Epoxy Film Board Mount Process for sheets or Custom Cut Preforms

Watch Video: Epoxy Film Board Mount Process

The epoxy film board mount process is the foundation for the electrical, thermal and mechanical performance of all assemblies.  Some experimentation may be required.

1. Allow film to reach room temperature before use.  This usually takes about 30 minutes.   Please note that many films have long shelf lives at room temperature and others can be refrigerated and do not require -40c storage.  Please see the data sheet.

2. Clean both the carrier/housing and circuit board (or other substrate) prior to starting the process.  A wipe with isopropyl alcohol will remove oils and FOD that may interfere with the bond adhesion.

3. Depending on preference, the process is facilitated if the film or preform is pre-tacked to the back of the circuit board before attaching to the carrier.  (Some prefer to tack to the carrier first). Make sure the hot plate surface is clean so any gold plating on the top side of the circuit board is not damaged during this process.  A layer of thin Kapton tape or Teflon can be put on the top surface of the hot plate to protect the circuit board plating. Depending on the film and application, the hot plate temperature should be in the 40c to 60c range. Place the circuit board upside down on the hot plate.  If it hasn't been removed already, remove the slip sheet from the top of the epoxy sheet/preform and attach the film to the circuit board.   Leave the other release sheet attached.   Align the preform to the circuit and remove any air bubbles or ripples by lightly pressing with a Q tip.  The film will now be attached (not cured) to the circuit board and will be easier to handle.

Note: If you are working with sheet stock and not a preform, the circuit board can be used as a template and the film can be cut to size with an Exacto blade.  

4. Place the carrier face up on the hot plate.  Remove the release sheet from the epoxy film.  Place the circuit board with epoxy facing down onto the carrier and align.  For RF module assembly, the alignment of the circuit within the carrier/housing is critical.  Press down on the circuit to make sure it is tacked to the carrier.  Use caution when pressing down so that the circuit traces are not damaged during the process.

5. The assembly is now ready for curing.  All epoxies sold by Bonding Source require pressure during cure. (Exception: some small ceramic substrates can be "scrubbed" in while on a hot plate and then placed in the oven to cure without pressure).  The pressure application is determined by the size of the assembly and customer preference.  Common industry practice includes the use of clamps, dead weights, vacuum bagging, or a press lamination process.  Some type of load distribution material is used between the circuit board and plate that applies the pressure.  The most common materials are red rubber silicon and Pacopad.  If using silicon rubber, it is recommended to also use a thin layer of Teflon sheet between the silicon and circuit board so that the silicon does not out-gas on the gold plating, which can cause problems at wire bond.  Once the pressure is applied, the fixtured assemblies are placed in the oven for the cure cycle.  The recommended cure schedules refer to the bondline value.  Large mass assemblies will require a longer time to achieve bondline temperatures.  It is good practice to keep the oven door shut for the duration of the cure cycle.

6.  Remove the fixtured assembly from the oven once the cure cycle is complete.  Let the unit come to room temperature before inspecting or the next assembly step.

How can you determine if  the board mount process is effective? For most of the films Bonding Source offers (with the exception of Coolspan TECA which is designed to be a low-flow product) there will be some evidence of epoxy flow on the edges or interior cutouts.  If there are unfilled via holes in the circuit, epoxy resin coming part way up the vias should be visible.  Please keep in mind that the amount of epoxy flow can depend on each batch of epoxy.   Sometimes one batch of film may flow more or less than a previous batch, even if the exact process and fixturing was used.  If there is excess epoxy flow out, or epoxy coming up through the vias that require cleaning, too much pressure may have been used.  If there is no evidence of flow at all, or if there is any voiding on the edges or corners, more pressure should be applied.  Sometimes the outer edges of a spidery RF circuit are the most susceptible to voiding.  Also, mounting multiple circuits at once or multiple circuits with different heights can be problematic.  This is where a good fixturing effort is needed the most.  If access to constant acceleration or a lap shear test is available for the assembly, that will provide good mechanical attachment data.  Electrically, the test engineers can provide feedback on how effective the board mount process was on the RF performance.

Bonding Source stocks the most popular films used in RF Microwave assembly, custom cuts preforms, silicon rubber sheet, Teflon sheet, and Pacopad board mount material.  We also sell board mount lamination presses and fixturing.  Please contact Bonding Source for recommendations on release sheet/slip sheet preferences, preform layout, and packaging for manufacturing efficiency.

We also offer pre-tacking of epoxy film preforms to circuit boards.

See the process here:
Pre-Tack Film Process