1.1　Introduction

Due to the excellent combination of thermal， mechanical， and electrical properties， aromatic polyimide films have been extensively used in many high-tech fields such as electrical insulation， electronic packaging， and aerospace industries. The typical applications include the tapes for winding of magnet wires， the base films for flexible print circuits （FPCs）， tape automated bonding （TAB） carrier tape， magnetic recording tape， and thermal-controlled shielding film for satellites， etc.

Polyimide film， first developed and commercialized in the late of 1960s by Dupont ［1］， was prepared by casting a solution of polyimide precursor-polyamic acid （PAA） on to the surface of the substrate followed by thermal-baking to give a self-supporting gelled film. The gelled film was then converted by thermal baking into polyimide film （Fig. 1.1）. The conversion can be accomplished thermally by baking at temperatures in excess of 300 °C （thermal imidization method） or chemically ［2］， by using of a mixture of an anhydride and an aromatic base， such as acetic anhydride and pyridine （chemical imidization method）. Moreover， the chemical conversion also needs a final heating treatment to remove the organic volatiles and to ensure the complete imidization.

FIGURE 1.1　Production process of polyimide films

The PAA solution was usually prepared by the polycondensation of aromatic dianhydrides and aromatic diamines in dipolar aprotic solvent at low temperatures （Fig. 1.2）， in which the important aromatic dianhydrides include PMDA， BPDA， BTDA， ODPA， etc.， and aromatic diamines include ODA， PDA， BAPP， etc. The solvents include dimethylacetamide （DMAc）， dimethylformamide （DMF）， and N-methyl-2- pyrrolidone （NMP）， etc.

FIGURE 1.2　Chemistry of poly（amic acid）and polyimides

The Dupont Kapton films were produced derived from PMDA and ODA or/and PDA using DMAc as solvent， while Ube Upilex films were based on BPDA and ODA or PDA. Ube Industries have developed an alternative casting method to produce BPDA-based polyimide films ［3-5］. Because the BPDA-based polyimide was first disclosed in US patent filing by Dupont ［6］， Ube had to find an alternative solvent such as phenolic instead of DMAc to cast the PAA-（BPDA/PDA） solution for the production of polyimide films. It was found that the PAA derived from BPDA and ODA is soluble in p-chlorophenol， which can be cast from this solvent on to a support surface， and then heated to >300 °C to complete the thermal imidization and drive out all of the high-boiling solvent. The PI-（BPDA/ODA） film was commercialized as Upilex R. The PI-（BPDA/PDA） film， another Ube product （Upilex S）， was also produced in a mixed solvent containing p-chlorophenol as the main solvent using a similar procedure to that of Kapton films.

In 1968， it was found that stretching orientations have obvious effects on the mechanical properties of polyimide films. At that time， the researchers at Dupont developed the chemical imidization method to produce polyimide films. A chemical conversion agent （a mixture of acetic anhydride and β-picoline） was first added to the chilled PAA solution in DMAc， which was then cast on to a heated drum to give a gelled film which was a partly imidized polymer containing large quantities of solvent. After being peeled off from the drum surface， the self-supported gelled film was then one-way or two-way stretched and simultaneously imidized under heating to produce a fully imidized polymer （polyimide） film. Experimental results indicated that the orientation of polyimide films produced by mono-or bi-axial stretching exhibited improved tensile strength and tensile modulus and significantly reduced water uptake and dissipation factor ［7］. For instance， after being mono-axially oriented， the BPDA-based PAA gelled film was converted thermally to the polyimide film. The resulting polyimide film was brittle transverse to the draw direction， but major changes of properties in the draw direction were observed. At draw ratios of 1.75×， the maximum mechanical properties were achieved with tensile strength of 140 MPa and tensile modulus of 7.0 GPa， respectively. It was also found that polyimide films derived from PMDA and m-phenylenediamine （MPD）， para-phenylenediamine （PDA） or benzidine showed very poor hydrolytic stability， however the bridged diamines such as ODA give polyimide films with very good hydrolytic stability. Hence， the chemical structures of polyimide films have obvious influences on the mechanical， thermal， and chemical properties.

For many years， Kapton H films have been accepted as the standard in commercial polyimide films. The Upilex S films derived BPDA and PDA exhibited higher tensile strength and modulus as well as very low coefficient of thermal expansion （CTE）. However， the elongation at breakage of Upilex S was reduced about 50% compared with Kapton H. In addition， Upilex R films derived BPDA and ODA exhibited comparable combined properties with Kapton H films.

In recent years， polyimide films have been extensively used in microelectronic manufacturing and packaging industries， especially as the metal interconnect board substrates for FPC， TAB， chip on film （COF）， chip scale packaging， ball grid array， etc. With the miniaturization and thin filming in electrical and electronic parts， line thinning of electric circuits is rapidly progressing. Dimensional changes of polyimide films during long-term servicing may cause accidents such as disconnection and short-circuit in the thinner lined circuit structures. Hence， highly accurate dimensional stability and high elastic modulus as well as low moisture absorption are required for polyimide films used for microelectronic applications.

In this chapter， the chemistry， preparation， and applications of the advanced polyimide films will be discussed.