Characterization of Molded Underfill Material for Flip Chip Ball Grid Array Packages Fenny Liu, Y. P Wang, Kevin Chai and T. D. Her Siliconware Precision Industries Co., Ltd. No. 123, Sec. 3, Da Fong Rd, Tantzu Taichung, Taiwan, R.O.C. Tel: 886-4-5341525 ext 3532, Fax: 886-4-5339099
[email protected]
Abstract Flip chip technology has been utilized for more than 30 years in the field of electronic package. It was developed to meet the requirements of excellent electrical performance, small device, high input/output (I/O) density and high speed. Underfill which is used to fill the gaps between solder bumps can enhance the fatigue life of solder bumps due to CTE mismatch between chip and substrate. Underfill materials usually need longer cure time to cure in process and with higher cost. Therefore, the molded underfill is studied to replace the capillary flow underfill for providing higher productivity, no particle setting and less expensive materials. The molded underfill material has to contain smaller filler size to fill the small gap between solder bumps and fast cured time. The development of molded underfill material is different from conventional underfill and molding compound material. The material characterization of molded underfill material was analyzed with various methods. The curing condition was measured by Differential Scanning Calorimeter (DSC). Thermo-gravimetrical Analyzer (TGA) was used to study the weight loss of materials. Thermal Mechanical Analyzer (TMA) was used to investigate the coefficient of thermal expansion (CTE) of the cured materials. Dynamic Mechanic Analyzer (DMA) was used to measure the storage modulus of the cured materials. In this paper, the curing conditions and materials properties such as CTE (coefficient of thermal expansion), Tg (glass transition temperature), storage modulus and weight loss were evaluated from the measurement results for molded underfill materials. The results were also compared with conventional underfill materials and molding compound materials.
problem all been reported in previous studies. Figure 1 shows the structure of conventionally underfilled FCBGA. Recent process advances is able to provide a better solution on aforementioned problems by using transfer molding technology and Figure 2 shows the structure of molded underfilled FCBGA. The transfer molded underfill technology is a simplified one step process to offer fast process, high throughput volume, high productivity and low cost of ownership for flip chip [1-10].
Figure 1.The structure of underfilled FCBGA.
Figure 2. Molded underfilled FCBGA structure.
Usually, the void is easy to cause at the undergap of die and substrate for capillary flow underfill material. The void may increase the stress on the chip. In addition, the CTE mismatch nature of capillary flow underfill between the chip, substrate and solder bumps cause the delamination, stress and warpage issues. To overcome issues, the molded underfill material can provides the lower CTE, no void setting and good warpage performance. The smaller filler size of molded underfill material is easy to fill the small gap between solder bumps. The undergap is completely filled by mold compound to ensure the solder bump reliability. Moreover, the molded Introduction underfill material also provides good moisture resistance. Flip chip technology was developed more than 30 years Therefore, the molded underfill material was developed to ago. It is widely used in the assembly of high performance replace the capillary flow underfill [2]. devices. Flip chip technology can offer excellent advantages The molded underfill material is a new material for such as highest input/output (I/O) density, enhanced electrical molded underfill technology. It is very important to evaluate performance, better thermal characteristics at smaller die the material properties, operating process and reliability areas. To improve the fatigue life of solder bumps due to CTE concerns. In this study, the properties of material were mismatch between chip and substrate, underfill process is measured by using thermal analysis instruments such as usually necessary. Traditionally, underfill material was Differential Scanning Calorimeter (DSC), Thermodispensed to the periphery of flip-chip die with designed gravimetrical Analyzer (TGA), Thermal Mechanical Analyzer dispensing pattern, underfill material then flows underneath (TMA) and Dynamic Mechanic Analyzer (DMA). The the die to fill the gaps between bumps and substrate by material properties such as curing condition, relation between capillary flowing phenomena. Due to the nature of this CTE with the filler size and filler content, weight loss and capillary flow, there are challenges on slow flow time and process quality such as underfill void, fillet coverage, modulus for the molded underfill material, capillary flow underfill and molding compound were discussed. adhesion strength between die and substrate interfaces
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Experiment The thermal analysis instruments will help to understand the material properties that are the functions of time and temperature for molded underfill material. The curing condition was measured by using DSC. The fast curing time will reduce the cycle time and increase the unit per hour (UPH). TGA was used to understand the weight loss and decomposition temperature of material. TMA was used to investigate the coefficient of thermal expansion (CTE) of the material. The lower CTE of material can reduce thermal expansion mismatch between the chip, substrate and solder bumps. The modulus helps to know the mechanical performance of the material. So the modulus was measured by using DMA. The detailed experiments are described as following.
