Power Integrity in Interconnect Design

The need for high-current power interconnect solutions inincreasingly smaller spaces continues to rise rapidly. As demand grows for morepower in smaller packages, solving the power equation on new architectures andsystem platforms can pose electrical and mechanical design challenges for OEMsystem and power engineers charged with specifying interconnect components thatensure both signal and power integrity.

Unlike signal connectors, which continue to get smaller at highertransmission speeds, power connectors require a specific amount of conductivematerial to carry specific amounts of current or amperage. There are no specialsecrets to design that will allow smaller power contacts to carry more current- as power needs increase, so does the amount of space needed for higherampacity interconnects.

Design Density and PowerHandling

Even though new systemdesigns often require more power to travel across a limited amount of space,several factors still affect the density of a design and how much power it canactually handle. A clear understanding of each of these elements is critical tosuccessfully designing power integrity and safety into the system - and willhelp streamline the overall design process. These key factors include:

Balancing Space and Power. First, it is necessary to determine how muchspace is required for a power interconnect versus how much available space hasbeen allotted in the finished product design. While saving space is a highpriority for most OEMs, the height, width and length of the connector, andparticularly its copper content, will directly affect the achievable currentdensity. System architects always want to get more power in the same space,which can pose a challenge for connector manufacturers.

However, leading globalconnector manufacturers continue to develop new and innovative designs that usehigher conductivity materials and utilize space more creatively to improvepower delivery and electrical performance without expanding space requirements.For example, in some cases, a lower profile connector may be preferable tomaximize air flow for cooling. In other cases, a taller connector offeringimproved contact performance may be the right solution to properly handle theamount of current generated in less card edge space. What's important isstriking the optimal balance of power and its resultant thermal effects in thePCB with the spatial design requirements to ensure the end product's safety andperformance.

Thermal Management. Thermal issues caused by contact or constrictionresistance and inefficient air flow are always a concern and should becarefully considered early on in the design process. PCB copper content is oneelement of this. Too little copper can restrict current flow, causingconstriction resistance. Appropriate copper trace sizes decreases bulk resistance,allowing for cooler temperatures and less loss. Otherwise that heat could be"sinked" to the connector interface, increasing reliability concerns. Powersupply manufacturers are very creative in supplementing PCB structures withfeatures to alleviate thermal and constriction concerns.

In addition, as systems arepackaged into smaller boxes with more components, it is critical to ensureproper management of air flow around connectors positioned at the intersectionpoint (such as between a power supply and server). Ample air flow around andthrough the connector helps cool the power contact, allowing for more currentand/or an increased margin of safety. At the same time, connectors aresometimes located at key points and block airflow. The process of coolingconnectors, however, is often not high on the list of priorities for designerswhen considering air flow.

With operational safety inmind, the designer needs to consider the entire system and its powerarchitecture to understand what potential may exist - from end to end - forconstriction areas and voltage drops that affect thermal and electricalperformance. Typically, a maximum 30mV drop defines the threshold of thermalstability for a power contact. Once this threshold is crossed, the probabilityof thermal instability increases significantly.

World-class connectorproducers are working with their customers to develop improved powerinterconnect solutions for safe, reliable operation in smaller spaces at highertemperatures over long product life. New designs incorporate new alloys andmolding resins, plating, improved contact technology - all to increase currentdensity without sacrificing safety and reliability.

Risk Mitigation. Connector manufacturers havetraditionally based current ratings on their products' electrical performanceon testing under ideal circumstances. These published ratings, while accuratefor what they measure, rarely tell the whole story because they fail to takeinto account the various conditions and interactions that will affect theenvironment in which the connector actually will be operating.

As a result, a common practice among OEMs has been to derate theconnectors in order to build in a thermal safety margin over product ratingspublished in the connector manufacturers' literature. Many use a simpleapproach, testing a smaller circuit count along with a longer one and chartinga range of T rise versus current showing lower current carrying capability asthe circuit count increases. In addition, some customers assign yet anotherarbitrary percentage, so if a connector supplier submitted a product rated at,say, 100 amps, the user would automatically derate it by 30 percent to ensure abuilt-in safety margin against the possibility of overheating.

Today's leading connector providers understand this and will workclosely with OEMs and their design team to match their connector selection tothe specific application, based on scientific testing and performance analysisunder real-world application conditions.

To provide accurate ratings, top manufacturers conduct extensivetesting and predictive modeling, such as Joule Heating FEA (finite elementanalysis) and CFD software (computational fluid dynamics) with inputspertaining to the connector and PCB geometry and material properties, current,contact resistances (actual test data) and air flow. In this way, they canestimate the performance of each of their interconnect products and providereliable counsel to customers as to which of their products would be the best match for the application requirements. It is not practical to simulate and/or test everypossible environment but these models and analyses can help guide designers tosmarter choices in a shorter amount of time. This is important in thefast-paced design cycles required in the electronics industry.

Power Integrity Planning YieldsBetter Results

With compact size,transmission speed, signal and power integrity being paramount in electronicdevice technology - the benefits of proactive power integrity engineeringsimply cannot be overstated. Increasing demand for computing power is drivingthe demand for more raw power. Meanwhile, product design cycles continue toshrink, giving power engineers less time to make critical decisions.

Gaining a clear understanding of all the requirements early inthe design phase, before specifying interconnect components, can help ensurethe right decisions and avoid costly missteps. Most important, high-qualitypower integrity engineering enables OEMs and their product designers tomaximize the performance, reliability and safety of their products.

Ken Stead is global new product development manager for powerproducts at Molex Inc.