Effect of Chain Flexibility of Epoxy Encapsulants on the Performance

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Effect of Chain Flexibility of Epoxy Encapsulants on the Performance and Reliability of Light-emitting Diodes Zhuo Chen, Zhuoyu Liu, Gebin Shen, Ruiheng Wen, Jie Lv, Jizhen Huo, and Yingfeng Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01159 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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Industrial & Engineering Chemistry Research

Effect of Chain Flexibility of Epoxy Encapsulants on the Performance and Reliability of Light-emitting Diodes Zhuo Chen, Zhuoyu Liu, Gebin Shen, Ruiheng Wen, Jie Lv, Jizhen Huo, Yingfeng Yu* State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai, 200433, China. E-mail: [email protected]; Fax: +86 21 6564 0293; Tel: +86 21 6564 2865. KEYWORDS. Chain flexibility, Epoxy encapsulants, LED devices, Reliability. ABSTRACT: Cycloaliphatic epoxy resin (3-4-epoxycyclohexane) methyl 3-4-epoxycyclohexylcarboxylate (ECC) was formulated with flexible hydrogenated bisphenol A diglycidyl ether (HBADGE) to inspect the influence of chain flexibility on the performance and reliability of epoxy packaged Light-emitting Diode (LED). The properties of epoxy encapsulants were characterized by using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), ultraviolet visible (UV−Vis) spectrophotometer, thermomechanical analyzer (TMA), scanning acoustic microscopy (SAM), and scanning electron microscopy (SEM). With the incorporation of flexible HBADGE, favorable properties were obtained, such as decreased thermal expansion coefficient (CTE), lowered storage modulus at

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reflow temperature, and depressed equilibrium water uptake for the epoxy packaging materials. The light transmittance of encapsulants differed after thermal and UV aging. Compared with Neat ECC and Neat HBADGE, the HBADGE-modified encapsulants endowed LED devices with good performance and high reliability. 1. INTRODUCTION Nowadays, light-emitting diodes (LEDs) are extensively applied in various applications, such as large area displays, traffic signals and lighting, due to their high efficiency, low power consumption, and high durability.1-6 The rapid development of LED technology appeals for high performance packaging materials. In this aspect, thermosetting epoxy resins are generally used as major encapsulation materials of LEDs owning to their good mechanical property, great dimensional stability, excellent adhesion performance and optimal operation process.7-10 Among epoxy resins, cycloaliphatic epoxies, especially (3-4-epoxycyclohexane) methyl 3-4epoxycyclohexyl-carboxylate (ECC) cured with acid anhydride are widely employed for the encapsulation of LED and optical devices because of their characteristic properties including excellent processability, high glass transition temperature and UV resistance, superior mechanical and electrical properties.11-14 However, the application of cycloaliphatic epoxy resin in microelectronic and LED packaging fields is often limited referable to its inherent brittleness arising from its high-crosslinked structure, leading to a low resistance to crack development and growth.15, 16 At present time, surface mount technology is widely employed to encapsulate the LED devices. When surface mount device (SMD) LEDs were attached to a printed circuit board, the induced thermal stress mainly affects reliability with respect to package cracking and delamination. It has been reported that moisture uptake and mechanical stress are two major


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factors that greatly impacts on reliability of the LED packages.17, 18 The mechanical stress, which ascribed to the cure stress and cooling stress resulting from the thermal expansion coefficient (CTE) mismatch, was developed during process and operation.19, 20 It had been found for quite a long time in the microelectronic packaging study that the generation of voids, crazes, micro-cracks together with moisture can induce interfacial delamination and pop corning through solder reflow process.17 The internal stress caused during the cold-heating thermal cycling and CTE mismatch can develop the delamination between the integrated circuit (IC) die and encapsulants.21 Therefore, tremendous attentions have been focused to meliorate the interfacial interaction between encapsulants and metal frame and lower the internal stress of packaging materials, for the purpose of performance improvement of the microelectronics.22 In the study and material application of IC encapsulation, multiple approaches have been adopted to improve IC devices reliability. For example, adhesion promoters, like silanes,23 and thiols,24 were applied to improve the interfacial bonding to metals or chips; flexible component parts is introduced to decrease the flexural modulus of the encapsulant25-27 at higher temperature; inorganic filler is loaded to lower the CTE28, 29 of the encapsulants; while modifiers, such as acrylic core-shell particles,30,

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silicon rubber,32 or dendritic hyperbranched polymers,33 can

decrease the internal stress generated during reflow and thermal cycling. Nevertheless, few of the aforementioned methods could be mostly suitable for LED packaging due to the high transparency requirement of LED encapsulants. In our previous work34, we have established a set of methods to evaluate the performance of LED devices and their reliability, furthermore, high-performance packaging materials had been prepared with loading of different kinds of fillers. From another aspect, the performance of an epoxy encapsulant is also determined


