With the wearable electronics market growing by the day, the electrical connectors used in these devices are more critical than ever. While gold is the best and preferred metal to use, its high cost has led designers to explore other metals as plating materials. Tin has proven to be a suitable replacement, due to its low cost and ability to withstand environmental factors that wearables commonly come in contact with, including sweat, high humidity, high and low temperatures, corrosive gasses and various types of debris. That being said, tin connectors are more susceptible to failure, and it’s important that electronic designers and engineers understand their primary failure mechanisms to help prevent issues from occurring.
By following the “Tin Commandments” below and using a Physics of Failure approach to testing, designers can help mitigate the potential for tin electrical connector failure risk – while also improving overall wearable design and reliability, reducing costly testing and speeding time to market. Before reviewing the Tin Commandments, however, it’s important to know why tin connectors can experience failure.
Common Tin Electrical Connector Failure Mechanisms
When looking at the cause behind a majority of wearable electronic failures, often the first problem that occurs is a loss of contact between two conductors. Conductor failure is directly related to poor connector performance, and there are several reasons connectors fail. For starters, connectors are frequently overlooked as critical components in wearable design, due to the fact that they are not as sophisticated as the integrated circuits and complex technologies used to package them. Likewise, most wearables have thousands of opportunities for contact failure — any of which could shut the system down. For example, a wearable product can have connectors used in antennas, sensors, power/grounding, battery connections, board-to-board connection, flexible printed circuit connection, wire-to-board connection and removable memory cards.
An initial lack of communication and understanding between product designers and connector manufacturers can also lead to electrical connector failure in wearable devices. Because there are dozens of companies producing electrical connectors, subtle product quality differences can occur, which can impact reliability. A connector’s contact force, the hardness of the metal finish, the oxide layer, adsorbed gasses, and any physical contaminants are all dependent on production, and can impact the asperities, or contact points, of a reliable connection. These variances require connector engineers to have a firm grasp of contact physics in order to help ensure that predicted reliability is achieved.
The composition of tin itself can also pose reliability risks. It’s important to remember that tin is a soft metal that can grow a hard and brittle oxide. At a microscopic level, it is comparable to ice on mud. Once contact is made, the oxide is easily broken and fresh tin oozes through. Then, once contact is broken, the exposed tin re-oxidizes and must be broken through again. If new contact locations are frequently made due to displacement, the tin oxide can build up and eventually create a debris field that can prevent adequate contact from being made. This process is called fretting, and typically occurs on tin-plated connectors that are used in a vibrating environment, or in an application with many insertion/withdrawal cycles. Fretting can also occur with frequent temperature cycling that can lead to micromotion of the contacts.
Follow the Tin Commandments
To help prevent connector system failure from occurring when using tin-plated contacts, there are 10 best practices that should be followed. A deep understanding of these guidelines, officially known as the Tin Commandments, is needed:
- Contacts should be mechanically stable in the mated condition
- Tin-plated contacts need at least 100 grams normal contact force
- Tin-plated contacts need lubrication
- Tin plating is not recommended for continuous service at temperatures above 100° C
- The electrical performance of contacts is not strongly affected by the choice of bright tight tin, matte tin, or tin-lead alloy plating
- Electroplated-in coatings should be at least 100 microinches thick
- Mating tin-plated contacts to gold-plated contacts is not recommended
- Sliding or wiping action during contact engagement is recommended with tin-plated contacts
- Tin-plated contacts must not be used to make or break current
- Tin-plated contacts can be used under dry-circuit or low-level conditions
As outlined above, there are many factors that engineers must be aware of when working with tin electrical connectors. However, by following the Tin Commandments and designing in reliability during concepting and design using a Physics of Failure approach, it’s possible to help effectively predict and solve for common failure mechanisms that could inhibit the reliability of a wearable device. Using reliability analysis tools, like Sherlock Automated Design Analysis™ Software, can help pinpoint areas of design weakness and potential failure sites using simulated testing, saving valuable time and money by enabling engineers to mitigate issues before physical prototyping occurs.
To learn more about optimizing wearable tech design and helping to mitigate device failure, watch our free on-demand webinar, Wearables That Work: Getting it Right the First Time.