Janting, Jakob

Petersen, Dirch Hjorth

Schonwandt, B. V.

Online Research Database In Technology

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V.Published in:Proceedings of Materials Week conferencePublication date:2002Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Janting, J., Petersen, D. H., & Schonwandt, B. V. (2002). Scanning Acoustic Microscopy Study of Flip-ChipUnderfill Cure Degree. In Proceedings of Materials Week conference   1Print PrintEpoxymouldChip InterconnectionEC1211Si chipAubumpCu conductor31 µm Figure 1: Cross section through bump – pad electrical interconnection between flip-chipped MEMS pressure sensor chip and print. Scanning Acoustic Microscopy Study of Flip-Chip Underfill Cure Degree J. Jantinga, D. H. Petersenb, B. V. Schonwandta aDELTA Danish Electronics, Light & Acoustics, Denmark bThe Microelectronics Centre (MIC), Denmark 1 Introduction The motivation for this work has been to gain a fast and reliable Compression UnderFill (CUF) based flip-chip interconnection process for MEMS components. In this process electrical contact between bumps and pads is established mechanically by the high curing shrinkage of the underfill, figure 1.           Although the studied CUF material EC1211 has been cure characterised by Differential Scanning Calorimetry (DSC), the actual temperature environments experienced by the underfill in the flip-chip process might be much different from expected. When trying to run the process as fast as possible it might therefore be that the CUF is not uniformly cured. Using DSC for evaluation is tedious and destructive because samples of the CUF will have to be taken out for the analysis. A better alternative is Scanning Acoustic Microscopy (SAM). Here detection of cure degree relies on the big mechanical (stiffness E) differences between cured and uncured material and the great advantages are that the inspection is 2D and non-destructive. Acoustic microscopy has many applications, SAM especially in the microelectronics industry to detect failures e.g. cracks, bubbles, delaminations in chip encapsulations, solderings, gluings etc. Generally SAM is a Non Destructive Testing (NDT) method for microinspection which functions like a sonar, though using much higher frequencies, MHz – GHz. Ultrasound is transmitted through a liquid medium, typically water, to the component and its interior where it is reflected and transmitted at an interface between two materials 1 and 2 with amplitudes A(R)12, A(T)12 because of the changes in acoustic impedance according to [1]:  )1()(21120012 12 ZZZZARARA +−== 2 where A0 is the incoming wave amplitude, R12 and T12 are the reflection and transmission coefficients respectively, Z1 and Z2 are the acoustic impedances of materials 1 and 2 respectively, figures 2, 3.                    The acoustic impedance is expressed by [2]:   where VL is the longitudinal wave velocity, B is the bulk modulus, E is Young's modulus, ρ is the density, and ν is Poisson's ratio. Both E and ρ will increase by curing and therefore Zcured > Zuncured. SAM can be considered as a supplement to other NDT methods like X-ray inspection. The contrast in X-ray inspection relies on absorption due to differences in the atomic mass. Bubbles, cracks etc. in polymers are for instance not easy to see with X-rays, though they are easy to see with sound. Results are most often presented as C-scans, which are 2D pictures of the sound reflected from a certain depth range corresponding to a certain echo time delay where a gate is placed in a sample, figure 3. Pulses and echoes are transmitted and received by the same transducer, which is scanned over the surface of the sample. In SAM resolution and penetration depth depends on frequency (focal spot size), focus position, stiffness and stiffness changes in the materials. High frequency gives high resolution but low penetration depth and vice versa. Lateral and depth resolution is expressed by equations 4 and 5 below respectively.  )4(22.1DFmateriallateral λ=∆   )2(2)(2120012 12 ZZZATATA +==)3()21()1()1()21()1()1()21()1()1( EEBBVZ L ρνννρνννρννρνρρ ≈−⋅+−≈−⋅+−=−⋅+−==TransducerTransducerIncoming waveTransmitted waveReflected wavePosition A Position BAirWater (Z1)Flip chip ( )Z2Interconnection ( )Z4Cured underfill ( )Z5Print ( )Z6Uncured underfill ( )Z3Figure 2: Illustration of SAM study on flip-chip with cured / uncured underfill. AmplitudensH O/Chip2100%Chip/underfillUnderfill/printPrint/H O2R  < 0R  > 0gate for C-scan Figure 3: A-scan on flip-chip in figure 1. Reflected sound at chip / underfill varies according to cure degree.    