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by its chemical composition, however, the effect of chain flexibility of epoxy packaging materials on the LEDs performance and their reliability has not been systematically studied till now. Due to the good mechanical strength and toughness associated with excellent thermal and UV resistance resulting from the stable alicyclic structure and flexibility, the hydrogenated bisphenol A diglycidyl ether (HBADGE) epoxy resin has been employed for light-emitting diode (LED) encapsulation, in attempt to improve the weather ability and to achieve long-term color stability.35-38 Numerous investigations on cycloaliphatic epoxy resin blends have been reported in the literature,39-43 but very little has been extensively discussed about their structure property relationship. In this work, HBADGE with relatively lower polarity and higher flexibility was selected to blend with cycloaliphatic epoxy ECC to investigate the influence of formula change on LEDs performance. The reliability and performance of LED devices encapsulated with these epoxy materials were tested according to the characterization methods established previously. The relationship between LED performance and the structure of epoxy encapsulants was discussed on the basis of physicochemical property study of the materials.

2. EXPERIMENTAL SECTION 2.1

Materials.

Cycloaliphatic

epoxy

resin

(3-4-epoxycyclohexane)

methyl

3-4-

epoxycyclohexyl-carboxylate (ECC, UVR-6110) was provided by Dow Chemical Co. Alicyclic epoxy resin hydrogenated bisphenol A diglycidyl ether (HBADGE) was provided by Emerald Performance Materials (EPALLOY™ 5000). Methyl hexahydrophthalic anhydride (MHHPA, Polynt, Italy) was selected as curing agents and tetrabutylammonium bromide (TBAB, Aladdin Reagent, China) was used as accelerator. High purity silica-filler (quartz, 99.999%) with a mean


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diameter of 5 microns was purchased from Lianyungang Donghai Silica Power Co. Ltd. The chemical structure of materials is illustrated in Scheme 1. All chemicals were used without further treatments.

Scheme 1. Chemical structure of ECC, HBADGE, MHHPA and TBAB. Table 1. The formulations of the epoxy packaging materials investigated. Sample

ECC(wt%)

HBADGE(wt%)

MHHPA(wt%)a

TBAB(wt%)b

Neat ECC

100

0

129

0.39

HBADGE-10%

90

10

124

0.37

HBADGE-20%

80

20

119

0.36

HBADGE-30%

70

30

114

0.34

Neat HBADGE

0

100

79

0.24

a

Epoxies and MHHPA are in stoichiometric ratio. bTBAB is 0.3 wt% of MHHPA.

2.2 Sample preparation. The investigated systems were fabricated by blending ECC and HBADGE with equal equivalence of MHHPA. The HBADGE content in the epoxy mixture was varied from 0 to 100% by weight, and the epoxy packaging materials are denoted as Neat ECC, HBADGE-10%, HBADGE-20%, HBADGE-30% and Neat HBADGE, respectively. For example,


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Neat ECC means the ECC content is 100%, while HBADGE-30% means the HBADGE content is 30%. The formulations of the studied systems are detailed in Table 1. For isothermal curing, the procedure was selected as: 120oC for 0.5h; 160oC for 3.5h; and 200oC for 2h. The mass fraction of silica filler was 40% in the packaging materials when used for LED encapsulation on the purpose of an optimal property. The curing procedure for LED encapsulation was set as: 130oC for 2h; and 150oC for 4h. 2.3 Characterization. The test procedure for LED packaging reliability characterization was detailed in our previous work,34 involving moisture preconditioning, thermal shock cycling test (T/C: -65 to 150oC), pressure cooker test (PCT: 121oC/100% RH/2 atm for 168 hours) and infrared (IR) solder reflow process (260oC×3 times). The encapsulated LED devices were subjected to different moisture sensitivity level tests, MSL 1, MSL 3 and MSL 5 upon IPC/JEDEC J-STD-020. The LEDs were primarily baked to remove residual moisture subsequently implemented with T/C tests or PCT, then exposure to damp environments at 85oC and 85 % RH for 168 hours as MSL 1, 30oC and 60 % RH for 192hours as MSL 3, or 30oC and 60 % RH for 72 hours as MSL 5 followed by three times IR reflow soldering. Differential scanning calorimetry (DSC) was performed using a TA Instruments DSC Q2000 at a heating rate of 10oC/min in a nitrogen atmosphere. Dynamic mechanical analysis (DMA) was measured with a Mettler Toledo DMA/SDTA861e in the double cantilever mode at a ramping rate of 3oC/min with fixed frequency of 1Hz. Thermal gravimetrical analysis (TGA) was collected with a Mettler Toledo TGA1 at a heating rate of 10oC/min under nitrogen atmosphere. Thermal mechanical analysis (TMA) was performed with a Mettler Toledo TMA/SDTA841e in the compression mode at a ramping rate of 3oC/min in a nitrogen atmosphere. The SONIX C-SAM LHF-200 scanning acoustic microscope (C-SAM) was


