The new generation package free LED - IEEE Xplore

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Taiwanse LED chip manufacturers Epistar, Forepi, TSMC. Solid State Lighting, Lextar, as well as package manufacturers, such as Unistars are preparing to ...
The new generation package free LED: The performance and reliability of White Chip

C.K.Y. Wong l , G.Q. Fan l , S.Y.Y. Leung l , C.C.A. Yuan l , G.Q. Zhang2 1 State Key Laboratory of Solid State Lighting (Changzhou Base), F7 R&D HUB 1, Science and Education Town, Changzhou, 213161, China 2 Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, Delft, the Netherlands Abstract Package free LED, whose characteristics as no leadframe/ reflector, is one of those emerging technology [1]. The package free LED has arose excessive attention in recent years because of its benefits in excellent color consistence, wide light distribution, low material cost and simple process. This study assesses the performance of "sheet forming type" package free LEDunder operational and humidity conditions. The lumen maintenance study shows insignificant degradation of opto-electrical performance of the devices. Color shift is also minimal.In the accelerated humidity condition, the device failed mostly due to die failure. Minimal optical degradation was found in the phosphor layer. Moisture induced failure which affects the interfacial integrity should be addressed so as to improve the reliability of the package free LED. Introduction Since Ql of 2013, package free LED (Figure. 1) has become a controversial topic among LED industry. Big companies including Toshiba, Cree, Philips Lumileds, Taiwanse LED chip manufacturers Epistar, Forepi, TSMC Solid State Lighting, Lextar, as well as package manufacturers, such as Unistars are preparing to enter this market by the end of year 2014 [2, 3].

Figurel. Different types of white chip/package free LED devices Recently, the performance of the package free LED from four manufacturers namely Lumileds, TSMC, Epistar and Sanan was evaluated by China CESI (Guangzhou) Laboratory and HKUST LED-FPD Technology R&D Center [4]. The report shows the initial luminous efficiency and chromaticity tolerance of thesepackage free LED samples at about 100lmlW and 4-8 SDCM respectively.While the report gives the preliminary parameters of the packages at time 0, the reliability of the packages was not studied. Being a new package concept, the reliability of thepackage free LED is a concern that determines the applicability. Package free LED consists of substrate, flipchip and a phosphor layer. The phosphor layer, as interacting with the

flipchip, is the key components in terms of both light extraction and color conversion. The optical as well as mechanical properties of thephosphor layer are key factor that defines the lifetime of package free LED. The delamination or crack thatinduced during usagecan deteriorate the device substantially. This paper evaluates the reliability of a high power package free LED formed by phosphor sheet molding. Accelerated testsof the package free LED according the IES LM-80 [5] and JESD 22-AII0-B[6]standard were obtained. The degradation in terms of lumen maintenance and color shift of the packages is reported. To understand the cause of degradation, material test in complementary to the accelerated Our results tests of the devices was conducted. showinsignificant deterioration in the LM-80 preconditioning. The study yet demonstrates the package free LED devices are moisture sensitive. The devices fail under humidity condition because of the blue LED failure. No significant change in the optical properties of the phosphor layer.Delamination of the phosphor layer from the LED was the cause of color shift in the moisture acceleration test. The study shows the application potential of the package free LED in high power device. Moisture issue needs to be addressed in order for wide application. Methodology Several methods are adopted in manufacturing of the package free LED, including electrophoresis phosphor followed by molding, spraying followed by molding, phosphor sheet molding, etc. Among these methods, sheet molding, due to the reduced material waste, simple manufacturing step, makes it to be more commercially viable. In the sheet molding process, a semi-cured silicone sheet loaded with phosphor would be formed first. The semi-cured phosphor sheet is then molded on top of well aligned LED dies. Following by curing, the assembled devices are singulated. In this study, we use the sheet molding package (White Chip) manufacturing by Nitto Denko to make white chip LED devices (Figure 2). To evaluate the new packages, the optoelectrical performance of White Chip and reliability upon accelerated test was investigated. Package reliability data of 1000 hours according the IES LM-80 [5] and JESD 22-AII0B[6]standard were obtained.

