Industry-standard connectors don’t always get the job done. A very public failure of a proven connector spurred a team of designers to engineer a flexible and dependable new solution using pogo pins. Here’s how they did it.
By Nobukazu Kato, ACES Electronics, and Sam Martin, ACES North America
When a major OEM brand of mobile phones exhibited a cradle device for their smartphone and tablet at the Consumer Electronics Show a few years ago, the interconnect failed on the first day of the show. Although the standard micro USB and micro HDMI connectors were widely used in the device market, the cradle device was designed for use across multiple smartphone and tablet product lines, so its structure could not be designed to a specific body. The failure occurred when removing the devices from the cradle. A new connection was needed.
The first step in the redesign process identified that the angle of insertion and extraction had to be controlled to protect the connector from damage. A wrapping structure would have restricted the direction and prevented damage problems but would have also limited the versatility of the design to a single, specific device. Another effective countermeasure could have been to design a guide that was slightly larger than the connector, but that is contrary to the small, thin connector size demanded by end devices like smartphones and tablets. The solution required a design that would prevent breakage yet deliver performance in a small, thin, and elegant end product.
Designers examined various concepts, including a structure in which the cradle side connector rotated and followed up when unmating, as well as an option with a shutter, but the most user-friendly connector seemed to be a so-called brand-mate-type connector with a magnetic port. However, this large connector wasn’t suitable for portable equipment and, in a miniaturized version, its contacts were too weak. To combine user-friendliness and miniaturization, the concept of a connector with a shutter that could protect the contacts was developed.
This concept, called the Mag-Fit, is classified as a surface type connection, rather than an insertion type, and was adopted in an industry standard connector. In this type, the plug side has only a contact pad. This is a very simple configuration that maintains a small size and can be produced at a low cost. The opposite/receptacle connector has a mating contact and a structure that is pressed against the pad by a spring. Through the use of a magnet, it is possible to guide the connector to the proper position with the magnetic force alone. Disconnection can be achieved with a relatively small rotational force or a slight twisting action.
The first version of the Mag-Fit concept sufficiently protected the connector and was user-friendly, but there was a limit to high-speed signal transmission. Although it was able to cope with the USB 2.0 specification that was widely used at that time, it had a problem handling signals faster than 5Gb/s for USB 3.0 devices. The connector’s signal contact had a spring structure, and in many cases it became too complex a structure to obtain sufficient memory and force. When there were two or more turns of the spring, coil characteristics were generated and it became inappropriate for high-speed transmission; although, with just two turns, it was possible for it to correspond to a certain speed. The number of turns of a spring can handle is determined by the required spring characteristics and physical space available. If there is a limit on the width, the number of turns will increase. In portable consumer electronics devices, both width and height are often limited. So, the original Mag-Fit required a new concept that was independent of the structure of the return spring and still allowed for high-speed transmission if a spring had two or more turns. The idea is the separation of mechanics and electricity.
For the high-speed signal route, the team selected thin coaxial and flexible printed circuit (FPC) connectors. For the surface type, they selected simple, telescoping springs, which are also called spring-loaded contacts or pogo pins.
Advantages and Disadvantages of Pogo Pins
A simple spring can be strong enough to withstand external forces if it is sufficiently large, but it can’t be miniaturized to fit into a small space because it can easily deform. The pogo pin, on the other hand, is a very traditional product that was developed as a test probe application. In test probe applications, pogo pins are set to be pressed from directly above by a jig. Today, pogo pins are employed in charging terminals for wearable watches, connections with a table keyboard, and other consumer electronics devices.
However, pogo pins can still cause some problems. To counteract them, a wall-type device can be designed around the pin to prevent side damage. However, then they are of little to no use for surface-type connections, which must enable easy attachment and detachment. If there is no freedom of degree in the attachment and detachment direction, the pogo pin can only be accepted in a straight direction, which isn’t always useful. Alternately, the Mag-Fit connector can be detached at any angle of 180°, depending on the structure protecting the contact, and can withstand an insertion and removal lifetime of 100,000 cycles or more.
For portable electronic devices, small size and light weight are the basic needs. As the functionality of equipment increases, the number of pins required of the external connection also tends to increase, as does the need to further reduce the pitch. To reduce the pitch of pogo pins, it is necessary to narrow the pin diameter, which is often already as small as 2.5mm. To further reduce pitch, one row of pogo pins can be rearranged into a two-row arrangement. But, in the two-row configuration, the rotation center and the contact arrangement line are displaced when detached and rotating and, in this state, a force in the diagonal direction is always applied to the pogo pin, which increases the possibility of creating a contact failure. A 0.8mm-pitch is the smallest solution currently available in the Mag-Fit line, but since the position of the pad and the contact can be suppressed, there is still a possibility of further reducing the pitch size.
A pogo pin consists of three parts: a core comprised of two external fixtures and a coil spring. Machining of the outer part requires cutting and it is necessary to apply gold plating to the entire outer part, which means that manufacturing costs are high. In addition, when using multiple pins, it is necessary to add parts to align those pins.
The Mag-Fit configuration allows signals to be passed through a return spring and contacts to be mass-produced by ordinary connector sequential press processing. These connectors consist of a protector and a main body (insulator), so their manufacturing costs are lower than pogo pin manufacturing costs. However, Mag-Fit connectors do require custom development and tooling costs. As such, pogo pins may have a cheaper total cost in the case of small quantity production and low pin-count numbers.
Advances in electronic components continue to move with tremendous momentum. By separating the return spring from the signal line as described above, the Mag-Fit is expected to achieve transmission speed of 20Gb/s, which a normal pogo pin could not currently reach.
Mag-Fit Connector Design
In the basic Mag-Fit, the contact (i.e., the return spring) is covered with the protector, the pad side touches the contact, the contact is deformed in the elastic formation, and the protector is also moved at the same time. The contact pushes the pad up in the mating configuration. When the mating pad is released, the contact pushes the protector up. The pushed-up portion is set for the contact in the protector. Contact height is set to protrude up to 0.2mm from the protector. In order to stabilize the contact height, the Mag-Fit is preloaded. That is, in the initial state, the return spring pushes the protector up.
The basic high-speed transmission Mag-Fit structure follows the basic type but the return spring and the transmission route are separated. The transmission route has the miniature contact and thin coaxial or FPC and they are soldered to each other.
Selection Criteria for Insert Type and Surface Type
When deciding whether to choose an insertion type or a surface type, it is necessary to consider the importance of ease of attachment and detachment of the connector. Insertion type connector solutions are preferable for application in which detachment is only required once to a few times. Once the fitting is completed, the insertion type can maintain the fitted state by itself. Although, in applications where an external force, such as tension, is applied, holding strength can be further increased by adding a mechanical lock. Surface-type connectors are more beneficial in module-to-module applications.
The second checkpoint is the available space. In the insertion type, both receptacle and plug are overlapped and fitted together, and both need the same height. In the surface type, only one side has a pad, so it is possible to reduce the height. When there is a need for low profile on the main device side, a surface type is selected. On the other hand, another side may require space for a return spring, so a corresponding size is required.
As consumer devices continue to increase in power while decreasing in size, standard connectors may not serve every need. Creativity and flexibility will usher in the next wave of connector concepts that, like the Mag-Safe, build on strong existing designs — but make them better.