Most applications that use ULTEMTM film require some degree of processing to enhance the functionality of the film. Often they are coated with adhesives, pigmented cover coats, or laminated with another material.
Heat Sealing and Thermal Lamination
According to tests conducted by the GE Advanced Materials technology department, ULTEMTM film shows adhesive property to a variety of materials, including itself or other ULTEMTM films, polyimide films, and LCP films. ULTEMTM film also demonstrates good adhesion strength to copper foils. However, because of the CTE mismatch between copper and ULTEMTM film, the potential use of this method is limited. The curling effect is elimated at thicker gauges of film, making ULTEMTM film a potential candidate for circuit board laminates.
Lamination conditions vary according to the selected grade of ULTEMTM film. ULTEMTM film must be laminated at least 40 to 45 ºC over the glass transition temperature of the film. Lamination pressures of 50 to 100 pounds per square inch are needed to achive the required bond strength. Lamination time can vary, depending on the thickness of the film.
These conditions have been used to bond ULTEMTM film to copper and to circuitized polyimide film without adhesives. The Global Research Center demonstrated the capability to encapsulate 35-µm-high traces by allowing the ULTEMTM film to “flow” around the traces.
Utilizing a metal sputtering process, GE Global Research Center has demonstrated very high metal adhesion strength with ULTEMTM 1000B and 5000B when compared to other materials. A design of experiments was conducted on ULTEMTM films to examine the effects of process conditions on metal adhesion strength. The results showed that the metal adhesion results for ULTEMTM films are not as sensitive to certain conditions.
There are several selection criteria used to choose adhesive. These include the method of application, mechanical properties, and temperature and environment requirements. ULTEMTM film is currently used in high temperature applications where acrylic-based pressuresensitive adhesives are utilized. Pretreatment of the film to promote adhesion, such as corona treatment, may be necessary to achieve acceptable adhesive anchorage to the film. The surface energy of ULTEMTM film has been measured to be approximately 35 to 40 dynes, depending on grade and the side of the film measured. The most important criteria used to choose the right adhesive system is the effect of the solvent on the ULTEMTM film. The use of methylene chloride and MEK-based solvent systems should be generally avoided. A list of recommended adhesive systems for all ULTEMTM films is currently under development.
Laser and Mechanical Drilling
ULTEMTM film can also be successfully laser drilled. However, the standard process conditions that are used for polyimide films will cause reflow of the material. A lower power setting has to be used in order to avoid the reflow. Processing parameters for the ULTEMTM film by using an ESI Laser system are included in the following table.
Unlike polyimide film, ULTEMTM film may be thermoformed in a variety of shapes because of the thermoplastic nature of the film. Applications such as speaker cones require the film to be formed into a specific shape.
Recommended thermoforming temperatures for ULTEMTM film are between 260 and 290ºC. The required forming temperature will be dependent on the grade of film selected to meet the application requirements. Typically the film temperature must be elevated greater than 40ºC above the glass transition temperature (Tg) of the material. Tool temperatures are suggested to be between 150 and 170ºC.
ULTEMTM film should also be dried, prior to thermoforming, to reduce the possibility of moisture bubbling during processing. The film should be dried 2 to 4 hours between 150 and 160ºC. After the film is dried, it will be useable for 2 to 4 hours (depending on relative humidity) before it will need to be re-dried.
Via Integrity Testing
Each ULTEMTM film test circuit contained over 1000 vias. Circuits were tested in a thermal cycle from -40 to 125 ºC for 1,000 hours. Testing of approximately 120,000 vias showed no change in contact resistance.