Processing LEDs

LEDs allow the creation of completely new products with fascinating characteristics and SMEs have been successful in developing new light sources or replacements for incandescent lamps. However, electronics for LED products are different to standard electronics because, in most cases, the circuit is the final product. Thus the substrate serves as a support for electronic components, it guarantees the correct positioning of the LED and it is an important element for temperature management. Generally the substrate is not a standard PCB material but is made from composite or is a film type circuit.



Because of this, printing, pick&place, curing and soldering systems used for the manufacturing of LED products has to be adaptable. Machines must be able to produce prototypes and series with little effort but high quality. A combination of semiautomatic printer, automatic SMD pick&place and full convection reflow oven has proven to be the most suitable for small to medium production.



Exact positioning and alignment of the LED in the circuit is key. Solder paste printing and placement are the two processes with the biggest influence on LED positioning. As a result, stencil printers and pick&place machines must be built robustly and feature optical aids for positioning.



Most stencil printers align the substrate and the stencil using a vision system. An alternative technology is the look-through method whereby the camera is positioned above the stencil and looks through it to the substrate. This is the preferred technology used by stencil printers for small to medium series production as it is simple, fast and reliable.



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An LED pick&place machine must be able to place LEDs of different sizes, shape, transparencies and with different connection layouts. For example, Essemtec’s Pantera-XV placement machine includes multiple alignment systems. It can accurately place any kind of component and LED, and even features a PCB holder that is adaptable to any shape and an exchangeable vacuum table for foils.

For LED handling, the vacuum tools must be able to hold the component firmly but must not damage lenses or surfaces. A Teflon coating on the tool tips is suitable for most cases. However, for odd shaped LEDs, product specific tools often have to be designed and manufactured.



Reflow, selective radiation soldering or curing

Several techniques connecting LEDs either use solder or conductive adhesives. The latter is mostly used on foil substrates that cannot withstand the high temperatures of soldering. There is no standard, the adhesive typically is chosen for a specific application and in some cases even developed especially for it. Some adhesives are cured by temperature whilst others are UV or even IR radiation.

In some applications, the LED is fixed with a non-conductive glue dot which guarantees the LED does not shift when handling the substrates or during reflow soldering. The application of such glue dots requires precision on position and amount as the non-conductive glue must not cover any contact area while still holding the LED firmly in place.



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For glue dot dispensing, there are specialised dispensing machines such as Essemtec’s CDS series and some highly flexible pick&place machines, including the Pantera-XV, incorporate a dispensing system.

Selective radiation soldering enables the low temperature soldering of LEDs on PET or Mylar foils. Essemtec developed the process and a soldering machine in collaboration with a membrane keyboard manufacturer. IR radiation is absorbed in the process only by the solder joint and not by the foil. The substrate foil must be transparent for the chosen IR wavelength and does not heat, only the solder joints do.



Efficient & economical turnkey solutions

Manufacturing LED products requires flexible and highly accurate machines and specific process know-how. Start-up companies as well as small and medium enterprises, however, must concentrate on product development and customer acquisition. They do not have time to build up production know-how from scratch. Therefore, it is advantageous to collaborate with a partner that can deliver machines, process know-how, education and support from one source.



Real turnkey solutions from the likes of Essemtec simplify the selection of machines. The effort to install, education and support is less than when using a patchwork line from different manufacturers and with such partners, the responsibilities are clear and the supplier knows all machines and processes.