3)5(44.22=∆DFmaterialdepth λ   where λmaterial is the wavelength in the study material, F is the focal length and ∆ is the transducer aperture diameter. The detection limit of delaminations or other gaps (air or vacuum) is below 0.1 µm since even such narrow defects have been demonstrated to reflect all the sound [3]. For best results plane and smooth surfaces are required. Using 200 MHz and the KSI WINSAM 200 instrument of this study a lateral resolution of 8 µm in the interface between bonded silicon and glass has been achieved. 2 Results Two approaches for determining the degree of CUF curing degree have been studied. First it was attempted to determine the cure degree from differences in sound speed. This requires knowledge about the CUF thickness and reflection time delay between the top and bottom interfaces. The method turns out not to be feasible for typical CUF thicknesses (30 µm – 70 µm) because of too low depth resolution. Even with thick bond lines and therefore well separated reflections from top and bottom interfaces it is difficult to determine the speed with sufficient accuracy because the waves do not have the same form and the places on the waves to measure from / to are not well defined. Though, using C-scan grey tone values good results can be achieved.  2.1 Acoustic impedance calculations from C-scan grey tone values The advantage of this approach compared to the speed of sound method is that only the degree of reflection at the same depth between different materials e.g. cured / uncured CUF is measured. Depending on the situation, with reference to figure 4, two different calculations can be used to find the acoustic impedance of a material under another:  • The acoustic impedance Z1 of the top material is unknown.  Calculation of Zx is based on the approximation that R12 has a linear dependence on Z2. Then Zx is given by:  )6()(ababbxbx ggZZggZZ −−−+=   I.e. to determine Zx two reference acoustic impedances Za, Zb for materials under the top material are needed. Their grey tone values and that for the studied material x are measured in the C-scan picture.  • The acoustic impedance Z1 of the top material is known  Calculation of Zx is based on the linear dependence of gx on R. Then Zx is given by:    45 mmDSC pan H O2IsopropanoleH O2 AdhesiveAdhesiveAdhesiveDSC panAirFigure 5: Isopropanole, air, water, and adhesive between two microscope slides. Z1ZxZa ZbIncoming wave(C-scan)Figure 4: Illustration of the variables in equations 6 and 7.   )7()()()()(11111 ZZgZZgZZgZZgZZaxaaaxaax −−+−++=   I.e. to determine Zx in this case only one reference acoustic impedance Za for a material under-neath the top material is needed. Again the corresponding grey tone value ga and that for the studied material gx is found in the C-scan picture. Normally underfill materials contain a silica filler to lower the Coefficient of Thermal Expansion (CTE). In an attempt to evaluate the uniformity of the filler in cured underfill the above equation 7 has previously been derived in a slightly different form [4].            2.2 Test 1: Determination of acoustic impedance of isopropanole To test the methods of determining acoustic impedances isopropanole, air, water, and adhesive were placed between two microscope slides (glass) with the purpose of determining the acoustic impedance of isopropanole, fig 5. Isopropanole and air were entrapped in small aluminum pans meant for DSC measurements.            The sample was placed in the water filled SAM inspection vessel and a gate for the C-scan cover-ing the whole reflected wave from the bottom interface of the microscope slide was placed, figure 6. The C-scan was performed and grey tone values determined with KSI WINSAM 200 software, fig-ure 7.           5152144170128126126142142Figure 7: 110 MHz C-scan. Grey tone values for isopropanole, air, water, and adhesive. Figure 6: 110 MHz A-scan in centre of figure 5 sample (adhesive under microscope slide).                             The grey tone and the reference acoustic impedance values used for the calculations are shown in table 1. The results derived by use of equation 6 and 7 show satisfactory agreement with the tabu-lated Z value for isopropanole, table 2.  