It is a major trend. Light-emitting diodes (LEDs) are becoming ever smaller. “This opens up entirely new possibilities for fitting LEDs even more closely together, which is particularly advantageous for applications requiring high luminance,” explains Stefan Mausner, Segment Marketing Manager at the 2018 International LED professional Symposium (LpS) in Bregenz ( see interview ). Examples include vehicle headlights and UV LED modules, which contain very tightly packed LEDs for exceptional brightness and high energy density.
Technical challenges with micro LEDs
“This progressive miniaturization, however, also brings with it a number of technical challenges, which must not be underestimated,” Mausner stressed during the 2018 LpS in Bregenz. The specific question facing manufacturers is how to successfully connect micro LEDs, which can have an edge length of less than 150 µm, successfully connected electrically when standard solder materials are no longer suitable?
Sinter paste for effective heat dissipation
Heat can be transported from the LED chip to the ceramic or metal substrate via the die-attach material, which connects the chip. Heraeus has specially developed a sinter paste for LED applications, which has a thermal conductivity value twice as high as gold-tin or tin-silver alloys. The heat loss can therefore be dissipated much faster, resulting in higher luminous efficacy and ultimately protecting the LED chip from damage.
Ultra-fine solder pastes for electrical connection
Extremely fine materials are required to electrically connect the micro LEDs on the backplane, as micro LEDs can have an edge length of less than 150 µm. Conventional solder pastes, with particle sizes of between 20 and 45 µm, cannot meet the bond line thickness and stencil opening requirements, meaning that solder alloys with a particle size of less than 11 µm are called for. Heraeus WS5112 type 7 solder paste, made with patented Welco solder powder, allows the printing of tiny solder bumps of up to 50 µm.