized Universal Power Module (UPM) contacts used throughout the telecommunications industry. With this blade and receptacle design, the contact mating points are well defined and controlled. The connector has a very high current density, exceeding 14.5 A with a 30°C temperature rise (see Fig. 4 and the table). It also has easy current distribution through the use of multiple compliant tails to pass up to 14.5 A per contact to the pc board (resulting in less than 4 A per plated through hole).

To address the hot mate and unmate requirements, the contact design uses a sacrificial extended tip-blade design. So, during hot mate and unmate cycles the arc, which occurs at the first point of electrical contact, is focused to one side of the blade tip. Upon further contact engagement, the additional mating points engage without any arc because initial contact has been established. The multiple points of contact on the gold-plated contact-surface finish result in less fretting wear and maintain the performance attributes of the contact interface.

With the extended tip-blade design, the contacts are suitable for up to 250 mating cycles. The nature of the stamped and formed contacts makes it easy to incorporate two levels of sequential mating via two different contact mating lengths.

With the MicroTCA, the design decision of whether to use a stamped and formed contact versus a round-screw, machined-power contact was based on several factors including higher performance, better reliability, less variation and lower piece part cost. Stamping and forming involves the process of progressively stamping a flat strip of copper alloy stock. The contacts are then processed on the continuous strip through plating and finishing steps until they are inserted into the molded connector housing.

This high-volume process is extremely consistent and repeatable, resulting in a high degree of uniformity. In a high-power application, a flat conductor will deliver more current at the same temperature rise than a round conductor of the same cross sectional area.

In the report “3.2 FCC Applications in the NASA TM X-53975, Flat Conductor Cable Design, Manufacture and Installation,” dated Jan. 9, 1970, it is shown that fully loaded flat conductors can carry up to 155% higher current loads than equal-sized round wire. Although flat wire would cost more than round wire, in the case of a precision-formed electrical contact, the flat stamped and formed contact can be produced today more easily than solid round contacts, thus providing better current handling with lower cost.

Since stamped and formed conductors are handled in a continuous strip, it is easier to plate specific portions of the contact with more accurately controlled plating thickness than when plating loose piece screw machined parts, resulting in higher reliability. The power and signal contacts on this connector are post plated with 30 micro inches of gold to ensure long term reliability in demanding environments.

The continuous strip process also allows for additional surface area treatments to be added as required by Bellcore/Telcordia specifications. The final reason the stamped and formed contact design was chosen is that it is lower cost than screw machining, given appropriate production volumes.

The 72 signal contacts called out in the connector use the Z-PACK 2-mm HM contact interface, which is used throughout the telecommunications industry, designed to IEC 6176-4-101 standards and rated for 0.625 A per contact. High reliability in the signal pins is achieved by using a multiple-contact point design for the signal contacts as well as 30-microinch gold plating. Two more levels of sequential mating are incorporated in the signal contact to provide the customer's requirement for four levels of sequential mating.

Lessons Learned

The design process of this MicroTCA connector represents a wave of change that is occurring within the power industry. In the same way that the development of standards-based architectures for telecom is a change in the way to get the job done, the implementation of power systems is undergoing tremendous change. The migration of digital circuitry into the once analog realm of power supplies has driven the industry to lower cost and smaller physical size. The power connectors also must follow this trend. As a result, it is important to find ways to achieve new levels of density and integration in the connector while still optimizing cost.

To achieve the overall density in the MicroTCA connector, it was necessary to integrate power, signal and alignment features into a single housing. In the past, this might have been done with standalone guide pins and alignment hardware mounted separately to the printed circuit board, as well as a separate power connector and a standalone signal connector.

To design for this new level of integration, a careful analysis was required to minimize the housing design for thinner walls and features, while still preserving the robust features required for the integrated mechanical alignment posts and receptacles.

Detailed modeling, using PROEngineer and Moldflow, was required to optimize all aspects of the housing and the candidate plastics that would be required to mold these diverse features, while still preserving the functions of allowing the contacts to achieve the thermal and speed functions as well as meeting cost targets.

One of the critical design elements of the MicroTCA connector that enabled the contact density was the addition of a second row of power contacts. This design decision required use of thermal modeling to consider the temperature rise of the contacts on each row (Fig. 5). Tyco Electronics uses both proprietary and commercially available finite element analysis (FEA) modeling software, such as ANSYS, to evaluate the thermal performance. The FEA data is used as a guide. Actual test results, using a test board design that mimics the intended applications, are needed to validate the temperature rise based on the specific environmental conditions (printed circuit board design and copper content, ambient temperature and airflow). This, in turn, caused design considerations with regard to material choices. Depending on the results of the thermal modeling and the predicted temperature rise, one might consider the use of higher conductivity metals to reduce the Trise. However this comes with the trade-offs that most higher conductivity metals add cost to the connector design.

A further issue to consider in terms of overall contact density is the interface to the printed circuit board. Again, modeling was used to determine the level of contact density that would still allow the routing of the circuit board traces without requiring additional layers of printed circuit board. In this case, a printed circuit board modeling tool was used such as Autocad or Mentor Graphics to layout the printed circuit and determine what trace and layer density would be required.

A power connector developed only a few years after the ATCA power connector, reveals that even higher levels of power density are possible using the same design techniques applied in the MicroTCA connector (Fig. 6).

In about the same space that previously housed eight 16-A power contacts for ATCA, we have now integrated 16 14.5-A contacts, an increase of approximately 81%. This illustrates the impact of stamped and formed rectangular contacts, careful spacing of contacts and wall thickness, and optimization of footprint on power density. As these trends continue, analysis of new materials to realize thinner walls and higher conductivities, and the use of next-generation EDA design tools, will continue in the power-connector industry.

The lessons learned in the development of the MicroTCA connector can be applied to all applications in the power industry. Given the customers' requirements in terms of volume (market size), one must consider the choices of integrated functionality, manufacturing technology, overall contact density and environmental requirements. Numerous tools are available to the connector designer that allow these requirements to be incorporated in a balanced manner, so that the overall objective is achieved in an economical manner.

As power applications evolve, these trends will continue. It will become increasingly important to incorporate these design tools, trade-offs and material choices in the design process for power connectors. It is basically the process of “rightsizing the connector” to match the trends in power-supply and distribution-system design. Where extra design margin was formerly tolerated (or even desired), it is critical that the power-connector designer strive to optimize the design to meet the customer's requirements, while continuing an uncompromising drive for reliability.

Table. MicroTCA connector specifications.ElectricalCurrent Rating (tested in accordance with IEC 60512-5-2 Signal contacts 0.625 A each Power contacts 14.5 A each Dielectric withstanding voltage 80 V rms EnvironmentalMaximum continuous operating temperature 100°C Durability 250 cycles