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employed to analyze fatigue crack, defects and delamination during reflow soldering process. Scanning electron microscope (SEM, Philip XL 39) was used to examine the surface cracking of encapsulated LEDs. Gravimetric measurements of water sorption were implemented at an 85oC deionized water bath, weighed on a TG332A microbalance with a readability of 0.01mg. Transmittance spectrum was recorded on a Perkin-Elmer Lambda 750 UV-Visible spectrophotometer from 300 to 800nm.

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Neat ECC HBADGE-10% HBADGE-20% HBADGE-30% Neat HBADGE

8

Heat Flow Exotherm →

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

2

0

50

100

150

200

250

300

o

Temperature( C)

Figure 1. Dynamic curing study of epoxy systems at a ramp rate of 10oC/min obtained from DSC analysis. 3. RESULTS AND DISCUSSION 3.1 Properties of the Epoxy Encapsulants. 3.1.1 Curing Behavior. The curing behaviors of epoxy/anhydride (MHHPA) catalyzed by quaternary amine (TBAB) were investigated by using DSC as shown in Figure 1. The onset temperatures, peak temperatures and normalized enthalpy were summarized in Table 2. Compared with Neat ECC, the curing onset temperature and peak temperature of Neat HBADGE were shifted to lower temperature, with a reduction of 10oC and


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14oC respectively. For the HBADGE-incorporated systems, it could be found that the onset temperatures and peak temperatures decreased with the increase of HBADGE concentration, while the normalized enthalpy of the polymerization showed little differences with each other. It indicates that HBADGE has higher polymerization reactivity than that of ECC, suggesting an increase of curing rate with the incorporation of HBADGE. Based on the general mechanism of quaternary ammonium salt-catalyzed epoxy-anhydride reactions, the reaction is initiated by quaternary ammonium cation and/or anion formed due to interaction of the quaternary ammonium salt with anhydride or epoxy and propagation proceeds via two reactions involving acylation and esterification. Compared to the alicyclic epoxy groups of ECC, the aliphatic epoxy groups of HBADGE which have less steric hindrance are more favorable to be attracted by nucleophilic anion to polymerization, resulting in higher curing rate and lower curing characteristic temperatures for HBADGE-modified system. Table 2. Curing Characteristic Temperatures and Normalized Enthalpy of Epoxy packaging materials Received from DSC analysis. Systems

Neat ECC

HBADGE10%

HBADGE20%

HBADGE30%

Neat HBADGE

Tonset(oC)

153

152

149

146

143

Tpeak(oC)

198

197

196

196

184

∆Hr(kJ/mol)

97

98

98

100

110

3.1.2 Thermal Properties. The glass transition temperatures (Tg) values of epoxy resins with various flexibility determined by DSC studies are listed in Table 3. The data indicates that Neat ECC provides the highest Tg, and Neat HBADGE shows the lowest Tg. It is observed that the incorporation of flexible aliphatic epoxy HBADGE lower the glass transition temperatures to


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some extent, while the Tg values still remain above 150oC with the addition of 30% HBADGE. Since the thermal aging test was always carried out at 150oC in air and T/C reliability test was conducted as -65oC×15 min and 150oC×15 min, a Tg above 150oC is thus favorable for better performance and higher reliability.


100

80

Weight(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0

100

200

300

400

500

600

700

800

o

Temperature( C)

Figure 2. TGA thermograms of epoxy resins with various flexibility. Table 3. Thermal Properties Parameters Obtained by the DSC and TGA Measurements, respectively. Systems

Neat ECC

HBADGE10%

HBADGE20%

HBADGE30%

Neat HBADGE

a

186

175

170

152

80

328

334

337

340

349

Tg(oC)

b

T5%(oC） a

Tg determined on DSC curves. bTemperature at 5% weight loss.

