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Table 1. Flex Circuit Update
December 31, 1969 |Estimated reading time: 1 minute
"Polyimide" is a generic chemical descriptor that applies to thermoplastic films as well as thermosetting resins. Polyimide films are marketed for FPC use by several manufacturers, including duPont, Kaneka, Ube Chemical. Properties can vary significantly between manufacturers and film types. Table 1. sets forth properties of two polyimide films and Type 1 (higher temperature) LCP.
Taking these in order:1. Tensile strength is extremely important for maintenance of dimensional stability. Production yield, particularly in high density circuitry, requires good stability. Polyimide is the clear winner here.
2. Elongation is undesirable in itself but higher elongation suggests greater tear strength. Tearing as a result of poorly defined outlines is a common failure in FPC.
3. See (2)
4. Moisture absorption complicates high-temperature processing and is undesirable. There are forms of polyimide film which absorb 1% or less. LCP's extremely low absorption means less pre-process bakeout is needed and leads to stable dielectric properties with varying environmental humidity.
5. Some authors emphasize humidity effects on dimensional stability but the real-world importance is quite small. For example, a 20% variation in humidity means a change in polyimide FPC of about 0.00025"/inch; 0.0025" in 10 inches.
6. Moisture transmission and its corollary oxygen transmission are important in the stability of foil-to-film adhesion with extended high temperature exposure. LCP offers excellent barrier properties which means a properly prepared "adhesiveless" LCP composite should have excellent long-term thermal durability.
7. Dielectric Constant and Dissipation Factor. These are the touted advantages of LCP over polyimide and the difference is not very great. Impedance is proportional to the square root of the dielectric constant which means that the difference is even smaller. The primary advantage of LCP is its uniformity with frequency and humidity variations. Whether this slight improvement is worth the significant (roughly 25% currently) increase in FPC material cost is a case-by-case question.
Table originally published in Rui Yang's "Liquid crystal polymers: A flex circuit substrate option," Advanced Packaging March 2002.