The present invention relates to a process for producing polyester films, in particular PET films, which, due to their shrinkage properties, are particularly suitable for producing SMD-technology (surface mount device-technology) capacitors. Particularly for capacitors used in SMD technology, there is a need for films with low thickness and resistance to temperature variation at the temperatures arising during soldering. Low thickness and resistance to temperature variation provides advantages in terms of space utilization in the capacitor and of the soldering procedure.
PEN films and PPS films are currently used for SMD-technology film capacitors. PEN films and PPS films have a markedly higher melting point than PET films (about 255xc2x0 C.). The melting point of PEN is about 265xc2x0 C. and that of PPS is about 285xc2x0 C. However, a decisive disadvantage is the high costs for these PEN films and PPS films.
For reasons of cost there has been a trend for some years in the electronics industry away from wired components to components with SMD capabilities. This trend also applies to the capacitor sector. SMD capacitors are mounted and soldered directly on the PCB (printed circuit board). While specific components making up SMD capacitors do not strongly differ from traditional leaded capacitors, the main difference is that a SMD capacitor has no leads. Additionally, there are usually minor differences in the contact areas, but this is not an essential difference. The manufacture of SMD capacitors is also similar to the production of traditional leaded capacitors. The film is metallized and slit. The slit rolls are wound to capacitors, schooped, and heat treated. Finally, the metal contacts are sprayed on the schoop layer or dipped in molten metal. No leads are attached to the metal end contacts. In addition to winding capacitors, it is also possible to produce stacked SMD capacitors. For stacked capacitors, the slit rolls are wound on big wheels, schooped, and then provided with a metal contact in a manner similar to that for wound capacitors.
A very high percentage of supplies to this market is represented by ceramic SMD capacitors, since capacitors made from other materials are damaged by the temperatures occurring during the SMD soldering process. (Peak temperatures are typically above 225xc2x0 C. during the widely used reflow soldering process.) Film capacitors are a particular case in point. The abovementioned peak temperatures are in a range in which PET, one of the traditional polymers for film capacitors, begins to melt. In the years past, capacitor producers and film producers have therefore concentrated on polymers with higher melting points, such as PEN and PPS. Other methods have been to use SMD film capacitors produced from PET which are protected by an additional casing from exposure to high temperatures, or SMD capacitors made from PET with a limitation on peak temperatures to hardly more than 200xc2x0 C. Both methods have markedly restricted application. As described in WO 98/13414 and WO 98/13415, it has been found, surprisingly, that SMD capacitors having particularly low transverse shrinkage can be produced without the abovementioned restrictions from PET films. It was found here that, in contrast to the transverse shrinkage, there must still be significant shrinkage in the longitudinal direction in order to ensure the consolidation needed to obtain an effective capacitor.
Surprisingly, it has now been found that, contrary to the disclosures of WO 98/13414 and WO 98/13415, effective SMD capacitors can also be produced from films whose longitudinal shrinkage is only minimal. Films with particularly low shrinkage values are generally particularly suitable for SMD capacitors since they give particularly low loss of capacitance during soldering.
It is known from EP-A-0 402 861 that a 75 xcexcm film with low shrinkage values in machine direction (MD) and transverse direction (TD) can be produced by relaxing the film in the transverse direction during setting and also permitting longitudinal relaxation. According to EP-A-0 402 861, this longitudinal relaxation is apparently only possible if, after leaving the setting frame, the film is wound up at a speed lower than the speed in the frame. This is possible with the relatively thick films (75 xcexcm) described in that publication, but impossible at film thicknesses  less than 10 xcexcm, since uncontrolled changes in dimensions occur during the free relaxation between the frame and wind-up, resulting in a uniformity of profile which is unacceptable for capacitor films (thickness variation in MD and TD).
Processes as described, for example, in U.S. Pat. No. 4,042,569, in which the longitudinally/transversely stretched film is then relaxed in-line or off-line between rollers in the MD give films with low shrinkage values but also give the difficulties described above, and are also completely uneconomic due to the addition of process steps and reduced process reliability.
WO 88/10188 describes an apparatus and a process for producing polymer films which can be stretched simultaneously longitudinally and transversely with the aid of the apparatus described. Capacitor films produced by a process of this type are described in U.S. Pat. No. 5,429,785. No information is given in this publication about the shrinkage performance of the films. However, it is unlikely that the shrinkage values are particularly low, since the films which it describes do not undergo relaxation.
It was therefore the object of the present invention to provide films with suitable shrinkage values, and in particular a process suitable for producing films of this type.