The TGA thermograms of epoxies with various flexibility are showed in Figure 2. Temperatures at 5% weight loss were employed as an important thermal stability parameter as


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collected in Table 3. It could be found that the decomposition behavior of epoxy materials appear similar to each other. The 5% weight loss temperatures increased with the addition of HBAPGE, suggesting a slightly increase in thermal stability of the HBADGE-incorporated systems, which might come from the stable structure of alicyclic structure of HBADGE without ester bond. 3.1.3 Optical Properties and Weather Resistance. High junction temperature between lead frame and LED die led to thermal degradation of packaging materials, and UV radiation came from LED dies or outdoor resulted in photo degradation, which are great challenges for the reliability of LED packaging.44 The yellowness caused by thermal and photo degradation can decline the transmittance of blue light at a wavelength of about 450 nm, whereas the transmission need to maintain during encapsulation assembly and service life for LED devices. From this perspective, optical properties and thermal and UV resistance of cured epoxies of various flexible parts were evaluated for development of high-performance LEDs encapsulants. The optical transmittances of the cured epoxies with various flexibility are shown in Figure 3. It could be observed that the transmittances of all the packaging materials at 450 nm before thermal aging and UV aging were maintained over 90%, which indicated a favorable optical clarity at the initial light transmission. Light transmission of the epoxy encapsulant is closely relevant to wavelength and may fluctuate with the degree of degradation that occurred during aging process. HBADGE-30% was selected as a representative of modified systems with various flexibility, and its aging behaviors during thermal and UV aging were investigated to verify the capacity of thermal and UV resistance. Upon aging, the transmission curves moved towards the higher wavelength and reduction in transmission occurred in lower wavelength, as Figure 4 presented. The transmittance of the HBADGE-30% at 450nm was still above 80% after UV aging, which is quite close to the


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performance of Neat ECC indicating fine weather resistance after addition of suitable amount of HBADGE.35

100

(a)

95

Transmittance/(%)

90 85


80 75 70 65 60 55 50 400

500

600

700

800

Wavelength/(nm)

100

(b) 95 90

Transmittance/(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



85 80 75 70 65 60 55 50 400

500

600

700

800

Wavelength/(nm)

Figure 3. Optical transmittance for epoxy packaging materials (a) before thermal aging and (b) before UV aging.


11


(a)

100 90 80

HBADGE-30%-0h HBADGE-30%-12h HBADGE-30%-24h HBADGE-30%-48h HBADGE-30%-72h HBADGE-30%-124h HBADGE-30%-175h

Transmittance/(%)

70 60 50 40 30 20 10 0 300

400

500

600

700

800

Wavelength/(nm)

(b) 100 90

HBADGE-30%-0h HBADGE-30%-12h HBADGE-30%-24h HBADGE-30%-48h HBADGE-30%-72h HBADGE-30%-124h HBADGE-30%-175h

80 70

Transmittance/(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 50 40 30 20 10 0 300

400

500

600

700

800

Wavelength/(nm)

Figure 4. Optical transmittance versus wavelength for HBADGE-30% (a) the thermal aging behaviors and (b) the UV aging behaviors.


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35


(a) 30

Yellowness Index

25

20

15

10

5

0 0

20

40

60

80

100

120

140

160

180

Time(h)

(b)

20


16

Yellowness Index

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

8

4

0 0

20

40

60

80

100

120

140

160

180

Time(h)

Figure 5. Change in YI for epoxy systems during (a) thermal aging and (b) UV aging, respectively. The yellowness index (YI) is a parameter used for evaluating yellow discoloration resulted from thermal aging and UV aging and could provide more detailed information for thermal and UV discoloration, which is calculated by the equation as below:34


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YI = 100 × ( − T )/T

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(1)

where the wavelengths at 620, 520, and 470 nm were corresponding to red, green and blue lights emitted by the RGB type LED devices respectively. Figure 5 presents the changes in yellow index for cured epoxies of various flexible parts during thermal and UV aging. It could be found that, Neat ECC system provided the best performance with the lowest degree of discoloration and smallest change ranges in YI in both thermal and UV aging. Compared with Neat HBADGE, the flexible HBADGE parts in the modified systems showed minimum effect on the discoloration during aging process, as change in YI appeared very close to that of Neat ECC. It indicates that the HBADGE-modified systems exhibit better resistance to discoloration than Neat HBADGE system, which is attributed to the better UV and weathering resistance of cycloaliphatic epoxy, and deterioration effect of flexibility on the optical performance of packaging materials. 3.2 Performance of LED Devices and their Reliability. 3.2.1 Luminance Change during reliability Test. The luminous intensity and luminous flux change are important aspects for the performance and reliability of LED devices, which has been discussed in our previous study. Based on the light transmittance study of the cured epoxies with various flexibility, the introduction of flexible parts showed some negative effect on the thermal and UV resistance, which may reduce luminous intensity of LED devices, decrease their luminous flux, and deteriorate their performance and reliability. The hygrothermal aging experiment was conducted in 85oC and 85% RH to investigate the performance of LED devices packaging with various flexibility encapsulants (as shown in Figure 6). Only the trend of blue light versus aging time was listed in this article, due to the little change of luminous flux change for red and green light. It could be found that Neat ECC system offered


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the best performance as the luminous flux maintained above 95% after 1000 hours aging, which agreed well with UV-vis results that cycloaliphatic epoxy possessed better weathering resistance. The HBADGE modified systems showed a slightly lower luminous performance, however, Neat HBADGE was much lower than the other systems.

