One of the most critical design requirements for military and aerospace user interface devices is the ability to withstand high temperatures. Everything is relative, especially when it comes to what it means to be high-temperature resilient.
With all the ways to define this, many projects will characterize performance by being able to survive extreme temperatures.
- How high of an elevated temperature is high?
- Is the device able to operate during this high-temperature exposure?
- Is the dwell temperature a prolonged period or just a short-term exposure?
Answers to these questions may impact the claimed temperature rating of a particular device and could drive an eventual redesign. Since most military and aerospace keypads are made from a mix of silicone rubber, FR4 laminate, copper, and other miscellaneous metals and hardware, one limiting factor that can drive the maximum temperature rating of a particular device is the adhesives that join everything together.
Temperature Limits - Useful Life
When it comes to selecting the temperature limits of a project allowing engineers to isolate potential adhesive choices, referencing military standards such as MIL-STD-810 is an appropriate first step.
Understanding how these high-reliability devices will be tested, and the way their performance will be scrutinized is vital information that is necessary to consider before finalizing any potential design. Section 501.5 of military standard MIL-STD-810 offers guidance, but ultimately the high-temperature limits of any new project will be determined by the appropriate design authority and end application.
Still, the MIL-STD-810 temperature limits of +85°C or greater can be used to define the worst-case upper-temperature limits that the majority of devices will face during their useful life. Targeting adhesives capable of withstanding +85°C or greater will cover most design scenarios.
Silicone rubber keypad overlays use adhesives to attach to their base circuits.
Temperature Conditions – Useful Life
There are many ways to evaluate a potential adhesive’s performance. One of the first steps is to determine the specific conditions that these designs will face throughout their useful life or the conditions that they will encounter during qualification testing. From this information, specific tests can be developed to characterize performance and determine the best adhesive option for the project. It should be noted that more than one adhesive type can be considered as a viable solution.
Guidance from military and aerospace specifications can shed light on the test methods used to verify high-temperature resilience. One of the most common tests involves temperature cycling exposure that ramps between high- and low-temperature extremes. The resultant thermal expansion and contraction of the unit under test, repeated over and over again, can stress the bond strength of adhesives pushing them to their limits. Dissimilar materials may exhibit different thermal expansion properties, inducing additional stress on the adhesive. Robust adhesive choices are a necessity when mounting two materials, especially ones with high rigidity such as metal backplates and circuit boards.
Some military and aerospace applications will subject keypads to lengthy exposures at extreme high temperatures. Whether the event represents the maximum operating temperature, the maximum storage temperature, or a combination of these, including design margin into the adhesive selection process is strongly recommended. Some adhesives have short-term high-temperature resilience while others can withstand extreme temperatures for long durations such as hours or days. For a maximum exposure temperature of 85°C, choose an adhesive that is rated well above this, in the order of 120°C or higher.
Adhesive Forms and Keypad Assembly Considerations
There are many variables that impact an adhesive’s strength. One of the simplest steps to ensure that the adhesive is performing as intended is to control the surface preparation of the two adjoining materials before applying the adhesive. Engineers must follow the manufacturer’s recommended assembly instructions and surface preparation guidance, no matter the adhesive type. The solvents used to clean the materials, the application methods, and even the type of cloth used to wipe the surfaces can impact an adhesive’s intended performance.
For example, some molded subcomponents may be built with a mold release agent, helping the new molded widget to be removed from its mold. These mold release compounds act as lubricants preventing the molded widget from sticking to its tooling. But mold release agents also will inhibit paints or adhesives from sticking as well. Other contaminants such as cutting oils, grease, or FOD (foreign object debris) can be invisible to the naked eye but have a drastic impact on an adhesive’s performance. A simple cleaning with acetone or isopropyl alcohol may be the best remedy to address a poorly behaving adhesive.
Adhesives come in various forms with a wide variety of curing techniques. Whether it’s a tube of fast bonding super strength instant adhesive, a two-part RTV sealant, or a pressure-sensitive sheet adhesive, understanding the specific assembly methods and curing techniques for the chosen adhesive is especially critical. Assembly jigs, alignment fixtures, and other job aids are valuable tools to help make certain that the adhesive is applied to the appropriate surfaces and given the proper time and pressure to cure. Some adhesive’s cure time can be accelerated by exposure to elevated temperatures giving manufacturers the option to speed up cycle times by placing assemblies inside a thermal chamber.
High-temperature-rated adhesives are widely available to meet whatever design challenges exist. Many of these adhesives come in various form factors, tube sizes, and can be ordered in numerous dispensing techniques. These options give designers plenty of choices to identify the ideal adhesive for their needs. But with so many choices available, and dozens of adhesive manufacturers to choose from, finding the best option may feel like searching for a needle in a haystack.
When in doubt, start with adhesives that are familiar and available. If these readily available options do not meet whatever design requirement exists, reaching out to the adhesive manufacturer for recommendations is a worthwhile next step.
Still, many questions about best practices and how to facilitate assembly may remain, adding risk and cost to a rapidly evolving development project. This is when full-service manufacturers like Epec can offer design consultation, manufacturing support, and qualification testing providing expertise throughout the entire development life cycle.