114 978-1-4799-6697-4 ©IEEE 2014

properties such as light transmittance and reflective index is related to light extraction of the LED dies, and thus the package performance. Light extraction is also influenced by the adhesion between the silicone material and the die surface. To undercover the cause of the device failure, aging in terms of material level under high temperature, high humidity and irradiation has been conducted. The experiment has been designed based on the testing condition in the device level. Several acceleration factors including high temp and humidity has been addressed. To simulate the operation condition of the device, a setup so as to accelerate the aging of the phosphor material under high temperature and blue irradiation has been built (Figure 4). Figure 2. White Chip LED device samples A flow chart describes the methodology of is shown in Figure 3. The accelerated test has been divided into device and material level. The device acceleration aims at investigating the package performance under high temperature and high humidity. Lumen maintenance and color shift was evaluated in the device level.

4l1blue light module of 23W each .-

..

.-\"~~m.;.., .~~.. 1It~"":.

Iig ht accelerate-d test samples

...

,

Figure 3. Flow diagram of white chip LED reliability test

1

2

25°C 55°C

3

85°C

4

85°C/85%RH

Samples White chip devices x6 White chip devices x6 White chip devices x6 White chip devices x7

Test LED LED

Condition

LED LED

1

85°C(with blue irradiation)

2

85 °C (without blue irradiation)

Opto-electrical @ Oh-l OOOh Opto-electrical @ Oh- IOOOh Opto-electrical @ Oh-l OOOh

3

Opto-electrical @ Oh, IOOOh

Table 19ives the experimental details for the LM-80 operational and the humidity test in the device level. In the operational test, the package samples was light-up in an aging chamber under 25°C, 55°C or 85°C. The sample size was 6 under each condition. Opto-electrical data of the samples was obtained at certain interval (Table 3, 4 and 5) until 1000 hours. The humidity test was conducted inside 85/85%RH chamber under non-operational condition. The sample size was 7. Opto-electrical data of the samples was obtained until the end of the test at 1000 hours. Silicone material is the key to produce the sheet molding package which determines the lifetime of a device. Optical