100

Percentage of luminous flux(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

80


70

60

50 0

200

400

600

800

1000

Aging time(h)

Figure 6. Blue light luminous flux change of LEDs packaged with various flexibility encapsulants under hygrothermal aging test at 85oC and 85% RH for 1000 hours. 3.2.2 Reliability Test. As long-term hygrothermal aging behaviors was investigated above, more standard reliability test methods were involved to evaluate the performance of LED devices and their reliability. In this work, moisture/reflow sensitive level 1, level 3 and level 5 tests upon IPC/JEDEC J-STD-020 have been employed as the reliability test criteria for LEDs, meanwhile the luminance change during the reliability tests was taken as an important index for performance evaluation. Reliability test outcomes of LEDs encapsulated with epoxy of various flexible parts were collected in Table 4. It could be found that, the reliability performance of LED devices


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encapsulated with different packaging materials suffered significant changes from each other during the test process. For example, 12% of LEDs failed during reflow packaged with Neat ECC, while the systems encapsulated with HBADGE-30% and Neat HBADGE passed without failure. 64% of LEDs packaged with Neat HBADGE was invalid through 300 rounds of T/C test, whereas the HBADGE-30% modified packaged system passed reflow, PCT and 100 rounds of T/C perfectly, with only 2% failure during 300 rounds of T/C. It indicated that HBADGE-30% modified system performed well under these test conditions and superior reliability performance were obtained with the incorporation of flexible HBADGE. Table 4. Reliability test outcomes of LEDs encapsulated with epoxy of various flexible parts. Test item

Neat ECC HBADGE- HBADGE- HBADGE- Neat 10% 20% 30% HBADGE

-65-150 oC, 5T/C

200/200*

200/200

200/200

200/200

200/200

30 °C, 60 % RH, 192h

200/200

200/200

200/200

200/200

200/200

MSL 3 Reflow Profile 3 rounds 176/200

188/200

194/200

200/200

200/200

PCT

147/200

163/200

176/200

200/200

200/200

-65-150 oC, 50T/C

152/200

160/200

183/200

200/200

200/200

-65-150 oC, 100T/C

136/200

151/200

175/200

200/200

194/200

-65-150 oC, 300T/C

95/200

112/200

147/200

196/200

73/200

*pass/test pieces. Luminous flux change for blue light of LEDs encapsulated with epoxy of various flexible parts during reliability test was showed in Table 5. It could be observed that, except for system encapsulated with Neat HBADGE, all the other systems consistently exhibited better performance during reliability test, which was in accordance with the previous UV-Vis test results.


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Table 5. Luminous flux change for blue light of LEDs encapsulated with epoxy of various flexible parts during reliability test. Test item

Neat ECC HBADGE- HBADGE- HBADGE- Neat 10% 20% 30% HBADGE

-65-150 oC, 5T/C

99.9%

99.90%

99.90%

99.80%

99.60%

30 °C, 60 % RH, 192h

101.20%

100.50%

100.80%

100.60%

99.80%

MSL 3 Reflow Profile 3 rounds 99.70%

99.80%

99.80%

99.80%

97.50%

-65-150 oC, 100T/C

99.5%

99.60%

99.70%

99.70%

96.80%

-65-150 oC, 300T/C

99.4%

99.50%

99.40%

99.40%

92.50%

Figure 7. C-SAM images of LEDs with various delamination level at the die-attach/pad interface during reliability tests. The samples in order are Neat ECC, HBADGE-10%, HBADGE-20%, HBADGE-30% and Neat HBADGE from left to right. 3.2.3 Delamination observation. After moisture/reflow sensitive level test, the C-SAM (scanning acoustic micrograph) images of LED devices at the die-attach/pad interface were presented in Figure 7. As one can see, almost total delamination was found in the LED device


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packaged with Neat ECC, while the introduce of flexible HBADGE ameliorates the status, no delamination was observed for Neat HBADGE after IR reflow, which may due to the lower internal stress of the packaging material system.

