As an airline passenger is waiting in the terminal for his flight, he decides to plug in his tablet to charge the battery. As soon as he connects the AC/DC adapter to the tablet he hears a snap and he sees an error message on the tablet’s screen communicating that there is a power failure. Upon inspection he discovers that the connector has jarred loose and thus his battery is not charging his tablet.

These problems are not uncommon for some of the mobile products that are being sold in the market place. Seeing the individual’s frustrations from meager designs can have a dampening effect in terms of revenue and a company’s reputation. To overcome some of the quality issues in connectivity the design engineer needs to consider many characteristics of the connector. Characteristics such as electrical, mechanical, reliability, and environmental will require a detailed examination and definition. This article will explore what the considerations are on how to specify or design a connector system that is appropriate for certain applications. Then the focus will shift to devise a general guideline in which the designers can utilize to insure that a connector system can be a reliable one.

Connector Structure

Connectors have been in the electronic industry for quite a long time. With the advent of more miniaturization of electronics their complexities have grown by leaps and bounds. Connectors today have to support many high-speed electronic circuits in a system, whereby at the same time, they have to utilize as little real-estate possible.

In electronics, connectors are electro-mechanical devices that transfer and join electrical signals and power to other parts of the electronic system. Connectors can range in size from just a few millimeters in low power applications to feet in high power generation. One can surmise that the number of circuits and the power usage through the connector will determine a connector’s physical size.

Some connector systems can be static where they are only plugged in once during the life of the product to connector systems, others are extremely dynamic where they are inserted and extracted many times during its life. An example of a static connector system is one used for video inside the display of a tablet. A dynamic connector system would be one for plugging in power for charging in a smartphone. Connectors are constructed with certain materials that can withstand different environmental and manufacturing conditions to enhance the connector’s performance and reliability.

In general, a connector system is usually constructed from two pieces: a male and female, or a plug and a socket. There are different types of connector systems, the primary ones are electronic board-to-board , wire-to-board, and wire-to-wire connector systems where some of these are in Surface Mount Technology (SMT). There are others, such samples are below:

Fig 1: SMT Board-to-Board Connector

Fig 2: Wire-to-Board Connector

Fig 3.: Wire-to-Wire Connector

Depending on the connector, a plug or a socket, are structured utilizing several components; the housing and the electrical contacts are the two fundamental components. The other components that make up a connector are the support accessories such as EMI shielding and grounding, a polarization feature, a bracket, or a strain relief feature. I’ll discuss further on a strain relief feature below.

Fig 4: Connector Housing with Contacts

Connector Components

The connector housing is usually made of some specific polymer and/or metallic housing. The plastic housings are made of high temperature thermo-plastic to withstand manufacturing and other environmental conditions such as in a SMT (Surface Mount Technology) reflow process. Some examples of high temperature thermoplastics are Liquid Crystal Polymer (LCP), Polyphenylene Sulfide (PPS), or a High-Temperature Nylon, and there are others. SMT connectors should be able to withstand temperatures up to 260c for at least a minute without degradation.

As for the contacts for both the socket and plug, they are usually made of certain base metals such as phosphor bronze, copper alloys, or perhaps, tin with intermediate plating deposited such as nickel or tin. Outer surface plating or finishes for contacts consists of inert metals for a certain thickness such as of gold or palladium for low current applications. For high current or power applications usually tin or silver are utilized. These contacts are usually formed or stamped into different geometries to provide sufficient contact normal force for a long-term reliable connection along with insertion and extraction cycle life depending on the application. Additionally, the contact geometries will determine the electrical bandwidth of the connector and how fast electrical signals can operate without seeing any type of degradation.

One of the serious errors that engineers often make is when they inadvertently mix metal platings on contacts. For example, a socket may contain gold as its outer surface finish while the plug may consist of tin. When the plug and socket are mated, the different platings will react slowly due to the galvanic properties of the metals. After a period of time, along with humidity and the environment, galvanic corrosion occurs and leads to open circuit connection in the contacts, thus a connection failure. A critical connector commandment is never mix contact interface platings in a mated connector system or you will compromise the reliability of the design.

Mounting Connectors
Depending on the type of connectors, they can be mounted on a PCB utilizing Plated Thru-hole (PTH) or SMT or a mix of the two mounting technologies including a press fit type. Other types of connectors can be mounted on brackets with wires attached or even a flexible circuit . Connectors can be mounted vertically, horizontally or even sideways on the edge of the PCB.