A. Differential Scanning Calorimeter, DSC The DSC instrument (TA Instruments, Model 2910) was used to measure heat flow into or out of molded underfill material. The sample was put into a hermetic DSC sample pan and weighted before and after the experiment. Then, the o sample was put in the DSC cell at 5 C/min heating rate from room temperature to 280oC. The curing profile was obtained after DSC experiment. After that, DSC curve can be analyzed to determine specific kinetic parameters using the Borchardt and Daniels kinetic software [ 11]. The heat of reaction of molded underfill material can be represented by the following equation: A
from unity. The values for the kinetic parameters n, Ea and Z are obtained by a multiple linear regression analysis of this data. Figure 3 shows the DSC measurement curve for molded underfill material. Using DSC kinetic software, it can obtain the curing degree verses time for molded underfill material as show Figure 4. From Figure 4, it can be found the curing temperature is higher with decrease curing time.
Figure 3. DSC measurement curve of molded underfill material.
k B + ∆ H
Where: A = The material before reaction B = The material after reaction H = Heat of reaction k = Arrhenius rate constant. The Borchardt and Daniels approaches also assumes Arrhenius behavior, it is expressed as: =Z e-Ea/ RT k =Z Where: Z = pre-exponential factor (sec -1) Ea = activation energy of the reaction (J/mole) R = gas constant (8.314J/mole K) k = Arrhenius rate constant T = absolute temperature (K) The reaction rate dα/dt is obtained from the DSC measurement curve. Assuming the reaction follows n th order kinetic, so the relationship is: dα/dt=k (1(1-α)n Where: α = fractional conversion k = Arrhenius rate constant n = reaction order Combining the above equations and assuming a n th order reaction, the equation becomes: dα/dt= Z e-Ea/ RT (1-α)n The unreacted fraction (1- α) is obtained by measuring the ratio of the partial area to the peak area, then subtracting α
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Figure 4. DSC curve of curing degree verses time for molded underfill material. B. Thermal Mechanical Analyzer, TMA TMA (TA Instruments, Model 2940) is used to measure the dimension change of material as a function of temperature or time. TMA can be used to measure the CTE (coefficient of thermal expansion) and Tg (glass transition temperature) of cured molded underfill material. The sample (length x width x thickness: 8.37mm x 0.4mm x 0.38mm) was placed in the TMA instrument with film/ fiber probe, and was heated from o o room temperature to 300 C at a heating rate of 5 C/min. The CTE is calculated as the slope of dimension change from expansion profile that is shown at Figure 5. Figure 5 shows TMA measurement curve of cured molded underfill material. CTE1 is the slope of first line and CTE2 is the slope of second line. The TgTMA is determined by the onset of the slopes of CTE1 and CTE2. The CTE and Tg TMA were determined from TMA measurement curve. The CTE1, CTE2 and Tg TMA of
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molded underfill material are 18.5 ppm/oC, 69.6 ppm/oC and 202.09 oC. The higher Tg can endure the higher temperature environment.
temperature of molded underfill material. The sample was uniformly put into the platinum sample pan. Then, the sample o o was heated to 600 C at a heating rate of 10 C/min under nitrogen purge gas. Figure 7 shows the weight loss and decomposition temperature of molded underfill material. It can be seen the weight loss of molded underfill material during cure temperature was determined from TGA measurement curve. And the decomposition temperature of molded underfill material was also obtained. From Figure 7, the weight loss of molded underfill is 0.2844% after curing. The decomposition temperature of molded underfill material is above 350 oC, so the curing temperature was accepted in process.