(a)

(b)

(c)

(d)

Figure 8. SEM micrographs of the interfaces for encapsulated LED devices after Level-5 test. (a) The cross-sectional micrograph for packaged LED; (b) HBADGE-30% epoxy encapsulant and Ag/Cu layer; (c) HBADGE-30% epoxy encapsulant and polyphthalamide frame; (d) Ag/Cu layer and polyphthalamide frame.


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After 300 rounds of T/C, except for the HBADGE-30% modified packaged system, complete delamination took place in the other LED devices, especially the Neat HBADGE encapsulated system, as its lowest Tg (80oC) deteriorating the reliability performance during thermal shock cycling test. It indicated that incorporation of flexible epoxy parts affords the packaging materials to withstand severe conditions, and HBADGE-30% showed the balanced performance. In order to determine the site of delamination of LED devices, SEM was applied to examine the surface cracking of encapsulated LEDs, and the cross-sectional micrographs of LED encapsulated with HBADGE-30% were showed in Figure 8. For HBADGE-30% modified encapsulated system, the least delamination was observed between the Ag/Cu and encapsulants or between polyphthalamide frame and encapsulants, except the crack observed between the Ag/Cu layer and polyphthalamide frame. 3.3 The Relationship between the Reliability of LED devices and Properties of Epoxy Materials. Up to now, in-depth discussion of the performance of LED devices encapsulated with epoxy of various flexible parts have been conducted as well as their reliability, from the aspects of luminous intensity, luminous flux change and reliability test. To better understand the relationship between flexibility of epoxy packaging materials and reliability of LED devices, more detailed properties of epoxy materials need to be investigated. 3.3.1 Internal stress factors. The CTE mismatch in LED encapsulants can induce a significant thermo-mechanical stress between wire bond and chip in the bonding zone and cause the generation of thermal stress through the IR solder reflow, resulting in fatigue crack propagation and delamination between the bonded surfaces.19, 45 Figure 9 illustrates the TMA experiment curves of epoxy packaging materials of various flexibility, and the results of CTE values both below and above Tg are collected in Table 6. The


19


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Page 20 of 35

Tg values determined as the point of intersection of the expansion curves of TMA are also collected in Table 6 and coincidence with those determined from the DSC test. As one can see, for the HBPAG-modified epoxy systems CTE below Tg were essentially the same, while a rapidly decrease with a reduction of 15% was observed for CTE above Tg as compared to Neat ECC. When temperature is below Tg, the chain motion is restricted, and thus CTE values of the epoxy with various flexible parts below Tg appear to very close to each other. However, as we have find in the TGA study of better performance of HBPAG-modified epoxy systems, the incorporation of flexible chain into the epoxy network facilitates conformational rearrangements and averts the highly restricted conformation caused by highly cross-link density of ECC network, and thus results tightly accumulation of polymer chains . When the temperature increase above Tg, the restricted conformation rearranges for Neat ECC generating a higher CTE, while better arranged polymer chains in modified systems results in a reduction of CTE.

Table 6. The Values of CTE and Tg Obtained from TMA Analysis. Systems

α (ppm*K-1) Tg(oC)

HBADGE10%

HBADGE20%

HBADGE30%

Neat HBADGE

Below Tg 69

69

68

67

74

Above Tg 212

184

181

180

201

198

188

179

165

82

Neat ECC


20

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Sample dimension change(µ m)

(a)

Neat HBADGE HBADGE-30% HBADGE-20% HBADGE-10% Neat ECC

70

60

50

40

30

20

10

0 50

100

150

200

250

300

o

Temperature( C)

(b)


350

300

-1

250

α /ppm• K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

150

100

50 50

100

150

200

250

300

o

Temperature( C)

Figure 9. TMA experiment curves of epoxy packaging materials with various flexibility (a) sample dimensional changes with temperature and (b) variations in CTE values with temperature. It was reported that internal stress between two material interfaces produced by CTE (thermal expansion coefficient) mismatch during thermal cycling test or reflow soldering process can be obtained by integral of modulus and CTE versus temperature.32


21


(a) 3.5


3.0

Log E'

2.5

2.0

1.5

1.0

0.5

0.0 50

100

150

200

250

300

250

300

o

Temperature ( C)

(b)


1.2

1.0

0.8

Tan δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

0.6

0.4

0.2

0.0 50

100

150

200 o

Temperature ( C)

Figure10. Dynamic mechanical study of epoxy systems (a) storage modulus and (b) loss tangent. Since the curing procedure for LED encapsulation is conducted at 150oC, which was lower than Tg values of packaging materials, the thermal stress at this temperature could be taken for zero, thus internal stress during T/C procedure and IR solder reflow could be calculated by:34 ⁄ = ⁄ × (150 − ) ×

(2)


22

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= × (260 − ) × + × ( − 150) ×

