Reliability verification of LED phosphor at the packaging end

LED (Light Emitting Diode) is a light-emitting component. Its structure is actually a PN junction of a semiconductor. The basic working mechanism is an electro-optical conversion process. That is, when a forward bias is applied to both ends of the PN junction, due to the reduction of the PN junction barrier, the positive charge in the P region will diffuse to the N region, and the electrons in the N region will also diffuse to the P region, and at the same time, an accumulation of unbalanced charges will be formed in the two regions. For a real PN junction device, the carrier concentration in the P region is usually much greater than that in the N region, resulting in the accumulation of unbalanced holes in the N region being much greater than the accumulation of electrons in the P region (for NP junctions, the situation is just the opposite). Since the minority carriers generated by current injection are unstable, for the PN junction system, the unbalanced holes injected into the valence band must recombine with the electrons in the conduction band, and the excess energy will radiate outward in the form of light, thereby achieving the effect of luminescence. LED has the advantages of long working life, low power consumption, fast response time, small size, light weight, impact resistance, easy dimming, color adjustment, high controllability, green, and environmental protection. It can be expected to be widely used in many light source markets. Especially the SMD white light LED, which can be used in backlight, decoration and lighting, and its future application market will become increasingly popular and gradually extend to other fields.

With the widespread use of SMD white light LEDs (the main raw materials are shown in Figure 1 below), phosphors have become an indispensable part; a large number of manufacturers have smelled this business opportunity (the cost structure of SMD white light LED packaging is shown in Figure 2 below, the information comes from Korea's Displaybank) and have begun to develop in the field of phosphor production, resulting in an endless stream of phosphor brands on the market.

LED phosphors can also be called rare earth phosphors. Because rare earth element atoms have rich electronic energy levels, there are 4f orbitals in the electronic configuration of rare earth element atoms, which creates conditions for multiple energy level transitions, thereby obtaining a variety of luminescence properties (as shown in Figure 3 below):

LED phosphors can be roughly divided into the following categories according to their preparation methods: high-temperature solid-phase method, combustion synthesis method, sol-gel method, spray pyrolysis method, hydrothermal synthesis method, and chemical co-precipitation method. Among them, most phosphor manufacturers mainly use the solid-phase reaction method to prepare phosphors (the advantages and disadvantages of each preparation method are roughly shown in the following table).

Preparation method	advantage	shortcoming
High temperature solid phase method	Simple process	High synthesis temperature and high energy consumption
	Phosphor crystallization is good	Severe product agglomeration
	High brightness	Coarse particles
Combustion Synthesis	Short reaction time	The product is impure
	Low synthesis temperature	Low phosphor brightness
Sol-Gel Method	Uniform ingredients	Complex process and long reaction cycle
	High reactivity	Phosphor crystallization is poor
	Low synthesis temperature	Low brightness
Spray pyrolysis	Uniform ingredients	There is a cavity in the middle of the particle, which is easy to collapse
	Spherical particles	Micropores on the particle surface
	Good dispersibility
Hydrothermal synthesis	Easy to control purity and particle size	The use is limited and the equipment requirements are high
		Low phosphor brightness
Chemical coprecipitation	Even mixing, low synthesis temperature	The technology is not mature enough
	The process is simple and suitable for industrial production	The product is prone to agglomeration

sort by color	Application
Blue phosphor	UV chip + blue phosphor + green phosphor + red phosphor
Green phosphor	Blue light chip + green phosphor + red phosphor
	Blue light chip + green phosphor + yellow phosphor
Red phosphor	Blue light chip + green phosphor + red phosphor
	Blue light chip + red phosphor + yellow phosphor
Yellow phosphor	Blue light chip + yellow phosphor

Phosphors can be roughly divided into the following categories according to their composition: aluminate phosphors, silicate phosphors, nitride (or oxynitride) phosphors, and sulfide phosphors. Aluminate phosphors and silicate phosphors account for the majority of applications (the excitation efficiency of each component of phosphor is roughly shown in the following table).

