Choosing Epoxy Film For Cost Reduction

Selection of an adhesive can take many paths and every application has unique constraints. We have provided a technical checklist for adhesive selection, but ultimately cost is a big factor. The fact is that film format epoxy is more expensive than a liquid format on a volume or weight basis. This is very simply due to the higher level of processing necessary to make a filmed product. But does that mean all users of epoxy film are automatically producing a more costly product? The answer is clearly no, but the question is why?

After reviewing numerous applications across many industries, the standout cost driver for epoxy film is first pass yield in production. Yield improvement of just a few points can reduce cost significantly. For example, moving from 90% to 95% yield means the effective cost of goods drops 5.3%. Considering that adhesives typically represent a very small fraction of the material budget for most products, a moderate increase in adhesive cost could lead to significant savings overall. Many processes using film adhesives approach 100% first pass yield. Some of the reasons for improvement are:
  • Reproducible bond line thickness
  • Reproducible amount of adhesive deposited
  • Elimination of rework due to insufficient or excessive adhesive
  • High strength bond
  • Reduced functional tuneup time of device (most notably in microwave applications)

If an assembly process is currently running 90% or better yield, there may be an opportunity for cost improvement that justifies looking at alternative adhesives. Any process running less than 90% probably justifies a critical assessment.

Yes, epoxy film adhesives are more expensive than liquid epoxies. Yet in considering the total production process, they can in fact contribute to the lowest cost solution.
Click here to review the many options available.

Delivering Unique Epoxy Film Structures

Sometimes an adhesive interface needs to be more than just a mechanical bond. Sometimes it needs to be electrically conductive, or conversely, electrically insulating. Sometimes it needs to be thermally conductive, or again conversely, thermally insulating. But there are also applications that require specific structures for assembly or design reasons. For example, to set a specific gap in a bond line the use of a carrier film such as DuPont™ Kapton® polyimide or Mylar® polyester coated with b-stage epoxy on both sides is very effective. Coating the epoxy on just one side produces an overlay adhesive that is readily cut and easily processed in an automated line. Both of these carrier materials also offer excellent dielectric and chemical barrier properties. For a high breakdown voltage, it’s hard to beat an epoxy-Kapton-epoxy bond line.

Resin Designs has the flexibility to coat any of their
B-stage epoxies on these carriers, either one side or both sides. With a distinctive set of resin systems, a wide variety of fillers, and multiple carriers, Resin Designs can produce a broad selection of b-stage epoxy products. What application do you have that might benefit from a unique structure?

B-Stage Film Epoxies With Low Cure Temperature

Most epoxy films are cured around 150°C and some can cure down to 125°C. This lower limitation is due to the solid resins that must melt and then reach a threshold temperature where polymerization can start. Resin Designs has broken this low limit barrier with the TechFilm 2700 series of film adhesives that cure as low as 90°C. This novel resin system allows the benefits of film epoxies to be realized in temperature sensitive applications. Some of these are piezoelectric transducer assemblies, solid state laser assembly, and medical devices.

We provide a complete line in the 2700 series, all of which are compatible with each other in co-curing applications. They all offer great adhesion to a wide variety of materials and their glass transition temperature is in the 100° to 110°C range. Starting off is the pure, unfilled
I2701F, which enables bond lines as thin as one mil. Alumina filled T2723F provides thermal conductivity with great electrical insulation while the silver filled E2713F offers both electrical and thermal conductivity. We also produce specialty products such as the aluminum filled T2781F which provides excellent thermal conductivity with a “resistor” level of electrical conductivity, and E2771F3, which is a graphite filled, carbon mesh fabric supported material used primarily for grounding or shielding applications.

In total the TechFilm 2700 series enable the use of film adhesives in temperature sensitive applications where they couldn’t previously be considered. On the flip side, they also enable faster cure at higher temperatures, but that’s a story for another day. For more information,
review all the film products here.

TechFilm Adhesives Demystified

Resin Designs draws on six core polymer formulations for its TechFilm line of b-staged epoxy films. From a user point of view these formulations provide a broad range of properties such as cure temperature (90°C - 200°C), adhesion strength with different materials, glass transition temperature from 80°C to 200°C, and work lives from 1 - 30 days. The next step in customization for a specific application is determining what filler, if any, should be used. Most commonly, alumina and silver are used for enhancing thermal or electrical conductivity respectively. In fact, other fillers such as aluminum or graphite are also available. Further customization is possible with addition of a carrier such as a fiberglass mesh, Kapton® polyimide, or carbon fabric. Unlike many competitors, Resin Designs can precisely locate the carrier either in the middle of the film product or on one side. Thickness of the film is another important consideration. The standard range is 2 - 5 mils, but with some unfilled products thinner is possible and in many cases, thicker is possible as well. The final step of customization is cutting the film to a preform shape specific to the application.