Figure 5. TMA measurement curve of cured molded underfill material. C. Dynamic Mechanic Analyzer, DMA DMA (TA Instruments, Model 2980) is used to measure the modulus of molded underfill material. The modulus was measured by DMA instrument as a function of temperature, time, stress, amplitude, frequency and static force. The cured sample (length x width x thickness: 14.13mm x 2.8mm x 0.4mm) was placed in the DMA instrument. The measurement was performed on a film/ fiber mode under 1 Hz. A dynamic temperature ramp of 5 oC/min was used from room temperature to 300oC. The storage modulus (E’) and loss modulus (E”) were calculated by the software. The ratio of the sample loss modulus to storage modulus properties is tan delta (tanδ). The peak temperature of tan δ was defined as the TgDMA. Figure 6 shows the DMA measurement curve of cured molded underfill material. The modulus of molded underfill o material is 11.108 GPa at 35 C.
Figure 7. The weight loss and decomposition temperature of molded underfill material. Results and Discussion In DSC measurement result, Table 1 shows the comparison of the curing condition of molded underfill, capillary flow underfill and traditional molding compound materials. It can be found the curing is faster with higher curing temperature. The curing time of molded underfill o material is much faster than capillary flow material at 135 C o and 150 C curing temperature. The curing time of molded underfill material and molding compound are almost the same.
Table 1. The curing condition for molded underfill material. Temperature o o o 135 C 150 C 165 C Time(min) Molded underfill 14.62 7.15 3.67 Capillary flow underfil l 244.74 14.68 1.07 Molding co compound 12.17 6.10 3.20
Figure 6. DMA measurement curve of cured molded undefill material. D. Thermogravimetrical Analyzer, TGA The TGA instrument (TA Instruments, Model 2050) is used to measure the weight loss and decomposition 0-7803-7038-4/01/$10.00 (C)2001 IEEE
Table 2 shows the material properties of molded underfill, capillary flow underfill and molding compound materials. It can be seen the filler content increased with lower CTE value for molded underfill and capillary flow underfill materials that are almost the same filler particle size. In addition, the viscosity increases with increasing filler content. It has been reported in literature that the viscosity as a function of filler loading [12-13]. Figure 8 shows the relationship of CTE and filler content for molded underfill and caipllary flow underfill materials. Figure 9 shows the
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CTE verses particle size for molded underfill and molding compound. The filler size decreases, the CTE also decreases for molded underfill material and molding compound that are almost the same filler content. However, the CTE of the filler is much lower than the resin. The total surface area will increase with finer filler particle size. The interface between the filler and resin is increased. The CTE decreases due to the increasing surface area. Table 2. The comparison of material properties for molded underfill, capillary flow underfill and molding compound materials. Capillary Type Molded Molding flow Properties underfill compound underfill CTE (ppm/oC)
18.5
55.6 o
Modulus (MPa)
25.1 o
o
o
o
1705(290 C)
80
50
77.4
202.09
141.68
178.18
Avg.
9~10um
0.7um
60 ~70um
Max.
50um
3 ~ 5um
100um
Heat of reaction (J/g)
46.13
180.4
55.86
Weight loss (%)
0.2844%
2.5250%
0.2206%
137.44
149.28
129.58
363.03
387.89
356.55
o
Particle size
Curing peak o temperature ( C) Decomposition o temperature ( C)
C / m p p (
50
E T C
30
40
Molded underfill material
20 10 0 30
40
50
60
70
80
90
Filler Content (%)
Figure 8. The relationship of CTE and filler content for molded underfill and capillary flow underfill materials.
o
68(290 C)
Tg ( C) (TMA)
Capillary Capillary flow un nd de er rfillm aterial
60
11108(35 C) 7243(35 C) 16624(35 (35 C) 1038(290 C)
Ash content (%)
70
From above results, the filler size is smaller and filler content is higher for molded underfill material. Moreover, the CTE of molded underfill material is lowest among capillary flow underfill and molding compound materials. Therefore, the lowest CTE of molded underfill material can easy to match the CTE of substrate. From Figure 6, it can be seen that the higher temperature would contribute to lower modulus. From results in Table 2, the higher filler content has higher modulus for molded underfill and capillary flow underfill materials that are almost the same filler particle size. From Table 2, the weight loss of molded underfill material is lower than capillary flow underfill material because of the solvent residue of molded underfill is less. The higher weight loss of the capillary underfill cause higher outgassing during curing temperature. The lower weight loss for molding compound and transfer mold underfilling, so there is no outgassing issue. It is very important to monitor the outgassing problem during the process evaluation for molded underfill material.
