Fluorescing Enables Easier Visual Inspection of Bond Lines

At one point in your life, you’ve probably experienced the disappointment of having a favorite object or gadget come apart because the pieces weren’t joined together very well. To a manufacturer of components used in critical applications, such as medical in-vitro diagnostics or sensors in automotive ADAS, the scenario of their devices failing is intolerable and potentially life-threatening to users. Pieces of a medical device may tear off and remain in the body or an optical lens used in vehicle LiDAR to detect other cars on the highway may become misaligned, send a false signal, and cause an accident.

There are countless methods of assembling parts, from rivets and screws to ultrasonic welding and adhesives, and many reasons their joints might break or separate. Causes of failure include shear stress, substrate fatigue, surface contamination prior to processing, compression, and environmental conditions such as high humidity or heat, and bond failure. As a result, close examination of parts during and after assembly for possible defects is vital to the overall manufacturing process.

So how do companies ensure that their parts won’t fail? Through consistent, continuous quality control and inspection. Typical methods include pre-production or piece-by-piece review, taking a product offline and testing a sample, or via automated visual inspection systems. In medical device manufacturing, as an example, single-use products such as catheters and syringes are made in quantities of thousands at a time, so inspection needs to be accurate, but also fast. When adhesives are used to join components together, it's imperative that the substrates being bonded have sufficient material dispensed onto them so there are no gaps and adhesion is secure.

To make this process easier, the formulation of a fluorescent compound into an adhesive allows manufacturers to ensure the right amount of product has been applied to components and to verify that the cured bond line is intact. When exposed to low intensity blacklight, the fluorescing color of the adhesive provides significant contrast and high visibility against the substrates. When used in an optical vision system, the fluorescing capability is quickly captured on screen and immediately indicates if the component passes or fails inspection. 

What exactly is fluorescing technology in light-curable materials?

The process of fluorescing in chemicals occurs when short wavelength energy is absorbed by the material and the resulting light is emitted at a longer wavelength, but lower intensity. Subsequently, when the material is exposed to a UV blacklight, it radiates a distinct color in the visible part of the light spectrum detectable to the human eye or an automated vision system. The fluorescence is a temporary occurrence and once the light source is removed the material will no longer glow.

There are different colored fluorescing agents found in light-curable materials currently on the market. Blue fluorescence is often used in conformal coatings for printed circuit boards, yellow fluorescence is found in SpeedMask® 731-REV-A, a maskant utilized in MRO surface finishing processes, and patented Ultra-Red® fluorescing technology is formulated into a variety of Dymax medical device adhesives. Each type of fluorescence is intended to provide a quick and simple alternative to manual visual inspection, but others offer additional benefits.

Blue fluorescence emanates a very bright glow and is especially helpful when used in conformal coatings on PCBs where there is the potential for raised or shadow areas. It is easier to detect the lack of material coverage and voids in these peaks and valleys with a brilliant blue colored coating than a coating that is clear with no fluorescing capability.

The yellow fluorescence of SpeedMask® 731-REV-A masking resin enables easy visual verification of material placement and indication that the part is clean from the maskant residue after processing and removal.

Dymax adhesives formulated with patented Ultra-Red® technology fluoresce brightly after exposure to UV light energy. The vivid red color contrasts extremely well on solder masks, components, and plastics that naturally fluoresce blue in color (like PVC), greatly assisting with visual inspection of the bond line or coated area. When measured, this compound produces a unique energy peak that cannot be reproduced by other fluorescing compounds, offering manufacturers the ability to assemble or mark their products so they can be positively identified.

The benefits of using light-curable materials with fluorescing technology for quality control are obvious, from confirmation of complete material coverage and visual verification of bond lines, but the most important benefit is a manufacturer’s confidence that their finished parts won’t fail their customers.

Want to learn how you can improve your quality inspection process with fluorescing adhesives and coatings?