Table 1: Determined grey tone values and reference acoustic impedances. Reference acoustic impedances / MRayl Grey tone values (0 => 256) Z1(glass) Za(H2O) Zb(air) ga(H2O) gb(air) gx(isopropanole) 15 1.48 0.0004 142 170 152  Table 2: Acoustic impedance of isopropanole. Zx(isopropanole) / MRayl Equation 6 Equation 7 Table 0.95 0.97 0.92 2.3 Test 2: Test on flip-chip with CUF Cured / uncured epoxy EC1211 CUF material between a 2 cm x 2 cm x 0.35 mm test flip-chip with approximately 30 µm bumps and a Printed Circuit Board (PCB) were studied, figure 8. The narrow gate around the middle of CUF material in the figure 9 A-scan results in a clear picture of the bump positions, figure 10. To get the grey tone values a gate is placed which covers the whole  6PCB, bottom upGlass sample supportAdhesive for fixture and sealingFlip chipPCB, bottom upRegion with curedunderfill under chipRegion with uncuredunderfill under chip2.5 cmPoint of A-scanC-scan areaFigure 8: Test flip-chip with 30 µm bump height on PCB’s with cured / uncured EC1211 CUF material. During investigation water is under the chip in the bottom left corner.  Figure 9: 50 MHz A-scan on flip-chip in figure 8 (white cross)..reflected wave at the chip / CUF / PCB interfaces, figure 9. The resulting C-scan picture with grey tone values is shown in figure 11.                                    7Figure 10: 50 MHz C-scan corresponding to narrow gate in A-scan  shown in figure 9. 188188219188212Figure 11: 50 MHz C-scan corresponding to broad gate in A-scan. Numbers indicate grey tone values.                  The grey tone and reference acoustic impedance values used for the calculations are shown in ta-ble 3. The results derived by use of equation 7 show satisfactory agreements with the tabulated Z value for typical cured epoxy, table 4.  Table 3: Determined grey tone values and reference acoustic impedances. Reference acoustic impedances / MRayl Grey tone values (0 => 256) Z1(silicon) Za(H2O) ga(H2O) gx(EC1211 cured) gx(EC1211 uncured)20 1.48 219 188 212  Table 4: EC1211 and typical epoxy acoustic impedances. EC1211 acoustic impedances / MRayl Equation 7 Table (typical epoxy)Zx(cured) Zx(uncured) Z(cured) 3.0 1.8 3 - 4     4 Discussion The speed sound in the [111] direction of single crystalline silicon is 8.4 µm/ns. I.e. sound travels forth and back in the 350 µm flip-chip within 83 ns. The broad gate for grey tone determination starts about 20 ns later. The surface which is detected automatically at some level which might be quite low can be the explanation for this. Further the explanation might be the roughness of the unpolished top side of the chip. The speed of sound in cured epoxy is around 3 µm/ns. The wave length at 50 MHz is 60 µm. With a bump height of 30 µm the reflections from the chip / EC1211 and EC1211 / print interfaces with a time delay of around 20 ns cannot be resolved according to (5). Though, this interference from the EC1211 / print interface reflection is not critical since the two materials are acoustically well matched, which means that most of the sound is transmitted (see figure 3). This is also evidenced by the fact that A-scan wave forms in the flip-chip test case, figure  89 resembles the A-scan wave form in the isopropanole test case where the thickness of the adhesive is 2 mm. On curing EC1211 the acoustic impedance changes about 1 MRayl unit / 20 grey tone values. By measuring at different places the uncertainty was estimated to be approximately 2 grey tone values corresponding to 0.1 MRayl. 5 Conclusion It has been demonstrated that SAM can be used to determine acoustic impedances of materials in layered structures. Two equations have been verified. The usability of one equation depending on knowledge about the top layer acoustic impedance and only one other acoustic impedance of a material in the same depth as the analysed one has been demonstrated on a CUF flip chip with 30 µm bump height. A clear difference between cured / uncured CUF can be observed. 6 Acknowledgement The work is part of the collaboration on development of microsystems (SUM) project between two Danish institutes DELTA, the Microelectronics Centre (MIC), and three Danish companies Capres A/S, Danfoss A/S and Grundfos A/S. The project is financially supported by the Danish Agency for Trade and Industry. 7 References [1] B. A. Auld, in Acoustic fields and waves in solids, 2, 37-38, John Wiley & Sons, 1973. [2] S. P. Timoshenko and J. N. Goodier, in Theory of Elasticity, 492-493, McGrawHill, 1970 [3] Tom Adams, in: Advanced materials and processes, September 2000, p. 38-40. [4] S. Canumalla and M. G. Oravecz, In situ Elastic Property Characterization of Flip-Chip  Underfills, in: International Symposion on Advanced Packaging Materials, 1998, p. 106-110. 

Scanning Acoustic Microscopy Study of Flip-Chip Underfill Cure Degree

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Scanning Acoustic Microscopy Study of Flip-Chip Underfill Cure Degree

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