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In the 1st century AD, there was significant debate among Jewish theologians as to whether resurrection was possible. PCB designers in the early 21st century have had a similar concern about future availability of 85NT nonwoven aramid laminate and prepreg. The stakes may be somewhat less critical, depending on your theological bent, but the future of a wide variety of programs designed around the properties of (dare I mention the name?) Thermount® have been hanging in the balance. What was Thermount exactly and why was there such a furor when DuPont announced its premature demise? And where do we stand now with the redevelopment of a nonwoven aramid product?
The original 85NT product was based on high tensile-strength para-aramid fiber with a meta-aramid fibrid binder. The para-aramid fibers have a high tensile modulus (fiber modulus of about 19 mpsi) and a negative linear coefficient of thermal expansion with a CTE of about -4 ppm/oC meaning that the material shrank when heated, and the fibrid held the fibers together in a uniform matrix. Produced in a high-end papermaking process, the resulting substrate was a yellow aramid paper material that produced laminates with a unique and useful set of properties. For a polyimide aramid composite the benefits look like this:
- Tg of pure polyimide (250oC)
- Low in-plane (X,Y) expansion of 7-9 ppm/°C
- Excellent dimensional stability and enhanced registration
- Decomposition temperature of 426°C, compared with 300-360°C for typical high-performance epoxies, offering outstanding high-temperature life
- Polymeric reinforcement results in PCBs typically 25% lighter in weight than conventional glass-reinforced laminates
- Laser ablatable for high-speed formation of microvias as small as 25μm in clad laminates as thin as 0.002”
- Electrical and mechanical properties meeting the requirements of IPC-4101/53
- Compatible with lead-free processing
- RoHS/WEEE compliant
The earliest military adoption of aramid reinforced laminates was in the guidance system for the Tomahawk cruise missile, a significant success in terms of test results (thermal cycled better than the prior woven aramid technology) and field performance. The combat-proven Tomahawk gave the then relatively new substrate a kick-start. I still have the jacket patch that the Tomahawk PWB development staff gave me. And there was some real pride in achievement in the ongoing field success of that program.
Later uses of 85NT materials included a wide variety of military and commercial avionics. The first-generation avionics for the Boeing 777 were designed on this platform, and it replaced heavy copper-invar-copper (CIC) in a number of older designs. In addition to these, the Iridium telecommunications satellite constellation used 85NT for a number of its PWBs.
In July of 2006 the imminent demise of the DuPont Thermount product was announced to its customer base. In November 2006, a letter from Ralph Hutton, president of DuPont Teijin Advanced Papers, said, “Due to a dramatic reduction in demand for Thermount reinforcement for consumer applications in Japan, and a corresponding reduction in global demand for the foreseeable future, we are unable to continue accepting orders for Thermount reinforcement of any kind beyond December 31, 2006.”
Working with senior officials at major military and commercial OEMs, Arlon procured and stocked sufficient NWA product to sustain existing programs during the period required to find a suitable alternative source.
Well, nonwoven aramid is back. Although I have tried to keep this column non-commercial, Arlon is, as best I know, the only company that has committed the resources and time to work with potential suppliers to develop a polyimide NWA that meets all the performance criteria of the original 85NT product.
In fact, what we have now is a drop-in replacement that will match both in process and properties of the original. That was not easy to accomplish and several very well qualified vendors tried and failed to meet the exacting requirements for the substrate. The project took longer than had originally been expected, though stocks of the old material have held up well enough to protect ongoing programs for the entire duration of the development program.
As you can see from the following microphotographs, the registration and hole quality of the new aramid reinforcement is essentially identical to that of the original DuPont product. The12-layer test coupons were made in the same press and lamination cycle using parameters developed for the original 85NT product.
For those who have been frustrated trying to find alternative technologies for their critical SMT boards, I think I can say with a great degree of confidence that Arlon’s 85NT is back and that you won’t be disappointed with its processability or performance.
Until next time, this is the Material Witness wishing you happy fabrication and a great fall 2015.
Chet Guiles is a consultant for Arlon Electronic Materials.
