Resin system choices

The specification sheets for many multifunctional epoxies (FR-5, for example) show that they may have glass transition temperatures in the 170–180°C range, but may not be fully characterised above 150–165°C. Uncompounded polyimide materials, on the other hand, have values of T_g above 250°C, and may safely be used.

Much depends on the exact requirement: “worst-case local surface temperatures during operation reaching 170°C” may mean anything from “at most 5 minutes during the whole of life” to “substantial proportions of each day”. Also, the temperature quoted may be more or less conservatively estimated. For example, has account been taken of what would happen if the filter on the cooling fan became clogged?

Cost is generally a less significant issue for a military application than for a commercial client, but polyimide will certainly be the more expensive option, compared with epoxy, not least because processing is less routine and the requirements more stringent. There may be some issues over poorer demonstrated flame retardancy compared to epoxies.

With an eight-layer board, one would need to be aware of the potential for excessive resin flow, and the need for careful control of heating rate during lamination to achieve consistency of flow and thickness. Where available, low-flow prepregs are desirable.

Other fabrication issues are that:

Prepreg must be thoroughly dried before lamination, preferably by an extended vacuum bake, to avoid significant deterioration of the quality of inner layer bonding
The needle-like structure of conventional black oxide does not survive internal shear forces caused by thermal stress, leading to inner layer bond failure, so the thinner ‘brown oxide’ treatment is required
High temperature cures are necessary to achieve optimum material properties
Only plasma or permanganate desmear/etchback processes are effective on polyimide.

Although not mentioned in the question, during your reading you may have come across BT resin materials, with intermediate values of glass transition temperature, good humidity performance (a parameter often attractive to military users) and lower permittivity and loss. Certainly. at this comparatively low temperature, one would consider BT as a possible material, and might also consider polyimide blends, with a lower T_g, but cheaper and easier to fabricate.

If nothing else, we hope that this question will have made the point that you need to consult widely when choosing the most cost-effective and best-performing laminate for your application. The range of options, combined with substantial variations in claimed performance and differences in processes and quality problems, are all reasons why many conservative end-users (such as military customers) choose to remain with well-proven materials, despite their limitations and higher costs.

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