Establish Process Window for Curing

Light Curing Technology offers a wide range of benefits that enable manufactures to reduce assembly costs, improve quality, increase production while improving work environment for employees. These benefits can be realized relatively easily and efficiently if the proper steps are taken during the process design stage.

In the very early stages, the design engineer should consider the joint design, desired assembly method, select appropriate adhesive and then ultimately select the most suitable light source. The extra time spent during this initial phase will inevitably shorten the process develop cycle and ensure that the process is set-up to be optimized.

This article will focus mostly on the process steps that pertain specifically to the light curing stages of the process development.

This stage typically starts with appropriate adhesive selection. It is important to recognize that adhesives are engineered materials and therefore need to be developed fully into the assembly process. Each adhesive is specially formulated to achieve specific performance results. One adhesive does not fit all jobs. Adhesive selection will depend on the substrates being bonded, performance factors (bond strength), process considerations (curing time, viscosity) and environmental conditions (operating temperatures humidity). Be prepared to answer questions regarding these questions when you speak with your adhesive supplier. Given this information they will be able to suggest the appropriate candidate materials.

Now that you have selected the appropriate candidate materials, it is time to investigate which type of curing system that will be used. The two main types of curing systems are flood cure systems and spot cure systems. The application and process will usually dictate which system makes most sense for your application.

Flood Curing Systems offer advantages such as high power levels, moderate speed continuous processing, 3D shape components, no size limitations. Typical applications include web printing, flat panel display A/R coatings, consumer packaging, tape and label converting, flooring industry, automotive panels, UV powder coating, etc. It is also important to consider that many of these systems also have limited spectral control, can cause unwanted heating of components, may not have consistent irradiance over the entire lamp footprint and may need additional ventilation requirements to remove ozone.

Spot Curing systems are typically deployed for precision light cure assembly of components. Advantages include very high irradiance levels possible for fast cures, small footprint, enhanced spectral control, multi-point light delivery, Step Curing, and is manual and automation friendly. Applications include medical device assembly (catheters, needle assembly, hearing aids assembly, tubing, etc), optical component assembly (camera lens assembly, optical pick-up assembly, hard drive assembly, etc), electronics assembly (LCD Assembly, component encapsulation, etc). These light sources are limited to cure areas that are typically smaller than 2-3 inches in diameter and need to be automated for high volume assembly.

Once you have selected the appropriate adhesive/coating/ink and curing system it is now critical to understand how the two interact. The chemistry will require 3 things to ensure it is fully cured. Minimum peak irradiance at the appropriate wavelength for a specific amount of time is needed to fully cure the adhesive/coating/ink. It is important to understand these three factors and to develop a process that can be well documented and repeatable. It is advisable to develop a design of experiences to develop an optimized process. There are three variables that need to be understood fully to optimize the process. A tip to help short circuit this is to lock down the two that are relatively easy to understand. The adhesive will require a specific spectral range to cure and it will be important to specify the lamp and system that will deliver this content. Secondly in most cases the cure time will be defined by the overall cycle time for the component assembly. UV curing should be an enabler and not a bottleneck in a process. Therefore it is advisable for the customer to select the desired cycle time for this process. Now the last variable is the actual amount of light that needs to be delivered to deliver the dose required. You will require a Radiometer of Light Meter to measure and document this part of the process development. If you have the ability to adjust the amount of light being delivered from your system it is advisable to set your process requirements to the minimum level of light required to deliver the required dose. Any additional energy is just being delivered in abundance and may not affect actual cure. This is also important to ensure that you minimize cost or operations for your UV process. UV lamps will degrade over time and the lower the demanded level of intensity the longer it will take to reach that point in your lamps life. It is advisable with most systems to keep the desired process intensity to less than 50% of what the total available energy is available since most lamps are rated to 50% of initial output for lamp life specifications.

The time spent on developing and characterizing your UV process initially will enable you to take the most advantage from this exciting technology. It also makes the process very easy to transfer from R&D to manufacturing and ultimately replication to ramp up for full production. This information can be transferred to operations and quality control to maintain and monitor the process.