SMT, PCB Electronics Industry News

Manufacturing LED Products Requires Holistic Concepts

Dec 18, 2013

A Holistic Concept
Electronics for LED products are different than standard electronics because, in most cases, the electronic circuit is the final product. Therefore, the substrate not only serves as a support for electronic components but it guarantees the correct positioning of the LED and it is an important element for temperature management. Building such products requires a holistic approach in the production chain.

Based in Dorset, SafeTest Ltd. is a leading UK manufacturer of customer-specific LED products. In 2003, the company started with electronics testing and repairing for Parker Hannifin devices and continuously expanded its design and production services and capabilities. Today, SafeTest’s portfolio includes all services from circuit design to finished product shipment. This enables the company to realize high-quality LED applications quickly from design to final tested products.

Special Requirements for the Equipment
In many cases, the substrate of LED products is not a standard rectangular PCB but is made in special shapes or from special material such as composites or film. Therefore, 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 low effort but prime quality. A mixture of semiautomatic printer, automatic SMD pick-and-place and full convection reflow oven has proven to be most suitable for small to midsize production.

Exact positioning and alignment of the LED in the circuit is a key quality feature for a LED product. Printing and placement are the two processes that influence LED positioning the most. Therefore, stencil printers and pick-and-place machines must be built very robust and must feature optical aids for positioning.

High Precision Is Needed for Printing and Placement
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.

A LED pick-and-place machine must be able to place LEDs of different size, shape, transparency and with different connection layouts. As an example, the FLX placement system from Essemtec includes multiple alignment systems. This machine is capable of accurately placing any type 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 sometimes 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 mostly is used on foil type substrates that cannot withstand the high temperatures of soldering. There is no standard — the adhesive generally is chosen for a specific application and, in some cases, is even developed especially for it. Some adhesives are cured by temperature, others by UV and some by IR radiation.

In some applications, the LED is additionally fixed with a nonconductive glue dot. It guarantees that LEDs do not shift when handling the substrates or during the reflow soldering process. The application of such glue dots requires very precise control of position and amount because the nonconductive glue must not cover any contact areas but still must hold the LED firmly. For glue dot dispensing, there are specialized dispensing machines as well as highly flexible pick-and-place machines that already include a dispensing system, allowing them to perform both dispense and placement.

A special process is available to solder LEDs onto PET or Mylar foils using low-temperature solder known as selective radiation soldering. Essemtec in Switzerland has developed this process and a soldering machine in collaboration with a membrane keyboard manufacturer. The principle is as follows: The IR radiation is absorbed only by the solder joint but not by the foil. The substrate foil must be transparent for the chosen IR wave length and does not heat; only the solder joints do.

A Successful Concept
To fulfill the demands for high quality and quick realization of LED electronic products, the equipment must be efficient, flexible and reliable. Production machines must meet high-quality requirements and must be productive even for small runs. When purchasing an automatic assembly line, SafeTest learned that most machines were fast but also large, inflexible and complicated in operation. But there was one exception.
Essemtec in Switzerland is specialized in the production of both flexible and efficient production systems. It is the only supplier that can deliver a complete production line and the necessary knowhow from one source. SafeTest installed a highly flexible production line consisting of an SP600 stencil printer, a FLX2011-V pick-and-place and an RO300FC full convection reflow oven. The size and user interface of these machines fit perfectly with SafeTest’s concept.

Highest Quality and Flexibility
SafeTest has certified its services according to ISO-9001. The company also qualifies for the production of safety-related products in the medical, aviation and aerospace industries. For example, SafeTest produces mobile LED lighting for aircraft runways.

“SafeTest has achieved the unachievable.” This customer feedback confirms that SafeTest has succeeded where others have failed and it proves that the company’s holistic concept and its advanced production equipment are ideal for the efficient realization of complex, high-quality LED products.

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