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.

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.

Cure In Seconds With UV/VIS Light

There are many ways that organic adhesives can be cured to their final, fully polymerized state. Two part epoxies just need to be mixed and they will react in minutes or hours at room temperature, depending on how they were formulated. Cyanoacrylates (“Super Glue”) polymerize rapidly on exposure to moisture in the air. One part epoxies cure when heated, typically in the 100° - 150°C range. And then there are the materials that cure when exposed to light, either ultraviolet (UV) or visible (VIS). An important class is the acrylate urethane chemistry that cures in a free radical addition polymerization. Once the reaction is initiated, polymerization occurs very quickly, within seconds. In order to trigger the reaction, a small amount of photoinitiator is included in the adhesive and it decomposes when exposed to UV/VIS light, creating the free radical. A second important class is the UV/VIS activated, cationic-cured epoxy, which also cures very quickly.

UV/VIS cure materials are most commonly used as encapsulants or coatings, where one surface is available for exposure to the light. The major advantage of UV/VIS cure materials is the rapid reaction, enabling complete assembly in just a few seconds. They are also used as adhesives for bonding clear materials such as glass or plastics. But even with opaque materials, modified UV/VIS adhesives can be used to advantage. For example, when bonding a mostly clear material there may be shadowed areas where light cannot be directly transmitted. In these cases, there are products available with secondary cures activated by heat or moisture. These two-step adhesives allow fast tacking of two components with UV/VIS cure and complete bonding through the entire structure with the secondary cure. Another approach for opaque materials is using a delayed cure UV/VIS epoxy. In this case, exposure to light triggers a “fuse” which allows a time delay for final assembly after which full cure is achieved. This “fuse” can be set for just a few seconds in a high speed, automated assembly operation.

While fast cure is the primary advantage of UV/VIS adhesives, long work life in a one-component product is a close second. As long as there is no exposure to the activation wavelength, these materials can be stored, dispensed, and assembled at room temperature for long periods of time.

Resin Designs specializes in producing custom formulations to meet specific application needs. While some representative products can be found listed
here, it is best to contact us and discuss the details of your project.

Nanoparticle Silver Attach Adhesive

I gave a presentation this month at the IMAPS New England Symposium on a new nanoparticle silver epoxy that can be aerosolized into droplets less than five microns in diameter. These droplets can then be jetted in multiple axes to create bond pads or interconnects with very fine features. Unlike traditional die attach epoxies that are needle dispensed by touching the surface and drawing out a relatively large single drop of liquid adhesive, an aerosolized epoxy is “painted” on a substrate with a laser-like beam of tiny droplets using a dispense head that can be 10 mm from the surface. Even ink jet dispensing does not have the resolution of aerosol jetting. For more details, download the complete presentation. In addition to explaining the aerosol jet process, it describes the epoxy properties, especially highlighting its very high electrical conductivity. For those who don’t currently have access to an Optomec Aerosol Jet® system, Resin Designs has created a needle dispense version. While the benefits of aerosol jetting won’t be realized, the finest needles can be used (32 gauge and smaller) to begin experiencing the potential of this new technology. As shown in this photo of a Kovar tab bonded to a gold surface, this adhesive looks more like liquid silver than a traditional die attach material. We’ll have more about these new materials in future updates. If you have a potential application, please let us know.

Pasted Graphic

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.

Does Two-Part Epoxy Have To Be Difficult?

Strong epoxy bonds are created by the reaction of an epoxide resin and a polyamine hardener.  Since this reaction is irreversible, the end product is known as a thermoset polymer (as opposed to a reversible thermoplastic).  The rate of reaction is highly dependent on temperature and, even more importantly, on the choices of resin, hardener, and other ingredients that have been designed in to a particular formulation.  Two-part epoxies are supplied with the resin and hardener packaged separately, to prevent the reaction from occurring.  While this type of packaging offers the benefit of long term storage at room temperature, two-part epoxies are sometimes perceived as difficult to process.  This doesn't have to be the case.

Work life of an epoxy, or the time available for dispensing and then assembling components, can vary widely from a few minutes to several hours after mixing, depending on the formulation.  The key to success is quick, thorough mixing of resin and hardener with the correct amount of each.  Common ratios are 1:1, 2:1, 4:1, 10:1, and others.  How is a user supposed to accurately measure and mix quickly and cleanly?  For measurement, the answer is using a supplier that provides pre-measured amounts in packaging appropriate for the application.  For mixing, most applications are best served with disposable static mixers.  A static mixer is a tube containing internal barriers that separate and recombine a liquid multiple times as it flows through.  It is ideal for thoroughly mixing epoxy with absolutely no mess, providing a single stream from the outlet.
Epoxy from Resin Designs
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