Mezzanine connectors are among the most useful in the connector arsenal to solve packaging problems. They come in many forms. This article focuses on the IEEE 1386 Mezzanine connector, one of the simplest and most-used mezzanine solutions. This simple connector had a strong influence in the direction and growth of electronic packaging.
In 2001, the IEEE published a specification for an industry-standard connector to facilitate the use of custom IO cards within the many Eurocard-based standard systems like VME bus and Compact PCI in use at the time. The IEEE 1386 standard called out a small card, 75-mm wide and 150-mm long, positioned 8.2 mm above the host card. An IO connector on the bottom side of the mezzanine card can fit through a standard opening on the card faceplate. The mezzanine connector has 64 SMT contacts in two rows with a simple spring beam mating with a flat blade. The IEEE 1386 further defined the front panel opening so that many cards from different manufacturers would all fit the same space and mating connectors. This enabled a variety of IO cards to be used in the front, using the standard mezzanine mating interface. The standard further defined how to use up to four of the mezzanine connectors to create even more capable systems. Although PCI (Peripheral Computer Interface Standard) was the primary protocol used, any protocol that fits can be used.
The effect of having a standard interface for mezzanine cards was to enable a new community of IO mezzanine card suppliers, many of which are small entrepreneurial companies. This standard interface also meant that the host board suppliers no longer had to make different cards for every IO possibility. This allowed the host board supplier to achieve an economy of scale that helped the overall VME and CPCI standards to prosper, grow, and evolve over decades.
A number of features made this connector an especially good solution for this application. It has the ability to absorb a number of chassis related tolerances with a very simple, but flexible (literally) connector interface. The mating part of the 1386 connectors consists of flat metal contacts captured on each side of a plastic blade. The mating connector accepts the plastic blade between single beam springs. The mating system is quite resilient and strong, thanks to the plastic housing features combined with gull wing SMT (Surface Mount Technology) tails on the host and mezzanine cards. SMT gull wing leads have the wonderful characteristic that, in light connectors like this one, the connector will float on the liquid solder during reflow, centering the connector to the array of SMT pads. If the connector body is not fixed to the board, it can lean left or right to gain mating tolerance with the mating part. This centering effect allowed multiple connectors to be successfully mated to form a very reliable mating system with up to 272 pins. The elegant simplicity of this mating system enabled this interconnect to be used for decades as systems, protocols, and speeds evolved. This principle continues to be used today in a variety of micro miniature connectors that have made laptops, tablets, and cell phones possible.
A postscript to this story
The competition for the IEEE 1386 mezzanine connector standard was a surprise consortium of vendors offering compression solutions that could mate with a standard configuration of gold pads on the mezzanine card and host board. The consortium included ATT, IBM, and a spring pin supplier. This threat forced the major connector vendors including Tyco, Molex, and FCI to rally behind the connector that was chosen. A compression solution needs forces in the range of 50 pounds per connector to assure reliable connection with no wipe. Dave Rios from Molex borrowed a 50-lb barbell from a local gym that was used to illustrate the magnitude of this force. At the point in the PowerPoint presentation that this fact was presented, the barbell was dropped on a folding table, causing water to spray out of all the glasses around the table. When the laughter died down, the point had been made that conventional mating connectors with mating wipe provide a simple, reliable solution that is hard to beat.
The creation of an industry standard can be both entertaining and educational. I encourage you to become involved in the process if you have the opportunity. You will be wiser for it.