Polyamide films for flexible printed circuit boards

1. A process for the production of a dimensionally stable polymer film, comprising steps of:

(i) melt-processing a polymer moulding composition at a temperature above a melting temperature (Tm) to form a polymer melt thereof, wherein the moulding composition comprises at least 80 wt. % relative to the total weight of the polymer composition of a semi-crystalline semi-aromatic polyamide having a melting temperature (Tm) of at least 290° C. as measured according to ASTM D3418-03 by DSC with a heating rate of 10° C. and a glass transition temperature (Tg) measured according to ASTM E 1356-91 by DSC with a heating rate of 10° C., wherein the semi-crystalline semi-aromatic polyamide comprises repeat units derived from dicarboxylic acids and diamines, the dicarboxylic acids consisting of 70-100 mole % terephthalic acid, and other repeat units in a total amount in the range of 0-5 mole %, relative to the total molar amount of repeat units derived from dicarboxylic acids and diamines;

(ii) extruding the polymer melt obtained in step (i) into a film and thereafter immediately casting and cooling the film to a temperature below the glass transition temperature (Tg) to form a cast film;

(iii) biaxially stretching the cast film obtained in step (ii) at a temperature in a range of Tg-5° C up to and including Tg+10° C.; and

(iv) heat-setting the biaxially stretched film obtained in step (iii) at a temperature between the glass transition temperature and the melting temperature to obtain a dimensionally stable polymer film having an average thermal expansion coefficient (TEC) in a temperature range of 20° C.-Tg, measured in a plane according to ASTM D696-08, of at most 40 ppm/K.

2. The process according to claim 1 , wherein the semi-crystalline semi-aromatic polyamide in the polymer moulding composition has an intrinsic melting enthalpy of at least 25 J/g.

3. The process according to claim 1 , wherein the process is conducted applying the following conditions:

during the melt processing according to step (i), the polymer composition is kept above the melting temperature (Tm) of the semi-crystalline semi-aromatic polyamide for a residence time of at most 1 minute;

during the casting of the film according to step (ii), the film is cast onto a surface with a surface temperature of at least 25° C. below the glass transition temperature (Tg);

during the biaxial stretching according to step (iii), the cast film is biaxially stretched by a factor of at least 2×2; and

during the heat-setting according to step (iv), the biaxially stretched film is kept at a temperature in the range from Tm-80° C. up to and including Tm-10° C.

4. The process according to claim 3 , wherein the surface temperature of the surface onto which the film is cast according to step (ii) is at least 50° C. below the glass transition temperature (Tg).

5. The process according to claim 3 , wherein the biaxially stretched film is kept at a temperature during heat-setting according to step (iv) in the range from Tm-60° C. up to and including Tm-25° C.

6. The process according to claim 1 , wherein the polymer film has a coefficient of humidity expansion (CHE) measured in plane at a relative humidity of 50%, of at most 140 ppm/% RH.

7. The process according to claim 1 , wherein the polymer film has a melting enthalpy of at least 15 J/g within a temperature range of 270-350° C., wherein the melting enthalpy is measured according to ASTM D3418-03by DSC in a first heating run with a heating rate of 10° C/min.