The beginning of the 21st century finds optoelectronics being one of the key disciplines on technological development. Substantial progress in computer technology, communication, imaging, illumination technology, sensor technology, medical technology or production technology is based on the availability of high-performance optoelectronic components. In particular, applications in consumer electronics presuppose an ever-increasing power density and continuous miniaturization of the components, while driving down costs per power unit. This goal can only be achieved through powerful mass production concepts.

Figure 1. Outgassing progress of adhesive samples at 120°C: Standard acrylates continuously lose mass and achieve high outgassing values, while the outgassing progress of DELO-KATIOBOND OB products comes to a stop virtually directly after heating and remains at a very low level.
Light-curing adhesives satisfy the needs of the optoelectronics industry. They allow manufacturers to reliably and quickly join their tiny components made of dissimilar materials without significant heat input.

Low Outgassing

Nearly every cured adhesive shows mass loss under the influence of temperatures. This effect is called outgassing. The reason for this effect may lie in the volatilization of non-reacted or only loosely bound constituents. In addition, whole fragments may get dissolved out of the polymer at elevated temperatures, or the network even depolymerizes as a consequence of the oxidative attack.

Aside from the fact that the bonded connection may fail as a result of thermal overload, the consequences of outgassing are not relevant for many users. The situation in optoelectronics, however, is different. Outgassing products may condense on the lenses of LED packages, and thus change the emission characteristics. They may also directly interact with the LED surface and reduce the light yield.

Figure 2. Progress of the LED intensity in the LED Bright Test™. The outgassing behavior of many adhesives reduces the intensity.
When evaluating adhesives, the absolute outgassing volume plays a role. However, it is also important whether outgassing occurs only for a short time during heating or permanently, whether the out gassing products are volatile or recondensable, and whether damaging interactions take place with active components.

Some adhesives show very low outgassing in the magnitude of 0.1 mass% at 120°C. Their outgassing stops short ly after heating, while most light-curing acrylates already durably outgas at 120°C (Figure 1).

For the quantitative analysis of the outgassing products, analytical methods such as gas chromatography/mass spectroscopy (GCMS) are available. Fragments of hydrocarbons can be identified as main outgassing components of DELOOB adhesives. Many users, however, do not even know which substances are critical with regard to their application. In this case, developing application-related tests has proven to be beneficial. For example, consider the following test setup for LED applications. An LED on a printed circuit board is hermetically sealed in a housing with glass lid. A larger adhesive quantity is inserted into the housing from which material can outgas when the LED is on. By monitoring the time progress of the light intensity at the glass plate, or by visual inspection, it is possible to assess if the LEDís performance is influenced by adhesive outgassing or not.

Figure 3. Transmission spectra of adhesive samples aged at 120°C.
DELO-OB products do not impair the LED intensity even at the maximum permissible electric power of the LED over a test period of more than 10 weeks, whereas many other adhesives lead to a total failure of the LED after 2 – 3 weeks. Interestingly, the test shows that there is no compelling link between the outgassing quantity and the LED’s intensity progress. Some strongly outgassing products, for example, deliver good results. On the other hand, products with very low outgassing behavior are available that show interactions after only a short period of time.

Optical Stability

Figure 4. DELO-KATIOBOND OB adhesives are stabilized against thermal yellowing. Virtually no discoloration can be detected during storage at up to 150°C for 1 week. Standard adhesives on the basis of epoxies or acrylates become slightly yellow already at 100 — 120°C. The figure shows an adhesive layer with a thickness of 100 μm on a glass specimen stored in the open.
Bonded connections are often designed in such a way that the adhesive layer lies in the beam path of the optical component. Therefore, it is essential that the adhesive is highly transparent for the relevant wavelengths, and also largely retains this transparency upon temperature stress, when exposed to UV radiation, or during soldering processes. Adhesives that are transparent and stabilized against thermal aging become significantly less yellow than other UV- or heat-curing products do.

Even if the adhesive is not located directly in the beam path, users are often bothered by strong discoloration of the adhesive joint as, for example, the high-quality design of mobile phones is disturbed.

Fast Build-Up of Adhesion/Active Assembly

Although light-curing acrylates do not have optimal properties for optoelectronic applications, they are attractive because of their extremely fast radical curing process. Classical light-curing epoxies normally have to be irradiated for a short time as well. However, network formation is comparably slow so that the bonded components are ready for handling only after a few minutes. Curing is completed after 24 h in most cases.

With some adhesives, the polymerization of light-curing is so fast that sufficient functional strength is achieved at the end of irradiation. Therefore, higher output rates can be achieved in series production, and the materials meet the requirements of active alignment. During this process, optical components are directly aligned in production and must be fixed within seconds. It is often sufficient to quickly cure only one spot for the time being.

Figure 5. Lens of a mobile phone flash light after storage at 120°C for 168 h. The left unit was bonded with an acrylate not resistant to yellowing.
The entire adhesive volume can be cured in another step by light or heat. In doing so, it is very beneficial for optical components that dual-curing types can be cured at just 80°C.

Low-Temperature Curing

Light-curing adhesives can be cured at room temperature. This is an ideal answer to the requirements of optical components in terms of positioning accuracy and thermal loading capacity. However, components may heat up upon intensive irradiation by curing lamps. Black components, for example, can easily heat up to more than 100°C if the radiation intensity is high. With respect to the control of the temperature increase, the use of LED curing lamps is ideal. As the wavelength is optimally adjusted to the adhesiveís photoinitiator, adhesive curing is efficient without delivering radiation in wavelength ranges that does not contribute to curing, but heats up the substrate.

Reliability

Figure 6. Build-up of adhesion of light-curing adhesives. The polymerization of acrylates (blue) is completed directly upon the end of irradiation, while conventional epoxies take several minutes to establish sufficient functional strength.
Especially when bonding optoelectronic components for consumer applications, such as smartphones, there are absolutely no compromises when it comes to the reliability of the connection as the image and reputation of the manufacturer would be severely damaged as a consequence of frequent component failures. Accordingly, adhesives are subjected to harsh reliability tests in order to ensure the durability of the bonded connection.

Reliable Dispensing of Minute Quantities

In the production process of optoelectronic components, the vast number of the components to be bonded per time unit is not the only factor that presents a challenge. In the specific case of these components, it is also necessary to reliably dispense minute adhesive volumes of often clearly less than one milligram. Adhesives for such processes must optimally be adapted to the selected dispensing systems in terms of rheology, and must be absolutely homogeneous.

Figure 7. Temperature progress of a PC lens with a halogen lamp of 60 mW/cm2. The temperature increase can be further reduced by using LED curing lamps.
Even the slightest contaminations or bubbles cause clogging of the dispensing system or faulty dispensing so that rejects are produced as a consequence, or the production line comes to a standstill. In such cases, look for adhesives that are homogenized and filtered by ultra-modern aggregates. They can be applied with all conventional dispensing methods, such as micro jet valves, dispensers or by pin transfer.

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Lighting Technology Magazine

This article first appeared in the July, 2013 issue of Lighting Technology Magazine.

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