Which is the best choice of chemistry, filler, carrier, and thickness for your application? You can start by reviewing the product lines at
this page, or better yet, contact us and we’ll get back to you with our recommendations.

Liquid Epoxy - One or Two Part?

Selecting an epoxy for a given application involves many criteria, generally starting with a determination of the surfaces to be bonded and factoring in desired mechanical, electrical, and thermal properties. Process and storage criteria also play a critical role. Most of the time, these pre-determined requirements will naturally drive to a one or two part epoxy. Just as in politics, the best choice normally comes down to a bipartisan compromise. What are the choices? Here is a brief summary table:


Two-Part Epoxy

One-Part Epoxy

Storage

Room Temperature

Cold (Preferably -40°C)

Mixing

Required

Not Required

Work Life @ Room Temp.

Minutes To Hours

Hours To Weeks

Cure Temperature

Ambient to 80°C

80°C to 180°C

Cure Time

48 Hours to < 1 Hour (Heated)

< 1 Hour

These are general guidelines and there are exceptions in virtually every category. But if user mixing is a non-starter, then one-part is the only way to go. Conversely, if ambient cure is required, two-part is the best option.

Both
one and two-part products are listed on this website. Within these broad categories many variations are available in terms of physical properties and process options. With regard to one-part systems, there are also the B-staged film adhesives. This website lists only a representative sampling of available products. In many cases a custom formulation can be developed. Whether a perfect fit is found or not off the shelf, it’s best to contact us with your requirements and we’ll help determine the optimum solution.

Spotlight On High Temperature Epoxy

With glass transition temperatures (Tg) around 200°C, the 2300 series of TechFilm epoxy adhesives from Resin Designs are ideal for high temperature applications. The Tg is an important parameter for characterizing adhesives since it marks the transition from a rigid crystalline / glassy phase to a more disordered rubbery phase. Mechanical properties change significantly; adhesion strength and storage modulus drop, while the coefficient of thermal expansion increases. The high Tg of the 2300 series means more operational stability up to 200°C. As an added bonus these epoxy systems also have great chemical resistance at both ordinary and elevated temperatures. There are three key products in the series that address particular requirements:
  • I2300F is the pure, unfilled epoxy in film form.
  • T2321F contains alumina filler, usually used for enhanced thermal conductivity, but also for reducing flow during cure and ease of handling.
  • E2313F is the silver filled version, used where electrical conductivity is required.
All three products have a one year shelf life when stored at -40°C, but they also have a long, 30 day work life at room temperature. The optimum cure schedule is one hour at 180°C.
These
film materials are available in sheet or roll format that can be cut as needed; or as preforms cut to the specific requirements of the application. Whether simple or complex, preforms enable the highest ease of use. Let us know your needs.

Adhesives for Thermal Management

We receive a lot of calls for adhesives with high thermal conductivity to achieve maximum heat transfer across an interface. Invariably the next question is whether electrical conductivity is required. The answer makes a big difference on thermal performance since filler material is the driver. For electrically insulating applications, the filler is a ceramic and the most common is alumina (aluminum oxide). The tradeoff with any filler is achieving a fill rate that yields good thermal conductivity without compromising mechanical or process performance. Very high fill rates produce such highly viscous materials that they can’t be effectively dispensed. A typical unfilled adhesive has a thermal conductivity around 0.2 W/mK. Ceramic fillers enable conductivities around 1 W/mK. If electrical conductivity is needed or allowed, then metal fillers further enhance effectiveness. Silver is the classic and conductivities from 2 to 20 (or more) W/mK are possible. The wide variety of particle shapes (spheres to flat flakes) and sizes allow for maximizing particle contact, and hence the conductivity. Ultimately, adhesive formulators strive to balance high fill ratios for increased conductivity with conflicting performance (adhesion, modulus) and handling (viscosity) needs. While alumina and silver are the most common fillers, many others are available, such as aluminum, graphite, copper, aluminum nitride, boron nitride, etc.

Adhesives compete with grease, pads, phase change materials, etc. as “thermal interface materials” in electronic assembly. While each product has its key benefit, only adhesives add mechanical support as part of the total package. This can be a significant advantage for size and weight reduction, eliminating clamps or screws. Thermally conductive adhesives bring thermal, electrical, and mechanical performance together in a single multifunctional answer.
Contact us to review options.
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