Connectors that are mounted on PCBs usually have certain features with them to enhance mechanical reliability and/or solderability of the connector system. Such features could be locating and retention leads and/or strain relief feature.

Strain Relief

This article begins with connector snapping off a tablet, a problem which I experienced, is an example of a strain relief issue. Strain relief is a feature that is built into the connector to relieve mechanical stress on any component on the connector system. Without strain relief a catastrophic failure may occur in the connector system over time. Strain relief can come in many forms and are too long to list in this article, however, I will discuss one example below.

Take the case above, a connector snapping off or where the solder joints were weakened, eventually cracking and failing. The connector that failed was a power connector that essentially was used to charge the battery for the tablet’s mobility. The SMT connector was a two-circuit type with dc power and ground. Both contacts of the connecter were part of the leads that were soldered on the pads of the PCB. The solder joints of the connector were subjected to a high number of mechanical stresses since the number of insertions and extractions that the connector was exposed to were going to be very high.

This failure was compounded by the fact that the grounding contact of the connector was sizable such that every time the power cable was inserted into the connector it caused the ground contact to deflect quite a bit. This deflection of the ground pin caused the solder joint to be subjected to abnormal dynamic forces thus causing the joint to eventually crack then fail (See Fig. 5).

Fig. 5: Power Connector Solder Repair

To overcome some of the abnormal dynamic forces on the solder joint, a strain relief feature or features are necessary. Depending on the number of insertions and extractions perhaps a redundant PTH lead on the ground SMT contact would assist to overcome some of the abnormal dynamic forces on the solder joint. Other features such as a strain relief bracket attached to the connector that attaches to the housing of the tablet may assist in relieving the stress on the solder joints. There are other improvements a design/component engineer can explore.

How to Specify and Design-in a Robust Connector System

With so many variables that can affect a connector design, I put together a list of tasks that a designer/connector engineer should investigate before a connector system can be specified and designed into a particular product. The list of tasks are as follows:

1. Determine product specifications along with any regulatory requirements whereby this new connector would operate in.

2. Determine what configuration or type of connector to utilize such as Board-to-Board, a wire-to-board, or a wire-to-wire connector system or other connector types.

3. Determine the environment that the connector is supposed to operate, such as in a commercial, industrial or aerospace environment.

4. Determine the basic requirements of the connector system such as number of circuits, method of attachment (SMT or PTH or a panel mount), and so forth.

5. Determine if the connector will be subjected to high number of insertions and extractions such as in an external I/O or power connector, such that a strain relief feature may be specified.

6. Determine detailed electrical requirements:

• Determine bandwidth or maximum speed of the signals that the connector system will be subjected to.
•  Power ratings of the individual contacts and the entire connector system. I.E. voltage and current ratings. Utilize any derating criteria for power.
• Determine shielding requirement for EMI/RFI especially for I/O connector systems.

7. Determine detailed mechanical requirements.

• Determine connector insertion and extraction forces.

• Determine the contact normal forces to determine what surface platings to utilize.

• Determine physical size and PCB footprint of the connector (if applicable).

• Determine strain relief features for the connector or cable (if applicable).

8. Determine detailed material requirements for the connector system.

• Determine plastic housing materials. Plastic will have to meet SMT or Wave solder manufacturing process requirements (if applicable). Housing may have to meet flammability ratings if the connector is being used for power, especially AC power. UL and other agency certifications maybe required.

• Determine the contact materia, the base metal and its platings. Outer surface plating or finishes should be tin for high current applications. For low current external applications ideally inert metals are utilized such as gold or palladium unless the connector utilizes a “gas tight” contact interface then a tin-to-tin contact interface can be utilized.

9. Once a connector is specified determine its mating connector. It should meet the criteria you outlined for the original connector.

10. With the worldwide green initiatives, ensure all materials meet Restrictions of Hazardous Materials (RoHS) certifications or provisions for R.E.A.C.H.

11. Last but not least, DO NOT MIX METALS IN THE CONTACT INTERFACE AREA when mating connectors. It will go a long way in insuring long-term connector reliability.

The list above is a general one as the Design/Component Engineer needs to treat the connector system as if it is a new one for their applications. Additional tasks that could be completed if necessary, especially for new connector designs are stress simulations, electrical simulations including utilizing a network analyzer for S-parameter transfer function, thermal profiling at peak rated power, testing and verifying the connector design including cycle life.