(3)

where ⁄ and are the internal stress during T/C procedure and IR solder reflow process, respectively. ⁄ , and are the concerned elastic modulus through the test process, and Tg, α1, and α2 are the glass transition temperature, and CTE under and over Tg. According to Eq. 3, concerning the Tg values of LED encapsulants is pretty close to 150°C, it could be concluded that is arch dependent on the elastic modulus and CTE above Tg. Table 7. Thermal and Mechanical Properties of LED Encapsulants. Systems

Neat ECC

HBADGE10%

HBADGE20%

HBADGE30%

Neat HBADGE

a

245

241

226

223

97

Tg(oC)

210

202

178

167

73

Er(Mpa)

30

13

11

8

2*

23

12

11

8

2

1.66

0.88

0.84

0.61

0.19

Tg(oC)

b c

d

G'(Mpa)

ρ (10-3 mol•cm-3) a

Tg defined as the summit temperature for loss tangent. bTg defined as the onset temperature for storage modulus. cThe elastic modulus at IR reflow temperature. dThe Elastic modulus in the rubbery region at Tg+30oC. The DMA analysis of the storage modulus and loss tangent of epoxy materials with various flexibility are presented in Figure 10. The values of glass transition temperatures and elastic modulus are summarized in Table 7. Based on the rubber elasticity theory,46 the cross-linking density ρ for the packaging materials was obtained from the equilibrium elastic modulus

!

in

the rubbery region above the α-relaxation temperature through following equation: ρ = G! /∅RT

(4)

where R, ∅, and T are the universal gas constant, the front factor and the Kelvin temperature, respectively. Tg+ 30oC is used as the temperature for the equilibrated elastic modulus, where Tg


23


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Page 24 of 35

is identified as the summit temperature for loss tangent. The data of elastic modulus of rubbery region G’ and the cross-linking density ρ calculated from Eq.4 were also summarized in Table 7. The Tg values identified as the summit temperature for loss tangent or the onset temperature for storage modulus were moved towards lower temperatures with the incorporation of HBADGE, which is line with the discoveries obtained from the DSC and TMA tests, i.e. the introduction of flexible parts decreased the Tg of the epoxy system. Based on the above discussion, lower elastic modulus at temperature over Tg is an possible solution for delamination, as minimizing the thermal stress during reflow soldering process. Concerning the elastic modulus of epoxy materials through solder reflow, it could be observed that HBADGE30% system (except Neat HBADGE) had the lowest storage modulus, with a 73% reduction of storage modulus as compared with Neat ECC. The storage modulus at 260oC of the modified epoxy systems showed the same tendency with Tg, as well as the calculated cross-linking density ρ declined with the addition of HBADGE. As is mentioned above, elastic modulus and CTE over Tg are two main factors that determine the thermal stress through reflow soldering, indicating the lowest thermal stress for HBADGE-30% during IR reflow. 3.3.2 Moisture study. Moisture uptake by the packages during lifetime as well as thermal stress induced by CTE mismatch is regarded as the major reason for the delamination in electronic encapsulants.47, 48 Moisture diffuses into the encapsulant, swells the encapsulant, and increases the delamination by stimulating hygro-mechnical stress development and deteriorating interfacial adhesion performance.49 From this point, it is of importance to investigate the water uptake of encapsulants and its influence on the performance of LED devices and their reliability. Figure11 presents the water uptake behavior of epoxy packaging materials with various flexibility conducted by gravimetric measurements in 85oC hot water. The properties of


24

Page 25 of 35

packaging materials are closely related to equilibrium moisture uptake and diffusion coefficient. It was asserted that the behavior of moisture absorption in epoxy network can be described by Fick’s second law, and the diffusion coefficient is well fitted the following equation:50 '(

')

=*

+ √-

√./ √0

(5)

where M2 and M3 are the moisture absorption content versus time and the equilibrium moisture content for infinitely diffusing, respectively. L is the sample thickness and D is the diffusion coefficient. The moisture absorption content versus time ( M2 ) of the materials is calculated by the below equation: M2 =

45 67( 75

× 100%

(6)

where W0 is the original weight for the dry sample, and Wt is the measured weight for the wet sample versus time.

5


4

3

Mt(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

0 0

2000

4000

6000 1/2 -1

1/2

8000

10000

-1

t d (s cm )

Figure11. The water absorption curves of encapsulants with various flexibility conducted by gravimetric measurements.


