micro-SIM Card Connectors – How low can they go?

The 3FF micro-SIM card (UICC) is the 3rd generation SIM (Subscriber Identification Module) card since this form of card first appeared twenty years ago. It has dimensions of 12mm x 15mm x 0.76mm and the card has the same contact arrangement as its predecessors, i.e. the standard (credit-card size) and mini-SIM (25mm x 15mm) cards. The micro-SIM is backward compatible with larger SIM holders and SIM readers (via additional plastic cut-out surrounds).

Since the micro-SIM was developed specifically for the purpose of fitting into devices otherwise too small for a mini-SIM card, there is clearly a lot of pressure on connector makers to minimize footprint and height of micro-SIM sockets. In this blog I’d like to briefly analyze some of the factors contributing to connector height. I will focus only on sockets that fully envelop the card within the connector itself, (e.g. Guide & Holder, Push-Pull, Push-Push and Hinged type). Also I will not speak of PCB “mid-mount” designs, which would clearly allow further height advantage gains.

Micro-SIM connectors grip the card between a top cover and the main body of the connector, i.e. the plastic housing which contains the contacts. Allow me to start my analysis of connector height from the top down.

1)      The top cover, usually a metal shell, may be reduced to 0.1mm and still retain the stiffness needed for overall structural rigidity. Of course, material selection is important and careful design will then be needed to ensure the shell is well-connected to the body of the connector and can take the stresses and strains of card mis-insertion and other tests. Going thinner than 0.1mm with conventional metals seems to run too high a risk, not just for connector strength, but also in terms of manufacturing and indeed metal strip supply. Tolerances on a 0.1mm strip are typically 0.005 to 0.010mm.

2)      The 0.76mm (nominal) card thickness is clearly a given and the mouth of the connector cannot be much narrower than 0.90mm (nominal) to allow it accept the maximum thickness cards. Anything less will inevitably lead to card insertion difficulties as the SIM card, (unlike the microSD card) has no lead-in chamfer to aid card insertion.

3)      The main body of the connector for ultra-low-profile designs is invariably an overmolded structure to help minimize the dimensional stack. The area of the connector and the other features the design attempts to incorporate will determine this thickness. Super high-flowing LCP (Liquid Crystal Polymer) materials can be used to create extremely thin walls, in places close to 0.10mm. However, some thicker sections are needed to maintain material flow and also to provide space for features to attach the shell and to hold the contacts in place. With contact material typically at 0.10mm (to obtain reliable forces over these tiny deflections) this means that a total thickness below 0.30mm is hard to achieve for the main body of the connector. Thinner is possible, but it will result in trade-offs in other areas of the design, possibly in efforts to meet a tight coplanarity specification. Orientation of contact beams and position of contact tip at full deflection will also determine how much of the PCB area beneath the connector can be used for circuitry.

4)      Finally, many customers like to see a stand-off between the main connector body and the solder tails to improve solderability. This can typically be up to 0.05mm but is often specified at a min. of 0.0mm to at least ensure that no part of the connector body is below the level of the solder tails.

Adding the key dimensions in the stack above shows that 1.35mm nominal height (0.10 + 0.90 + 0.30 + 0.05) is typically as low as one can go with these designs. Of course, designers can play with 20 or 30 microns, particularly in the main body of the connector or in the stand-off area, but it will not significantly change the height. When overall tolerances are taken into account, we typically reach maximum socket heights of 1.40 to 1.50mm, similar to some ultra-low-profile microSD sockets on the market today.

To date, Molex has only manufactured custom micro SIM designs but will soon be releasing general market products. For more information about Molex card connectors, visit: http://www.molex.com/product/pccard/simmob.html