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Electronics Production | September 23, 2009

Trends in the electronic industry: bonding technology enables increasingly shorter process times

Today, fast processes are in demand, in particular, in the electronic industry. Design and production engineers have to deal with ever reducing cycle times and the need for economical assembly processes.
Therefore, adhesives are required which can be cured as fast as possible after application and joining of the components. Products like highly optimized lightcuring acrylates or epoxies and new fast heat-curing epoxies make a considerable contribution to added value for companies – for example, in relation to: the bonding of mobile phone loudspeakers, the sealing of OLED displays, or the fixing of miniature components to printed circuit boards.

Well-established products, such as light-curing acrylates and epoxies are generally distinguished by short curing times and are, therefore, required in the optimization of processes, for example, if components have to be fixed rapidly. Although they have already conquered numerous application areas, new technologies or changing needs of the customers require continuous enhancement. Their greatest advantages are easy processing, which is especially suitable for highly automated processes and curing in seconds without temperature application, which enables fast in-line production.

Fast light curing in seconds
Therefore, the application opportunities in electronics are numerous. Today, elements of miniature loudspeakers (for example, for mobile phones) are nearly exclusively bonded with light-curing acrylates. Several adhesives are used which bond the different materials – both metals and plastics. One important requirement: The adhesives must be flexible in order to absorb vibrations.

Only that way, good acoustic properties of the loudspeakers is ensured. At the same time, the adhesive must keep the loudspeaker components together even under harsh conditions. For this purpose, tests are carried out, for example, a drop test from a height of 1.5 m or climatic change tests.

In the production process, the individual elements are bonded rapidly in 2 to 3 seconds by means of the light-curing adhesives. Working with light curing enables the complete integration into the automated production process and fast cycles.


Illustration 1: Bonding of mobile phone loudspeakers

UV-curing adhesives for OLED technology
New display technologies like OLED (Organic Light Emitting Diode) open up completely new opportunities, for example, for color-intensive mobile phone displays or, in the long term, even for TV screens. The component consists of a luminous array of semiconductive layers, similar to a classic LED. However, the individual layers are composed of organic chemicals.

The extremely sensitive structure of the OLEDs must be protected against humidity, for example, by enclosing it between two glass plates to prevent its decomposition. The OLED layer structure is produced on a glass substrate and a cover glass plate is bonded to this substrate by means of an adhesive which is dispensed around the OLED layer structure. Normally, the adhesive is irradiated with UV light and cured under pressure, from through the glass substrate. After a typical period of time of 30 to 60 seconds the adhesive is so firm that OLED processing can continue.

Significant amounts of humidity or oxygen cannot reach the inside, neither through the glass substrate nor through the adhesive layer. The development of suitable adhesives with extremely low permeation rates for oxygen and humidity is, therefore, a decisive precondition for the realization of OLEDs with a sufficient lifetime. Previous UVcuring products were not up to the requirements for low humidity transmission.

Therefore, a new generation of UV-curing epoxies has been developed. Contrary to many currently available products, postbaking of these encapsulation compounds is not necessary. They reach their full efficiency exclusively through light curing. That allows much shorter process times in mass production and, therefore, leads to significant cost savings.


Illustration 2: In the first step of the encapsulation process, the adhesive is applied. Afterwards, the cover glass plate is attached and pressed. In the last step, the OLED sealants are cured by means of UV light. Many other standard products require a further heat curing step.

Highly efficient curing lamps
Processes can also be optimized by the development of highly efficient curing lamps. With this, polymerization of photoinitiated-curing adhesives and UV lacquers are possible, which until this point, have been nearly exclusively cured with classic discharge lamps. An LED can be switched on and off as often as required and can light only in that period of time when actual curing is required.

As switching on and off does not have any influence on the service life, but only the total hours of operation, the effective service life of the LED increases by a multiple. A further advantage is that the intensity of light emitting diodes can be continuously controlled between 0 and 100 percent via the applied current. Moreover, it is easy to integrate this lamp into a production process as the outer edge of the LED head is only slightly larger than the lamp head itself. Therefore, the space requirements within the production plant are extremely low. Thus, this new development makes a considerable contribution to optimization in fast and reliable processes.


Illustration 3: Highly efficient LED curing lamp DELOLUX 80

Shortened production flows with fast heat-curing epoxies
Heat-curing epoxies, which have been in use in the automotive and electronic sectors for years, normally cure slower than UV- or light-curing adhesives. Therefore, their enhancement is a special challenge for the developers as properties like high strength, good chemical resistance, and usability at high temperatures must be maintained. With the especially developed one-component epoxy resin adhesives considerably shorter curing times can be reached.

At the same time, the important product properties are maintained. As it is almost impossible to process two-component adhesives in dispensing amounts of tenths of milligram, the use of one-component heat-curing adhesives is ideal – in fast production flows, minimal curing times from six seconds on are possible by means of thermode processes. These adhesives are in special demand in industrial production, for example, if miniature components are to be fixed to printed circuit boards – fixing of coils, bonding of sensors, and sealing of housing halves are only some example applications. The majority of components to be produced require especially fast processes and short curing times. Also in microelectronics, e. g., when bonding flip-chips in smart label applications, epoxies are used.


Illustration 4: Bonding of miniature components to printed circuit boards

The applications of adhesives in electronic production are numerous: the bonding of miniature components to PCBS, the sealing of housings, the encapsulation of new display technologies like OLEDs or the bonding of mobile phone loudspeakers. These applications confront engineers with ever new challenges. Tailor-made products for special applications, which comply with shortest cycle times and fast process times, are in demand. Light- and UV-curing acrylates, new curing technologies as well as new heat-curing epoxies make a considerable contribution to increased added value by reducing the processes during in-line production.
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Author: Dipl.-Ing. Rainer Dörfler, Product Manager, DELO Industrial Adhesives

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