Revised: June, 2008
covers the handling and processing procedures that are suitable for small batch usage of these products. Large-scale production usage of the products requires techniques that are suitable for automatic handling and high speed processing, but require too much setup to be practical for a small scale or experimental application.
covers bonding process development for flip chip applications.
TP-1/-2/-3 are advanced ACF adhesives that have electrically/thermally conductive nickel fibers running through the Z-axis thickness of a thin thermoplastic film adhesive. The nickel fibers have an electrically insulative coating that limits electrical conductivity to the Z-axis direction and allows 11 micron pitch with high X-Y plane electrical resistance. The adhesive is produced as a 50-200 micron (0.002-0.008 inch) thick film. It is produced to a specific fiber volume. The second dash number indicates the fiber volume (e.g. TP-2-40 has a 40% fiber volume).
The NTP-1/-2/-3 series use the same thermoplastic resins as the TP series, however the nickel fibers do not have a special extra electrical insulation coating and, thus, they can be used down to a 200 micron pitch limit with high X-Y plane electrical resistance.
The fibers have a small tilt that allows proper consolidation of the adhesive during bonding. This is typically an angle of 5 degrees off the Z-axis that runs vertically through the thickness of the adhesive film. In order to facilitate alignment of circuits to the adhesive the direction of the tilt is shown on the release paper. There is also a small 45 degree angle cut into the top right corner with the fibers tilting away from the corner along the right edge. The observed tilt angle for a specific lot of film adhesive is shown after the lot number (e.g. Lot 14-175, 5° has an observed angle of 5 degrees).
If the tilt direction indicators are lost the best way to determine the tilt direction is to examine it under a microscope. At 400X with through transmission light, a distinct shadow can be seen attached to the fiber ends in the opposite direction from the tilt. For a reflected light microscope examination, slightly trim two edges that are 90 degrees to each other by placing a single sided razor blade over the adhesive film near the edge and taping the back of the razor to cut a clean edge. When both of these edges are examined at 100X one edge will show the tilt direction with fibers having a very distinct tilt. The other will show fibers straight up and down.
For small quantities the adhesive film typically comes between release papers clamped together in a package. The sheets may be stacked on top of each other.
TP/NTP adhesive films are storable in a controlled environment at room temperature.
Like all thin adhesives, the adhesive may warp and pucker if stored free standing. It should be stored between two sheets of release paper with a light dead weight on top or returned to the same clamping box it was shipped in with the original bubble wrap plies top and bottom. Carefully putting the adhesive between two sheets of release paper and putting a dead weight on top normally eliminates any puckering or warping.
Read the MSDS before handling. Powder free gloves should be worn to avoid contamination of the bond line.
The adhesive is subject to distortion and splitting if handled improperly. The adhesive can be easily cut by tapping on a one sided razor placed over it. This should be done on a clean surface such as glass, plastic or release paper. Like all adhesive films it will pick up dirt from any surface it touches.
The surfaces to be bonded should be clean enough to meet normal electronic assembly standards. TP/NTP will accommodate the surface flatness of most common electronic assemblies. Thicker versions of the film may be required for large bonding areas or surfaces with poor flatness.
TP/NTP has been designed to accommodate local intrusions (from flex circuit traces, die pads, etc.) up to 15% of the Z-axis thickness without compromising Z-axis conductivity. For example, 100 micron (0.004 inch) thick film can be successfully bonded to a 15 micron thick trace (with a flat substrate on the other side) or to a 10 micron thick trace on one side and a 5 micron pad on the other side. However these intrusions cause the nickel fibers to tilt over further so alignment compensation is needed for small pad connections. 100 micron (0.004 inch) thick TP/NTP with 5 degree fiber tilt has a 9 micron offset from top to bottom - that increases to ~40 microns when connecting flat pads and consolidating the bond line by 10% during bonding. This offset increases to 65 microns when connecting a 15 micron thick trace (height above the substrate) to a flat (same level as substrate) pad.
The bonding process is a function of temperature and pressure and is affected to a lesser degree by heatup rate, surface area, surface flatness and substrate material. Because temperature is usually the easiest variable to measure and control, we recommend starting your bonding experiments at a nominal point (such as 125-130°C, 50 psi for TP-1/NTP-1; 150-155°C, 50 psi for TP-2/NTP-2; and 185-190°C, 50 psi for TP-3/NTP-3) and varying the temperature at a constant pressure until suitable bonds are produced. All these bonding temperatures are the actual resin temperature and must be verified with a thermocouple attached to the film during bonding.
We recommend using two pieces of glass microscope slides to verify that full resin wet-out is produced at the temperature/pressure bonding conditions prior to experimenting with real test packages. Higher temperature produces higher consolidation of the adhesive and would be the direction to go if incomplete bonding is observed. The thermoplastic resin will start to degrade during bonding if heated above 150°C for TP-1/NTP-1; 170°C for TP-2/NTP-2 or 210°C for TP-3/NTP-3 (all ok for solder reflow exposure). Lowering the temperature will reduce excessive consolidation as observed by less squeeze-out of the adhesive around the bond edge. If temperature can’t be varied easily, pressure variation can be used. Again, higher pressure produces higher consolidation, lower pressure less.
A suitable bond will have wetted out both surfaces and won’t show squeeze-out of the adhesive around the edge. This can be determined by: (1) bond line compression that is ~10% of the original adhesive film thickness (for example, 0.004” film will compress down to ~0.0036”); (2) Z-axis electrical conductivity meets your requirement; and (3) die shear testing shows >500 psi bond strength. Some applications may need to trade higher pressure for lower temperature (>200 psi at ~135°C for a TP-2/NTP-2 example) or, to lower pressure, use a higher bonding temperature (165-170°C at 35 psi for a TP-2/NTP-2 example).
The temperature and pressure should be applied for long enough to ensure that the adhesive film and both substrates reach the proper temperature range as confirmed with a thermocouple attached to the film during bonding. The bond line should be cooled 30-40°C before releasing pressure.
Tacking the adhesive film to one of the substrates prior to bonding is the best way to assure proper film positioning for some applications. This can be accomplished by putting the adhesive film on release paper, putting the substrate on top, heating them on a hot plate (within 20-30°C of normal bond temperature) and applying light fingertip pressure to the substrate before removing it from the hot plate. The adhesive film should stick to the substrate enough so that it can be trimmed around the edges if desired.
Bonding pressure for laboratory experimental applications that do not require precision alignment is most easily accomplished with a simple spring fixture with a gimble mounted block on the end (Figure 1 below). A film such as Kapton is sometimes used under the block to protect the substrate. We use this type of fixture in our own lab…due to thermal absorption it takes about 15 minutes to reach ~150°C bond line temperature (for TP-2/NTP-2 applications) and 3 minutes to cool down below 120°C.
Pressure application with a dead weight is not recommended because the weight must be much larger than the bond area to get enough pressure. This leads to tipping of the weight, which creates a bond that is over compressed on one edge and debonded on the other. Very large bond areas of >6 cm2 (1 in2) are best bonded with a heated vacuum press or autoclave.
The bond is reworkable by exposing it to 15-25°C above the bond temperature and separating the bond line. The film can be scraped off the substrates at this temperature or removed at lower temperature with isopropyl alcohol. New film must be used for a new bond. |