Ingredient Classification	Stimulation efficiency
Aluminate	excellent
Silicate	excellent
Nitride/ Nitrogen Oxide	Difference
Sulfide	excellent

Generally speaking, the evaluation items of phosphors include efficiency evaluation, chromaticity evaluation, reliability evaluation and evaluation of other related parameters. Most LED packaging manufacturers focus on efficiency evaluation and chromaticity evaluation.

According to the current market situation, due to the different process technology capabilities of various phosphor manufacturers, their product performance also has its own advantages and disadvantages. Of course, some merchants, in pursuit of huge profits, add some organic powders or inorganic salts (such as sulfides) to phosphors to pass off inferior products as good ones. Therefore, the focus of phosphor evaluation is no longer limited to the efficiency and chromaticity of the phosphor itself, and its own reliability evaluation has become increasingly important.

At present, the relevant industry organizations issued the corresponding standards on November 17, 2009 to regulate the production of phosphors (People's Republic of China Electronic Industry Standard SJ/T 11397-2009 "Phosphors for Semiconductor Light Emitting Diodes"), and its official implementation time is January 1, 2010. This standard stipulates the terms and definitions related to phosphors for semiconductor light emitting diodes, and also stipulates the requirements, test methods, detection rules and marking, packaging, transportation and storage requirements for aluminate and silicate phosphors for semiconductor light emitting diodes. However, I personally think that this standard is actually more suitable for phosphor manufacturers to verify their own products, but it cannot better guide LED packaging factories to verify the reliability of phosphors.

So how do SMD LED packaging factories choose phosphor products with better stability among the numerous phosphor brands? Here, I propose the following four phosphor stability verification schemes for everyone to discuss:

A. Heat resistance verification of phosphor:

You may encounter this problem: some end customers report that the color temperature of our products shifts after REFLOW. What material causes this shift? Is it related to the phosphor? It is difficult to separate the various raw materials and analyze them.

So how should we determine the heat resistance of the phosphor in the early stage? As we all know, when the phosphor is packaged into a SMD product, it needs to be baked at about 150℃, and when the end customer uses it, the SMD product will be assembled on the PCB through REFLOW, and the highest reflow temperature is 260℃. In other words, when the phosphor is used in the early stage, the highest temperature it withstands is 260℃. Therefore, we can set the temperature of the heat resistance experiment at 260℃.

From the above, we can see that we have the following two verification schemes to choose from:

	Solution 1	Solution 2
Program Content	Place the phosphor in an oven at 260°C and bake directly	Place the phosphor in REFLOW for baking
Heating method	Constant temperature heating	Saddle curve heating
Heating curve	Figure 4	As shown in Figure 5 (this information comes from JEDEC)
advantage	Sustainable heating	How to get close to customers

Since our end customers actually use a saddle-shaped temperature curve, in order to better simulate the customer's usage method, it is the best choice to use Option 2 to verify the heat resistance of the phosphor. Perhaps before you conduct this experiment, your phosphor supplier will tell you that the sintering temperature of the phosphor is above 1000℃, so it is unnecessary to bake the phosphor at 260℃, and the phosphor will not decompose no matter how long it is baked. But don't forget that there are many post-processing steps in the phosphor manufacturing process (such as coating). Can the phosphor particles after the post-processing steps also withstand baking at 260℃ or even above 1000℃ without affecting the excitation efficiency of the product? If other substances are added to the phosphor, can this substance withstand 260℃ without decomposition? This depends on the post-processing process of each company.

The following are related experiments conducted by our company using Nitto's eight-zone lead-free reflow soldering (upper eight and lower eight zones). Figure 6 is a phosphor sample with better reliability, and Figure 7 is a phosphor sample with poorer reliability. The experiment proves that this solution can effectively verify the reliability of phosphors.

B. Verification of the moisture resistance of phosphors:

When our end customers use our products, the surrounding environment has a certain humidity. If our products have color temperature deviation, is this caused by the phosphor itself? And how should we verify the humidity resistance of phosphors in the early stage?

