Mezzanine connectors have been an important enabling technology for 1U packaging. Several electronic functions can conveniently fit into a 1U box. If you recall from previous posts, the vertical dimension in industry standard racks is based on “Rack Units” or multiples of 1.75 in. Somewhere along the way, the Rack Unit (RU) got shortened to just “U.”
For this reason, the 1U or 1.75-in. high box designed to be stackable in 19-in. rack architecture has become one of the most popular form factors in electronics, especially in telecommunication and computer systems that commonly use the standard 19-in. rack.
In order to maximize the amount of electronic content in the 1U form factor, mezzanine connectors are used in most 1U boxes. Basically, if one row of IO connectors is on the primary card, a second level of IO connectors can be placed on a mezzanine card positioned above the primary card with the IO connectors protruding from the front of the box.
An alternative to mezzanine cards is to use double deck connector blocks of four, six, or eight connectors or cages in one assembly. These are most often found in boxes that support high volumes of industry standard interconnects, notably Ethernet where stacked connector blocks are commonly available for modular jacks and cages for pluggable interconnects like SFP (Small Form Pluggable) or QSFP (Quad Small Form Pluggable.) If you are a major router builder, you will use truckloads of these standard connectors.
There is a world of companies, however, that make boxes in the 1U form factor who want to be able to customize the front panel layout in many different ways. This enables them to offer many different models of their system with the front panel IO specialized for each. For this kind of application, mezzanines are the answer. Stacked connectors do struggle sometimes to handle the highest speed interconnects. With a mezzanine solution, the chip can be right behind the IO connector, potentially giving a significant signal integrity advantage over stacked connectors.
If you choose the right mezzanine connector, much is possible, as seen in the following images.
These mezzanine cards can be full chassis width, or just a smaller mezzanine card providing just the right amount and type of IO necessary for the application. This approach allows the equipment manufacturer to use just the right building blocks for each of his system configurations. He can accommodate a variety of IO configuration; for example: Ethernet, Infiniband, or other legacy IO connectors to accommodate legacy customers as well as forward looking ones, using the same primary card. When the primary card is updated, you can still use all of the existing IO cards if you keep the same mezzanine connector.
These mezzanine connectors can be simple two-row configurations like IEEE 1386, but more likely, they will be array connectors, often with hundreds of pins to accommodate whatever function might be put on the IO mezzanine cards. The connector industry now supplies a large array of solutions. Following are some suggestions on how to choose the right one for your application:
• The distance between the primary board and the mezzanine board is probably the best place to start. You may want to choose the board on top approach to optimize cooling and provide room for tall components like heat sinks.
• Next parameter is probably the number of differential pairs that will be needed. Keep in mind not only the current requirements, but also potential future uses of the mezzanine card and connector. For example, you may want to have more pins than you need today to allow for future applications that are not yet defined
• By the way, also pay attention to air flow in your box. You need to make sure you are not creating a wall that blocks airflow. The mezzanine connector should be oriented so that maximum airflow is possible (put the narrow end of the connector in the direction of the airflow.)
• Think about the speed you may need now, and in the future. You will want to reuse IO mezzanine boards for multiple generations of host cards. You definitely do not want to re-architect the system, just to accommodate a higher speed IO because you did not anticipate that earlier. You will find that a high-speed mezzanine does not cost that much more than a low speed version. Err on the higher speed side for future applications
• Consider the routing pattern in the PCB (printed circuit board). In general terms, the more pairs within a column in the connector, the more layers that will be needed to route out. Sometimes you can route both ways, but for mezzanine connectors, it is typical to route one way from the connector to connect to the chip. For this reason, array mezzanine connectors typically are available in 2-pair and 4-pair variants, requiring one layer per pair to route out.
• Lastly, think about what termination you will want. SMT is common with BGA (Ball Grid Array) connectors. The process has been finely tuned at the major contract manufacturers so that high yields are common with SMT BGA connectors with low heights.
• In high stack height applications, press fit mezzanine connectors can have an advantage. They are rugged and can tolerate more abuse than a comparable BGA version, especially when the mezzanine board will experience forces that might break a solder joint. In the 1U form factor, it is common to place the boards as far apart as possible in the box, with a mezzanine between. This works out to be a distance of 38-42 mm depending on metal work. One typical tall mezzanine application is in 1U blade servers where the high speed signals travel from the CPU (Central Processor Unit) up to the mezzanine card where a chip converts the protocol to one that is best for the backplane (Ethernet, for example.)
I think you will find that judicious use of mezzanine connectors can enable you to create a box that can be simple today, allow lots of enhancement capabilities, and can also be future-proof to ensure long product life cycles. By the way, mezzanines can also preventa a lot of “Woops” events by avoiding a complete redesign by just adding a mezzanine card with the upgrade.
Mezzanine connectors are your friends.