I

I,

At

~~~i~

6"-

Figure 4. Light accelerated and high temp test setup The aging has been conducted under 3 different conditions as illustrated in Table 2: Table 2. The pre-conditioning and test details of the material test

Table 1. The pre-conditioning and test details of the device test Condition

'-, . Uf

~

-1-- ~ ~ i,;·~· .b·-.

-~I'~

,

----=---.

'i .

-~

85°C/85%RH

Samples Silicone sheet x3 Phosphor sheet x3 Silicone sheet x3 Phosphor sheet x3 Silicone sheet x3 Phosphor sheet x3

Test Transmittance @ Oh, 1000h Reflectivity @ Oh, 1000h Transmittance @ Oh, 1000h Reflectivity @ Oh, 1000h Transmittance @ Oh, 1000h Reflectivity @ Oh, 1000h

Figure5 shows the samples in the material test. We have prepared two types of sheet samples: the silicone sheet (without phosphor loading), Figure 5 left; and the phosphor sheet (silicone sheet blend with phosphor powder), Figure 5 right. The material degradation under different aging conditions was quantified by the light transmittance and reflectanceof the samples before and after 1,000 hours. Transmittance of the bare silicone sheet and the reflectivity of the phosphor sheet were measured by Konica Minolta Spectrophotometer(CM-3700A). The change in transmittance and reflectivity is reported.

115

Figure 5. Silicone sheet samples (left: silicone sheet; right: phosphor sheet)

o.2269±0.0006 0.5046±0.0007 o.2266±0.0006 0.5065±0.0008 o.2266±0.0006 0.5068±0.0008

264

3.067

106.23±0.96

3884±17

432

3.088

106.41±0.90

3864±17

696

3.087

108.47±1.21

3860±20

912

3.075

106.05±l.l 0

3855±21 0.2270±0.0006 0.5063±0.0008

1000

3.080

107.35±1.01

3863±20

o.2269±0.0006 0.5061±0.0008

With the opto-electrical data, the lumen maintenance and color shift has been calculated in Figure 5 and Figure6.Figure 5 illustrates the lumen maintenance of white chip under an operation test in 1,000 hr at 25, 55 and 85 DC respectively.As shown in Figure 5, lumen maintenance of white chip LED devices are all greater than 98% which is very close to the performance of the conventional LED devices (Table. 6). 1.1

Results

Reliability test for White Chip device Table 3-5 summaries the opto-electrical results of the operational test. Table 3. Operational test result at 350mA/25°C Time (hr) ViV) Luminous eff.(lm/W) CCT(K)

u'

y'

0

3.0695

107.93±1.64

3913±10 o.2262±0.0003 0.5044±0.001

24

3.0785

105.37±1.21

3901±10 o.2262±0.0003 0.5051±0.001

48

3.075

106.21±l.l9

72

3.076

106.32±1.41

o.2260±0.0003 3903±10 o.2260±0.0003

144

3.079

107.57±1.36

3889±11 0.2261±0.0003 0.5059±0.001

264

3.071

108.02±1.34

3892±10 0.2261±0.0003 0.5058±0.001

432

3.104

107.74±1.11

o.2263±0.0003 3870±11 o.2262±0.0003 3868±8 o.2265±0.0003 3881±9 o.2263±0.0003

696

3.099

109.70±1.36

912

3.077

107.46±1.32

1000

3.089

108.85±1.41

3907±9

3868±7

0.5051±0.001 0.5054±0.001

0.5069±0.001 0.5071±0.001 0.5065±0.001 0.5061±0.001

Figure5. Lumen maintenance of white chip LED devices in the operation test Figure 6. summarizes the color shift of white chip LED devices after accelerated reliability testfor 1,000 hours. As we can see, it is all less than 0.003 under all the test conditions.

Table 4. Operational test result at 350mA/55°C Time (hr) ViV) Luminous eff.(lm/W) CCT(K) 0

3.084

107.72±0.75

3926±32

24

3.077

107.53±1.62

3919±30

48

3.065

108.14±l.l9

3925±30

u'

0.005

y'

o.2264±0.0007 0.5032±0.0016 o.2265±0.0007 0.5035±0.0013 o.2263±0.0007 0.5036±0.0013

72

3.071

108.53±0.80

3928±30 0.2261 ±O.0007 0.5036±0.0015

144

3.067

108.31±1.02

3917±30

264

3.078

108.00±0.94

3922±27

432

3.102

107.59±0.68

3897±28

696

3.082

109.56±0.94

3880±24

912

3.088

105.98±0.37

3879±23

1000

3.086

107.70±0.66

3884±241 0.2267±0.0006 0.5051±0.0011

0

3.064

107.89±1.25

3911±18

24

3.064

107.66±1.06

3908±19

48

3.062

107.48±1.03

3906±18

72

3.065

106.36±0.94

3901±19

144

3.072

107.25±0.97

3895±18

0.003.

>,

o.2263±0.0007 0.5039±0.0014 o.2263±0.0007 0.5036±0.0014 o.2264±0.0007 0.5049±0.0013 o.2264±0.0007 0.5060±0.0013 o.2268±0.0007 0.5051±0.0012

u'

:::;l

.......... 25"C

Cl

"""-S5 DC

0.002.

---S5 DC

0.001

Time/h

Table 5. Operational test result at 350mA/85°C Time (hr) ViV) Luminous eff.(lm/W) CCT(K)

0.004

y'

o.2264±0.0006 0.5040±0.0008 o.2264±0.0006 0.5042±0.0008 o.2264±0.0006 0.5044±0.0008 o.2265±0.0006 0.5046±0.0008 o.2265±0.0006 0.5048±0.0007

Figure6. Color shift of white chip LED devices in the operation test From the above reliability test result, we found that in the high temperature opertional test (LM-80), both the lumen degradation and color shift is minimal. The lumen loss is less that 2%. The color shift, duv is less than 0.003 which corresponds to 2 sdcm as given in Figure 7. 116

Transmittance 1OOOh (Silicone sheet) Reflectivity Oh (Phosphor sheet) Reflectivity 1000h (Phosphor sheet)

000

0. ')35

u'

Figure 7. Color shift of a sample under 85°C operation test Table. 6 compares the performance of the White Chip with the commerialized device. Both the lumen maintainence and color shift are comparable with the conventional device. The result implies that application potential for the White Chip ™ packages. Table 6. Lumen maintenance and Duv compare between white chip LED and Luxeon rebel [7] White chip LED 1,000 hours

---------------------Luxeon Rebel

99.63@25°C Lumen maintenance

98.50@55°C 98.63%@85°C 0.0019@25°C

Duv

0.0019@55°C 0.0023@85 °C

98.5%@85°C

0.00 15@55 °C 0.0028@85 °C

In the moisture sensitivitytest, out of the 7 samples only 2 of them can still be lit up after the 1,000 hours aging. The opto-electrical parameters of the survived samples is summarized in Table 7. Table7. Opto-electrical data of 85/85 samples NO.

1# Oh 1# 1000h 2# Oh 2# 1000h

VF (V)