LEDs | electrical and electronic
Numerous materials have been developed specifically to tackle this issue. Polymers used for LED packages, for example, need
to have exceptional high temperature performance, since they are very close to the energy source. They also need to be able to form a perfect bond with the lead frame and they must be highly reflective to extract the most benefit from the LED. That reflectivity needs to stay high over the lifetime of the product, which means that discoloration caused by thermal degradation has to be as low as possible. Typical polymers used for this application include polyphthalamides, liquid crystal polymers, and high-heat polyesters. DSM recently added to the material offering with
Stanyl ForTii LED LX, a special grade of its high temperature resistant polyamide 4T. Stanyl ForTii LED LX is a halogen-free flame retardant grade. It is mineral-filled, but still has good flow, and DSM says its excellent UV resistance sets a new standard for LED packages. “Stanyl ForTii LED LX delivers a good reliability performance over time and, more specifically, high initial reflectivity, as well as excellent mechanical strength,” says Stijn Meijers, Global Segment Manager for Lighting. High reliability and LED lifetime are enabled through
the material’s high adhesion to silicones and epoxy encapsulates as well as the metallic lead frame, avoiding chip degradation due to moisture or air diffusion through interfacial delamination phenomena. Earlier this year Ticona introduced a new generation of its Thermx polycyclohexylene-dimethylene tereph- thalate (PCT) thermoplastic polyesters that are said to deliver outstanding initial reflectance and reflectance stability required in LED packages found in display backlight and general lighting. Two high performance super-white fiber reinforced
grades, Thermx LED 0201 and Thermx LED 0201S, are claimed to meet the challenging requirements of LED reflectors, especially for high brightness, medium- power LEDs. Ruth Castillo, Thermx PCT and Vectra/ Zenite LCP global business director, says Thermx PCT “provides superior colour and reflectance stability under heat and light compared to commonly used high temperature polyamides.” Apart from the very good thermal and reflective
properties, Ticona also cites excellent silicone adhe- sion, processability and mechanical properties; surface mount technology reflow capability; and low moisture absorption. Thermx LED 0201S is intended for more demanding applications, such as flat panel screens, ow- ing to its higher break strain and impact strength. PCT resists temperatures up to 260°C for short periods, making it lead-free solderable. It has very good electrical performance, high CTI and arc resistance, high dielectric strength and low dielectric constant, good flow, and the same shrinkage as PBT. “These grades provide customers with much better
reflectance stability than PA 9T and PA 6T nylons at high temperatures,” Castillo says. “Now, they can mould reflectors with excellent surface characteristics that maintain UV and color stability, providing superior long-term performance.”
Conductive options For LED designers, improving thermal management is a leading factor towards improving LED efficacy (light output per energy input) and lowering overall cost. Designers are challenged to increase lumens while preventing overheating of solid state circuitry, which will drastically shorten the service life of the device (now expected to last for up to 20 years). Typically metals such as aluminum are used for heat sinks, sock- ets, trim rings and housings to help conduct heat away from the solid state device. But aluminium parts are costly to make, and their carbon footprint is quite large. Thermally conductive (TC) compounds provide an
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Above: DSM’s Stanyl ForTii LED LX
polyamide 4T is said to have the heat resistance required for LED packages
Left: RTP uses special additives to create diffusing compounds to soften light from LEDs
April 2013 | INJECTION WORLD 23
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