Why Low-temperature Cure Epoxy Adhesives are important
Today’s mobile and electronic devices have come a long way since the first mobile phone was created nearly 40 years ago. The very first devices were big and bulky with limited capability. User experience improvements have driven the industry to produce lighter, thinner, and more style products. Today, creating new designs requires new materials, leaving the manufacturer with several challenges including:
- Difficult to bond materials
- Thinner geometry
- Sensitive component surfaces
Let’s start with the newer materials. Consumers want their phones to be smooth and sleek feeling. Smooth surfaces tend to have lower surface energy. As any material technologists and application engineers will tell you, maintaining strong bond strength when your materials have lower surface energy is a serious challenge. For example, we all know Teflon is “non-stick”, and it has a surface energy of around 19mJ/m2. Common materials in cell phones include polycarbonate plastics, ABS, and Poly ABS – all of which have lower surface energy. Inside the phone, metals like palladium and platinum are equally difficult to bond.
Another key consumer trend is a desire for a larger screen area but also a lighter, thinner device. This push for bigger and thinner compresses the geometry. The thinner a component is, the faster it will absorb heat. Thicker components have more mass for better heat dissipation. The rate of heat flow is also proportional to the thickness of the material. This is known as the relationship between wall thickness on heat conduction. When there is excess thermal stress created within the components, plastic deformation will occur. This in turn changes the physical properties of the material and can lead it to become brittle and not fit for its original purpose.
Epoxy technologies for mobile devices offer excellent adhesion solutions to such difficult-to-bond materials. Their low solvent content ensures that they do not damage these sensitive surfaces. The catch here is traditional epoxies are notoriously slow cures. Newer adhesive formulations allow for faster curing with the exposure of slight heat. Manufacturers enjoy the benefits of epoxy technology without compromising on the speed of cure. However, a manufacturer must ensure that there is no detrimental thermal stress created in their assemblies which could lead to plastic deformation and component failure.
Components such as those assembled onto a PCB board are a mixture of plastic and metal, e.g. an FR4 epoxy board is assembled with capacitors and resistors that are manufactured from metals such as aluminum. These materials are very different in their mechanical properties. One property that is inherently different is the Coefficient of Thermal Expansion (CTE). CTE describes how much a material will expand when a certain amount of heat energy is applied. Now, when we combine an assembly manufactured from electronic (metal) components and a printed circuit board (polymer/copper), and an epoxy adhesive (polymer). These assemblies have two very different CTEs. The issue comes when we apply heat to this assembly. If these components have vastly different CTEs, one will remain rigid while the other will expand. This is known as Differential Thermal Expansion (DTE). It is a concern as this can cause excess stress to be built up within the assembly during the heat curing process. Therefore, if we are going to use two materials with different CTEs then we need to ensure that one is flexible and capable of absorbing the loads caused by the curing process.
Krylex KE1092 is a high-performance, low-temperature curing epoxy. It offers good adhesion when bonding dissimilar materials such as PC, SUS, PA, and AL. It offers good flexibility to ensure that there is no damage to assemblies during the curing process.
Benefits of Krylex KE1092
- low-temperature curing, e.g. <80C.
- Excellent Adhesion to plastics and metal (various types and grades).
- High Reliability.
- Low solvent content (prevent damage to substrate surfaces)
- Flexible and able to tolerate CTE Mismatch.
- ROHS Compliant.
April 20, 2021