Light Emitting Diodes

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Light Emitting Diodes

Light emitting diodes are light emitting dies that get their color or hue through the phosphor that is coated on top of the die. They are typically blue tinted and are getting their color from a combination of the phosphor, the die light intensity and the overmolded optoelectronic molding compound. There are multipl LED types for a huge variety of applications, ranging from simple through hole mono chrom LEDs to backlight, organic, miniLEDs and industrial baylights. All of these LED types have the same principle but completely different application and reliability requirements.

General Considerations For LED Encapsulants

As LED encaplants, besides general consideration of packaging materials, optical performance with long-term reliability is specially emphasized.

Challenges & Solutions

Challenges	Problem Description	Encapsulation Solutions
Light Extraction	Refractive index mismatch between LED die (~1.58) and encapsulants, affecting energy output efficiency	High RI (refractive index) encapsulant
Delamination	Stress failure caused by CTE mismatch among encapsulant, die and lead frame during curing or temperature shifting; Moisture absorption and ions corrosion Adhesion issue	Matched CTE and low modulus materials Excellent adhesion to substrate Moisture resistance material with high purity
Thermal-Yellowing	Thermal degradation and discoloration induced by high junction temperature between die and leadframe	High Tg epoxy encapsulant with high cross-link and strong INP Add anti-yellowing agents Silicone based encapsulants
UV-Yellowing	Photo degradation induced by UV radiation from LED die and outdoor UV	UV transparent encapsulant Silicone based encapsulants
Aging/Lifetime	Aging caused by thermal and UV degradation Aging will result in mechanical failure, transmission decay and discoloration	Above All
Sensor Transmission	High transmission within application wavelength to avoid disturbing signals	Add dyes as cut-off or bandpass filter

Application requirements

RGB SMT Display Applications:

Junction temperature is low; Packages are 0.2W – 0.5W; 20mA current
Contrast is more important than brightness (hence the black body)
Silicone is not used as it is clear molding compound application

Mid Power White LED Applications:

Junction temperature is 85°C-125°C; Package: 0.1W – 0.5W;
Replaces more expensive alternative (silicone) as encapsulant
Stability against heat & UV
Highest transmittance in visible & IR wavelengths; Earliest transmittance onset (280 nm)

Other Considerations --- Indoor vs Outdoor Application

Indoor uses diffusant while outdoor uses a low reflectivity (reduce glare)
Indoor Packages: 1010,2121,3528; Outdoor Packages: 3535 & 5050
Outdoor requires UV resistance & low moisture absorption
Outdoor applications must pass these tests:
- 85°C /85%RH + 3x 260°C reflow
- Thermal Cycle tests: -65°C to 150°C for 200 cycles

LED Product overview

	Monochrome LEDs		HD RGB LEDs		White LEDs
Product	TC-8020 (Star Product)	TC-8040	TC-7600 (Popular Product)	TC-7800 (In Development)	TC-8030	TC-8060 (Popular Product)	TC-8090 (In Development)
Summary	Good adhesion to BT substrate/metal leadframe Adequate flexural modulus/strength Excellent anti-reflow performance. Good processability and moldability		Great moisture resistance property Enable LED display pass hash durability tests Ensures uniform colours and uniform emitting light at large angles Reduce warpage		Excellent anti-yellowish performance at high temperature Reduced light decline by blue/white light Excellent adhesion to Ag/Cu and PCB substrate Good moldability and higher reliability Two optional forms: cylindrical tablets, and fine powder
Applications	7-segment LED display, compact LED indicator, monochrome Chip LED, photo-link		High density (fine-pitch) RGB LED display		Decorative White LED	Backlight	High Reliability Backlight
Light Decline Aging Test	-	Medium 10-50% RT20mA500hrs	Medium 10-50% RT5mA1000hrs		Low <50% RT20mA500hrs	High <10% RT5mA1000hrs
Reliability Tests	Reflow, TCT, 85℃@85%RH		Reflow, TCT, 85℃@85%RH, PCT		Reflow, TCT, 85℃@85%RH
Highlight	Low stress and filter versions available	High reliability Mid blue-ray decay	Low blue-ray decay	Low blue-ray decay High anti-yellowish	Low cost	Low blue-ray decay	Low blue-ray decay Anti-moisture