Normally, when the phosphor and encapsulation glue are fully mixed and cured, the encapsulation glue itself will play a certain role in moisture-proofing and moisture-proofing, thereby protecting the phosphor from hydrolysis. However, each encapsulation glue itself has a certain degree of airtightness, that is, water vapor can penetrate into the encapsulation colloid to varying degrees and react with the phosphor; therefore, the moisture resistance of the phosphor is greatly affected by the airtightness of the encapsulation glue. The airtightness of existing encapsulation glues is roughly as follows:

Packaging glue	Airtightness
Epoxy	excellent
Silicone Rubber	Difference
Silicone	middle

Since the air tightness of each packaging glue is different, the results of moisture resistance verification using the same phosphor will be different. In this way, we cannot verify the moisture resistance of the phosphor more objectively;

In order to more objectively verify the moisture resistance of phosphors (including possible added substances), we have two options. One is to soak the phosphors in neutral water, and the other is to store the phosphors in a high humidity (90%RH) machine. When the first solution is used for verification, the humidity can be regarded as 100%RH, but this solution is more demanding for phosphors (especially silicate phosphors. From our company's current experimental results, as shown in Figure 8 below, almost all phosphor manufacturers' products cannot pass this experiment); when the second solution is used for verification, the water contacts the phosphors in gaseous form, which is closer to the actual product failure mechanism. (Even for silicate phosphors, from our company's current experimental results, as shown in Figure 9 below, it is found that the products of some foreign phosphor manufacturers have better moisture resistance than those of the same industry.)

C. Thermal stability test of phosphor

The heat resistance of phosphor is different from thermal stability. Heat resistance focuses on the early performance of phosphor, which is a transient verification; while thermal stability focuses on the later performance of phosphor, which is a relatively long-term verification. Although some phosphor testers in the industry can test the excitation efficiency of phosphor samples at different temperatures, most testers only heat the bottom of the sample powder tray. In fact, when the phosphor tester tests the sample, it receives the spectrum excited by the phosphor on the surface of the sample powder tray (the principle is shown in Figure 10). Since the thermal conductivity of the phosphor itself is relatively poor, if only the bottom of the powder tray is heated to test the phosphor sample, the value obtained is very inaccurate. This is because the machine may set the heating temperature to 120℃, and the actual temperature of the powder tray is also 120℃, but the phosphor on the surface of the powder tray is far below this temperature.

A relatively better way is to use space heating, that is, to place the entire powder tray in hot air so that the phosphor sample on the surface of the powder tray is fully heated. The test results of this method are more accurate. However, the equipment cost will be relatively expensive. If the LED packaging factory conducts this verification, the initial equipment investment cost is relatively high. Therefore, a more reasonable verification scheme is to fully mix the phosphor with the packaging glue to solidify it into a finished product, and then perform high-temperature aging. (Of course, when using this scheme for verification, you should use chips with better reliability and packaging glue with better airtightness and better anti-attenuation performance, etc.), and then compare its attenuation data after a certain aging time (usually aging 1000HRS).

D. Test of the UV resistance of phosphors
Most LEDs in the industry now use blue light chips and phosphors to produce white light. However, the blue light chip itself has a certain amount of energy in the ultraviolet part. The shorter the chip band, the more energy there is in the ultraviolet part. The phosphor itself absorbs part of the ultraviolet energy and converts it into visible light (as shown in Figure 11).

When the phosphor absorbs ultraviolet energy, it will also accelerate the aging of the phosphor itself. Especially when the post-processing technology of the phosphor is poor and some organic powders or inorganic salts (such as sulfides) are added to the phosphor, its anti-ultraviolet performance becomes more important.

Based on the above four solutions, their respective characteristics are as follows:

project	name	effect
Solution 1	Heat resistance	Verify the high temperature resistance of the phosphor in the transient state in the early stage
Solution 2	Moisture resistance	Verify the moisture resistance of phosphors
Option 3	Thermal stability	Verify the high temperature resistance of the phosphor in the later stage when it is in a stable state
Option 4	UV resistance	Verify the performance of phosphors against UV energy

The heat resistance, moisture resistance, thermal stability and UV resistance of phosphors determine the reliability of phosphors. Only when these four properties reach certain standards can they meet the requirements of white light LEDs. I hope everyone can evaluate the most suitable phosphor for their products among the many phosphor manufacturers.