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30 M olding com pound 25 M olded un unde derfillm aterial 20
C / m p 15 p ( E10 T C 5 0 1
11
21
31
41
51
61
71
Filler Particle Size (microns)
Figure 9. The CTE verses particle size for molded underfill and molding compound. Summary The molded underfill material was studied to replace the capillary flow underfill material. In this study, thermal analysis instruments such as DSC, TMA, DMA and TGA are useful tools for analyzing the material properties of molded underfill. It is very important to understand the basic material properties before evaluation of process and r eliability. In DSC measurement result shows the material cures faster at higher temperature. The molded underfill material can be cured in shorter time. From TMA results showed that as the filler content is increased, the CTE is decreased. As the filler particle size is decreased, the CTE is also decreased. So the CTE of molded underfill material is lower than capillary flow underfill and molding compound materials. Therefore, the CTE mismatch is lower to the substrate. The higher temperature would decrease the modulus of the material. The higher weight loss during curing process would increase the possibility of outgassing which creates the reliability concerns. From this study, molded underfill materials do not only solve the drawback of molding compound and underfill but also inherit the advantages of those materials. Therefore, the molded underfill material is an excellent alternative material used transfer molded underfill technology to shorter the processing time and enhance the reli ability.
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Acknowledgments The authors would like to thank Plaskon and Dexter for providing materials and Johnson Hsu for the assistance in providing thermal analysis instruments. Su-Yung Pan and Dragon Sue are greatly appreciated for preparing and measuring of the sample. Finally the material technology section’s colleagues are acknowledged for their kindly supports. Reference [1] Yu Po Wang, Kevin Chai, Steve Chiu, H. P. Pu, T. D Her, and Randy Lo, “Underill study of Flip Chip Ball Grid Array Package ”, Semicon Taiwan 2000, pp.133-139. [2] Yung Sen Lin, Kevin Chai, T. D Her, and Randy Lo, “Transfer Molded Underill for FC-BGA ”, Semicon Taiwan Taiwan 2000, pp.153-157. [3] M. K. Schwiebert and W. H. Leong, “Underfill flow as viscous flow between parallel plates driven by capillary action”, Int. Manufact. Technol. Symp., 1995, pp.8-13. [4] Zoba and Edwards, “ Review of underfill encapsulant development and performance of flip chip applications ”, ISHM’95 Proc., 1995, pp. 354-358. [5] H. P. Pu, S. K. Chiu, Y. P. Wang and T. D. Her, “ Assembly Development for Large IC Flip Chip Ball Grid Array Package”, to be published at InterPack ’01, Hawaii., July 2001. [6] H. P. Pu, Y. C. Tsai, Y. P. Wang and T. D. Her, ” Development of Flip Chip CSP Package”, ”, to be published at InterPack ’01, Hawaii., July 2001. [7] J. H. Lau, Flip Chip Technologies, McGraw-Hill, New York, NY, NY, 1996. 1 996. [8] C. P. Wong, S. H. Shi and G. Jefferson, “ High Performance No Flow Underfills for Low-Cost Flip-Chip Applications”, Proceedings of IEEE Electronic Components & Technology Conference, May 1997, pp. 850-503. [9] S. H. Shi, T. Yamashita and C. P. Wong, “ Development of the Wafer Level Compressive-Flow Underfill ”, 1999 International Symposium on Encapaulant ”, Advanced Packaging Materials, pp. 337-343. [10]C. [10] C. P. Wong, Michael B. Vincent, and S. Shi, “ Fast-Flow Underfill Encapsulant: Flow Rate and Coefficient of Thermal Expansion”, IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part A, Vol. 21, No. 2, June 1998, pp. 360-364. [11]T [11] TA Instrument DSC B & D Kinetics Analysis Handbook. [12]R. [12] R. J. Farris, “ Prediction of the Viscosity of Multimodal Suspensions from Unimodal Viscosity Data ”, Transs. Soc. Rheology, Rheology, vol. 12, no. 2, pp.281-301, 1968. [13]Ignatius [13] Ignatius J. Rasiah, P. S. Ho, M. Manoharan, C. L. Ng, Michael Chau, “ Rheological Analysis of an Underfill ”, IEEE/CPMT Electronics Packaging Material ”, Technology Technology Conference, 1998, pp. 354-358.
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