How to Choose the Right Polyurethane Hot Melts for Electronics Assemblies

The #electronics market is rapidly expanding, and there is a high demand for adhesives for bonding different parts and components during the electronic assembly process.  #Structural_adhesives are being utilized more and more in place of mechanical fastening methods and tapes due to their high strength, ease of use and low total cost of ownership.  In particular, Polyurethane hot melts are becoming one of the most popular adhesives selected by electronics manufacturers, owing to their wide range of benefits.  These products can be found in almost every consumer electronics product today including smartphones, wearable devices, automotive touch panels, tablets and notebook computers.  Among many other benefits, PUR hot melts provide: 

  • Very fast fixation for high speed production
  • Single component formulations allow for flexible dispensing methods
  • Excellent adhesion to variety of porous and non-porous surfaces
  • Good gap filling capability for greater tolerances in part design
  • Ability for thin bond-line or dot dispensing in micro applications
  • Excellent moisture and chemical resistance
  • Excellent durability and impact resistance for tough yet flexible bonds
  • Stable formulations with good shelf life

Applied in a molten state, these solvent-free, 100% solid adhesives solidify and crystallize on cooling, providing green strength and instant fixation on the parts being assembled.  Over the course of the next several days, PUR hot melts further react with ambient moisture to increase in strength and environmental resistance until a fully cured, robust bond-line is created. Since they are available in different viscosities, colors, adhesion packages and impact properties, among several other specifications, choosing the right polyurethane hot melt holds a great significance. This blog lists a few things to consider when choosing polyurethane hot melt adhesives for electronic assemblies.

Factors to Consider When Choosing Polyurethane Hot Melt Adhesives 

Although most types of polyurethane hot melt adhesives share similar qualities, they differ between one another in curing time, viscosity, cost and flexibility, among others. The following are some of the things to consider during the selection process. 

  • Colors: Do you know hot melt adhesives featuring polyurethane are available in various colors to match the application requirements? White, transparent, black, gold, yellow, silver, gray, and blue are common colors. However, you need to check with the manufacturer, if these adhesives can be customized in any other color of your choice. Many manufacturers also provide satin and glossy finishes of these adhesives, which are widely preferred in wearable devices for clean aesthetics and bond-lines.
  • Check Whether the Chosen Product Adheres to your Substrates: Every hot melt adhesive is not made the same and they may not fit well with your application. Products have different adhesion to porous or non-porous surfaces, various plastics, glass, metals ceramics or composites. Choosing the best product that adheres well to your surfaces should be your primary concern. Hence, before making a purchase, it’s always a good idea to contact an application specialist for testing and validation.
  • Electrical and Thermal Properties: Basic adhesive formulations are generally considered to be electrically and thermally insulative.  Some applications, however, require increased thermal conductivity to remove heat away from sensitive components or especially insulative electrical properties to not interfere with performance of transmitting devices. Chemence® has developed several specialty PUR hot melts that can provide thermal conductivity or low dielectric constants to suit your application, but some performance tradeoffs may exist such as longer open time or lower overall adhesion. 

PUR Hot Melt adhesives are typically dispensed using precise valves for electronics applications in highly automated production lines.  The fast fixture time of PUR Hot Melts allows for high production throughput and enables mass production of consumer electronics devices

  • Application Methods: As mentioned earlier, hot melt polyurethane adhesives are available in a varied range of rheologies, making the application method and processing parameters different from one application to another. Once an adhesive is selected that will work from a performance perspective, it is important to fine tune the dispensing and curing process to optimize long term performance. Will the adhesive be sprayed? Dispensed as a bead?  Jetted through a specialty valve?  Working with our technical experts to optimize rheology and dispensing parameters is critical for success in your dispensing system and ensure that stringing, clogging and other issues do not arise.
  • Processing Requirements: The recommended application temperature for polyurethane hot melt adhesives usually falls between 85°C to 140°C (185°F to 284°F). Also, the adhesives are manufactured with open times varying from 10 seconds to over 10 minutes. Some substrates such as aluminum have high thermal conductivity and act as a heat sink when a PUR hot melt is applied – decreasing open times significantly.  All these process requirements must be closely examined before making a final adhesive selection. 
  • Green Strength and Curing Times: PUR adhesives have different stages of curing. Once dispensed the adhesive has a specific open time that allows for mating of parts.  Upon cooling a certain amount of green strength is obtained that fixtures parts together.  Over the next several days the PUR hot melt will react with ambient moisture to further crosslink and build final adhesive properties such as environmental resistance, impact strength and full adhesion strength. Some adhesive formulations may form strong green strength bonds in a few seconds, whereas some act more as gasket materials and remain soft, reworkable and tacky similar to a pressure sensitive adhesive. Hence, it is important to concentrate on their curing times while selecting the adhesives for your application. 

Traditionally, PUR hot melt adhesives have been used in high volume, low cost applications such as panel laminating, book binding, woodworking and furniture manufacture and even kitchen sponge assembly. These products have stood the test of time because of their ease of use and durability in these applications.  Chemence® has taken the formulation of PUR hot melts a step further by customizing these products to fit the rigorous requirements of electronics manufacturers while maintaining the important benefits that have been outlined above with regards to performance and ease of processing. Our Polyurethane hot melts for electronics are available in syringes and cartridges to fit many existing electronics production lines.  Get in touch with Chemence®, a market leader in the development of polyurethane hot melts for the electronics industry.  Our KRYLEX® polyurethane hot melts come in a variety of formulations to meet most of the requirements discussed in this article. Contact us and we will be more than willing to work on customizing a formula specifically for your needs.

November 13, 2019