An LED Processing System Must Be Accurate and Flexible

By Adrian Schärli, Azular GmbH, for Essemtec AG

LEDs allow the creation of completely new products with fascinating characteristics. Small and medium enterprises have been successful in developing new light sources or replacements for incandescent lamps.

Special Requirements

Electronics for LED products are different than standard electronics because, in most cases, the electronic circuit is the final product. Therefore, 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 material or is a film type circuit.

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

High Precision Is Needed

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

Most stencil printers align the substrate and the stencil with a vision system interposed between them. An alternative technology is the look-through method in which 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 midsize series production because it is simple, fast and reliable.

An LED pick-and-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. This machine is capable of accurately placing any kind of component and LED. This pick-and-place system 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-shape LEDs, product-specific tools oftentimes must be designed and manufactured.

Reflow Soldering, Selective Radiation Soldering or Curing

There are several techniques to connect LEDs, either using solder or conductive adhesives. The latter is mostly used on foil type 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, while others are cured by UV or even IR radiation.

In some applications, the LED is fixed with a non-conductive glue dot. This guarantees that LEDs do not shift when handling the substrates or during the reflow soldering process. The application of such glue dots requires precise control of position and amount because the non-conductive glue must not cover any contact area while still holding the LED firmly in place. For glue dot dispensing, there are specialized dispensing machines such as Essemtec’s CDS series. Some highly flexible pick-and-place machines including the Pantera-XV already include a dispensing system, allowing them to dispense as well as place.

A special process, known as selective radiation soldering, is available to solder LEDs onto PET or Mylar foils using low-temperature solder. Essemtec has developed this process and a soldering machine in collaboration with a membrane keyboard manufacturer. The principle is that IR radiation is absorbed 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.

Real Turnkey Solutions Are Efficient and Economic

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 a supplier such as the Swiss manufacturer Essemtec simplifies the selection of machines. The effort to install, education and support is less than when using a patchwork line from different manufacturers. With Essemtec as a partner, the responsibilities are clear and the supplier knows all machines and processes.

For 15 years, Essemtec Switzerland has been developing and manufacturing production systems for electronics, especially for SMD assembly and semiconductor manufacturing. For manufacturing LED products, Essemtec has built many standard machines and customer-specific solutions for dispensing, printing, placement, curing and soldering. Today, Essemtec is one of the most experienced suppliers for LED assembly solutions and knows exactly what is important for this market.