Q & A: Adhesives for Optical Assembly Applications

Join Rohit Ramnath, Senior Product Engineer at Master Bond, as he answers questions about optical assembly and how to choose the best adhesives for these applications.

 

Q: In selecting an adhesive for applications involving optical assembly, what are some of the most important parameters to consider?

A: In a broad sense, optical assemblies requiring adhesives can be classified into two main application categories:

1. those requiring an optically clear adhesive, and
2. those where optical clarity is not required.

For applications requiring an optically clear adhesive, properties such as transmission at various wavelengths and index of refraction play a vital role. Some examples include bonding glass lenses or glass to acrylic/polycarbonate. For applications where optical clarity is not required, the adhesive can even be opaque. Some examples include bonding glass to aluminum for a mirror assembly or stainless steel to ceramic to name a few. Having broadly classified the two main types of applications in optical assemblies, please be aware that while optical clarity is usually important, engineers must also be mindful of other properties such as low outgassing, low shrinkage, and dimensional stability.

 

Q: What makes epoxy EP42HT-2LTE a good option for use in bonding applications in optical devices?

A: EP42HT-2LTE offers very rigid, dimensionally stable bonds, with an extremely low CTE (~9-12 ppm/°C) thereby, offering precise alignment capabilities. Also, it is capable of maintaining this alignment during temperature fluctuation through a wide temperature range. In addition, it passes the NASA low outgassing specifications, which might be vital in some sensitive optical devices. Finally, since the EP42HT-2LTE is a paste, it is a good fit for optical applications where minimal flow during the application and curing process is required.

 

Q: In one of your recent case studies, a customer was looking for an adhesive with low shrinkage and low CTE, as they were using glass and ceramic based substrates. Can you explain the importance of low CTE in such applications?

A: Basically, both glass and ceramic have an extremely low CTE, and the customer was looking for an adhesive which would offer high stiffness/rigidity, close match to the CTE of the substrates, and precise alignment post cure. A filled, low CTE adhesive would meet such requirements, and offer superb dimensional stability. These are some of the reasons to prefer a low CTE in such applications.

 

Q: What challenges do your customers typically face in this area, and how are these issues resolved?

A: There are many issues involved when selecting the right adhesive for optical assemblies. Here are a couple of scenarios: a. In applications where precise alignment post cure is very important, typically rigid curing, dimensionally stable adhesives are used. Adhesives with very low CTEs can provide these properties. b. On the other hand, there are applications, where low or minimal stress needs to be imparted on the substrates from the adhesive. In such cases, adhesives which provide a good degree of flexibility are used. In both cases, there might be a need for an optically clear system with specific refractive index and transmission requirements. There are optically clear adhesives which can cure rigid or flexible, and depending on the other requirements, engineers must select the appropriate compound.

 

Meet Our Tech Expert: Rohit Ramnath

Rohit Ramnath is a Senior Product Engineer for Master Bond Inc., a custom formulated adhesives manufacturer. He analyzes application oriented issues and provides product solutions for companies in the aerospace, electronics, medical, optical and oil/chemical industries. He graduated from Carnegie Mellon University with a Masters Degree in Chemical Engineering.