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HT 1.5 RF & Microwave Bonding Film Laminating Procedure
Surface Preparation and Handling
- After copper etching to form circuit traces, it is important to minimize
handling of the PTFE surfaces. Cotton gloves should be worn by operators and
protective slip sheets placed over each surface for transport to the next process.
- The PTFE surface from which ED foil has been etched has a sufficient roughness
for bonding. Where rolled foil has been etched or unclad laminate is to be bonded to,
it is recommended that the PTFE surface be treated to provide adequate adhesion. The
same chemistries which are used for PTH preparation are also recommended for surface
treating. The sodium based chemistries such as FluroEtch® by Acton or TetraEtch®
by Gore are recommended. The specific processing techniques are provided by the supplier.
- Copper surfaces should be treated for optimum bond strength. A brown oxide treatment
of the copper circuitry will enhance the surface topography for mechanical bonding with
the TAC BOND adhesive film. The first process step requires a cleaner to remove resist
residues and handling oils. A copper microetch follows to provide a uniform roughened
surface area. The brown oxide creates needle like crystals which anchor to the bond
layer during lamination. As with any chemical process, adequate rinsing is essential after
each process step. Salt residues will inhibit bonding. The final rinse should be monitored
and maintained at a pH less than 8.5. The layers should be dried and handled without
imparting surface contamination such as hand oils.
Lay-up and Lamination
- Bake the layers for one hour at 250 ºF (100 °C) to remove moisture. Store the layers
in a controlled environment and use within 24 hours.
- Press pads should be used between the tooling plate and first separator plate to
distribute the pressure evenly across the panel. High pressure areas which exist in the
plates and circuits to be filled are absorbed by the pads. The pads also provide temperature
uniformity from outside to center of the stack. This results in panel to panel thickness
consistency.
- The board should be constructed with TAC BOND film as supplied. Take care when handling
the film during cutting and lay-up to prevent contamination. Depending on the circuit design
and filling requirements, one to three sheets of bonding film may be required. The area to
be filled and the dielectric requirement should be used to calculate the number of .0015" (38 micron)
film sheets needed. Clean, polished steel or aluminum separator plates are recommended to be
used between the boards in multiple stack heights.
- For vacuum assist lamination, pull vacuum for 20 minutes prior to heat up. Maintain vacuum
throughout the cycle. The evacuation of air from the stack will aid in assuring complete
encapsulation of circuits.
- A thermocouple placed in the outside border area of the center board in the stack will
allow for temperature monitoring and determination of adequate cycle time.
- This stack may be loaded into a hot or cold press to start. The heat rise and cycle
profiles will differ if not compensated for with press pad lagging. The heat input into the
package is not critical, however, it should be controlled to minimize the differential between
the outside and center of the stack. Typically, the heat rate is between 12-20 ºF/min (6-9 °C/min)
from ambient to 425 ºF (220 °C).
- Pressure can be applied immediately upon loading into the press. The pressure will vary
with panel size and should be used within the range of 100-200 psi (7-14 bar).
- Maintain the stack at 425 ºF (230 °C) for 15 minutes. The temperature should not exceed
450 ºF (235 °C).
- Cool down under pressure in the lamination press or transfer to a cool down press. If
transferred, minimize the time that the stack does not have pressure. Maintain under pressure
until the stack is below 200 ºF (100 °C).
 Introducing HT 1.5
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