Last Updated: September 15, 2021

When I meet with customers, I field a variety of questions regarding epoxy processes in many applications, including fiber optic cable assemblies. This article addresses commonly asked questions:

  • Why is bond line thickness important?
  • What is the recommended minimum bond line thickness?
  • What is the recommended needle size to dispense epoxy?
  • If the epoxy has filler or glass beads, will that interfere with the strength of the bond – and create the need for a thicker bond line?

My foundational advice: Start with the general guidelines, then take time to test and adjust.

In the following paragraphs, I present general guidelines to answer the above questions. Use these as starting points. Since everyone’s application is different, the general guidelines may not work for you. Plan to test your bond line thickness – and other epoxy processes – to ensure they are optimal for your application. Remember this rule of thumb: “The application dictates.”

When developing a new process, engineers often undergo multiple steps to ensure proper bond line thickness. They evaluate parts under a microscope, test them, and may even ship parts to their end customers for testing in the application. If you’re implementing a new process, it could take weeks – or months – to fine-tune an epoxy process that achieves the required adhesion and performance.

If your process is set and you are changing epoxies, recognize that the properties of the new material are different. For example, if you previously used 2 mils of epoxy, you may need to adjust bond line thickness due to the new epoxy’s material properties. Also, take the time to understand the needed cure time for the new epoxy. Don’t assume you can swap out epoxies without testing and adjusting your process – it’s not apples to apples.

As promised, here are answers to commonly asked epoxy questions.

 Why is bond line thickness important?

Proper bond line thickness ensures you’ll achieve the adhesive properties AND performance specified in the epoxy’s datasheet. If there is inadequate bond line thickness, the risks include low strength (shear and tensile), poor electrical properties, high thermal resistance, optical scattering, and failure during operation.

 What is the recommended minimum bond line thickness?

A general guideline for minimum bond line thickness is between 25 to 150 microns (1 to 6 mils) – as a starting point. I recommend you choose a certain level to start, then make a few thicker and thinner versions of the bond. Take time to inspect completed bonds carefully to learn how a bond can fail, evaluate the performance for your application, and adjust bond line thickness as necessary to achieve a quality bond.

For every application, bond line thickness depends on:

  • The geometry of the bond area
  • The substrates being bonded
  • The material itself (the epoxy’s chemistry can dictate a specific minimum bond line thickness)
  • The temperature cycling the bond will need to survive
  • Proper surface preparation (improper cleaning can negate adhesion)
  • The cure time and method of cure
    • Bond line thickness and curing time go hand-in-hand to achieve sufficient adhesion and performance. Carefully review the epoxy manufacturer’s datasheet, including any charts showing bond line thickness versus cure time for the various curing methods.


What is the recommended needle size to dispense epoxy?

With so many unique applications as well as connector sizes, the answer to this question truly does depend on your application. Needles range in gauge size and length; some have a tapered tip to allow higher-viscosity materials to flow more easily. A general guideline is to try a 1.5 inch, 20-gauge dispensing needle. As noted above, test and adjust as necessary.

If the epoxy has filler or glass beads, will that interfere with the strength of the bond – and create the need for a thicker bond line?

If an epoxy has a filler or glass beads, the manufacturer has included the filler for a reason. The filler is part of the material’s properties – a component of the whole mixture – that contributes to achieving the adhesion and performance listed on the datasheet. Theoretically, there could be reduced strength if the dimension of the filler approaches the thickness of the bond line. Most fillers are much smaller than 1 mil (25 microns) and should be no cause for concern. However, you should always test the bond lines as described above to have confidence the adhered joint achieves the strength and stability needed for the application.


Still have questions about epoxy? We’re here to help!

I encourage you to carefully read the datasheet for the epoxy you use – you’ll find helpful information specific to the material used in your application. If you have additional questions, we offer technical assistance to get the specific information you need. At Fiber Optic Center, we’re committed to helping you manufacture the best fiber optic cable assemblies possible.

Additional resources from the FOC team include:


Follow Kelly @EpoxyExprtsFOC



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