25


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Page 26 of 35

Table 8 summarizes the equilibrium water sorption of all samples, and the diffusion coefficient gained by curve fitting Eq. 5. The content of moisture absorbed is closely dependent onto the polarity, free volume, cavities and structural defects, and the interactions between polymer and water molecules including the hydrogen bonding capability.17 In our previous study,51, 52 we have discovered that free volume and polarity are two major factors which influence moisture absorption behavior in epoxy resins. The equilibrium water absorption is primarily determined by polarity, while free volume mostly influences the diffusion coefficient. Table 8. Calculated Results from Gravimetric Measurements.

Systems

Neat ECC

HBADGE10%

HBADGE20%

HBADGE30%

Neat HBADGE

Mmax/%

3.95

2.89

2.64

2.37

1.69

D/10-8·s-1cm2

2.66

3.08

3.19

5.21

7.51

Adamson et al.53 have postulated that moisture diffusion below Tg is carried out by following process: firstly, the absorbed moisture occupies the free volume; secondly, water becomes bound to network sites, causing swelling; finally, water enters the densely cross-linked regions. Nevertheless, the availability of a larger free volume within the polymer results in increasing diffusion coefficients in the first state, a higher resin polarity accounts for more absorbed water molecules in the second and third state. It could be found that, the equilibrium water sorption is positively correlated to the polarity of the epoxy materials. The high polarity of ECC is beneficial for moisture diffusion and absorption onto high polar region. The hydrophilic nodules attract water molecules and attach them by hydrogen bonds, leading to more equilibrium water uptake. The equilibrated moisture uptake of the modified encapsulants declined with the incorporation of HBADGE due to its lower polarity.


26

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Neat HBADGE had the lowest equilibrium water sorption among the other epoxy systems, with a reduction of 57% of the Mmax value compared to Neat ECC. However, the diffusion coefficients of the epoxy materials showed reversed tendency compared to the equilibrium water uptake, it increased with the addition of HBADGE due to its higher free volume. The free volume of the cured epoxy resins is strongly related to the flexibility of the epoxy resin.54 The higher the flexibility of the epoxy resins, the greater the molecular chain mobility is, leading to looser molecular chain packing and the higher free volume. Compared with ECC, HBADGE exhibits more flexibility due to the flexible ether linkage resulting in higher free volume for the HBADGE-modified systems. As free volume mainly determines the transport of moisture, the free volume of the epoxy network increases with increasing HBADGE content resulting in faster diffusion process for water molecules diffusing through the vacancies of the free volume. The properties of CTE, internal stress and water uptake behavior of the epoxy with various flexibility have been investigated, it could be concluded that the reliability of LED devices and their performance have closely connection with the properties of the encapsulants. The HBADGE-30%, which possessed the minimum values of CTE both under and over Tg, as well as the lowest elastic modulus and crosslinking density, indicated the best reliability performance and least thermal stress within all the packaging materials. Combined with the lower equilibrium moisture content of the modified system, delamination and pop-corning could be significantly diminished during reflow soldering with the improvement of reliability and performance. 4. CONCLUSIONS Combined with reflow soldering process, thermal cycling test, moisture measurements and luminous test, the performance of LED devices and their reliability showed close relationship


27


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Page 28 of 35

with the flexibility of epoxy packaging materials. The glass transition temperatures were shifted to lower value with the incorporation of flexible parts, and the Tg values obtained from DSC, TMA, and DMA corresponded well with each other. The temperatures at 5% weight loss form TGA demonstrated the improvement of thermal stability for the epoxy packaging materials by the incorporation of flexible parts. The values for CTE have been declined with the incorporation of flexible parts. Compared with Neat ECC and Neat HBADGE, all the modified systems exhibited a reduction of CTE values, especially a rapidly decline above Tg. The system of HBADGE-30% showed a reduction of 15% in CTE above Tg, which performed best among others. The storage modulus at reflow temperature showed the same tendency of CTE values, as well as the crosslinking density of the epoxy systems. The UV-vis test and luminous intensity test showed that the introduction of flexible parts showed moderate effect on the optical performance of the LED devices during thermal, UV and hygrothermal aging. The incorporation of HBADGE lowers down the equilibrium water sorption content as compared with Neat ECC. The system of HBADGE-30% showed favorable properties for LED packaging materials, as its balanced properties as the lowest CTE values, minimum storage modulus at high temperature, relatively lower equilibrium water uptake, good UV and thermal stability, to withstand the reliability test. ACKNOWLEDGMENT This research work was supported by the National Natural Science Foundation of China (Grant 21274031).


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For Table of Contents Only

The performance and reliability of LED devices showed extensive relationship with the chain flexibility of epoxy packaging materials, which showed an optimized performance of HBADGE30% due to the balanced property of lower internal stress, water sorption and better transparency.


35

Effect of Chain Flexibility of Epoxy Encapsulants on the Performance

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