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Procedia Engineering 75 ( 2014 ) 134 – 139

MRS Singapore - ICMAT Symposia Proceedings

Synthesis, Processing and Characterization III

Hardness Measurement of Copper Bonding Wire

Johnny Yeunga, Loke Chee Keonga

aHeraeus Materials Singapore Pte Ltd, Block 5002, Ang Mo Kio Ave 5, #04-07, Singapore 569871

Abstract

In semiconductor packaging industry, thin metal bonding wires, such as gold and copper, in the diameter range of 18 to 25 microns, are commonly used to connect between IC chip and connector pins through thermosonic and ultrasonic welding (bonding) in various types of packages. Copper bonding wire posts great challenges to industry users when they need to bond it onto IC bond pad of sensitive construction underneath. Any hard impact due to the material property, i.e. hardness, or excess ultrasonic parameter setting in the bonding process can easily cause the pad to crack. It is therefore of interest to the users on how hard the copper wire is and most importantly, the subsequent solidified molten ball as a result of melting the tip of the wire in the process. A Vickers micro-indentation hardness tester with minimum load of 0.5 gmf (5mN) capability is used to measure the hardness of copper wire along its length and in the free air ball (FAB). To determine the suitable load to be used to measure hardness of such fine diameter and minimize variation in the measured results due to different sample preparation effects, a range of load from 1 gmf to less than 50 gmf was studied.

© 2013 TheThe Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Selection and/orand/or peer-review peer-review under under responsibility responsibility of theof thescientific scientific committee committee of Symposium of Symposium [Synthesis, [Advanced Processing Structural and and FunctionalCharacterization Materials III] – forICMAT. Protection] – ICMAT. . Keywords: Vickers; micro indentaion hardness; copper bonding wire; free air ball

Nomenclature 4N 4 Nine (99.99%) FAB Free Air Ball HV Vickers Hardness HAD Hard As Drawn

1. Introduction

In semiconductor packaging industry, the use of thin metal wires such as gold and copper to connect between IC chip and connector pins through thermo- and ultrasonic welding (bonding) are common for various types of packages [1]. These so called bonding wires are mainly gold, copper and aluminium with silver beginning to be used for low end LED devices. In late 2008 to early 2009 when gold price was soaring after the economic downturn, the demand to replace gold bonding wire with much lower cost material was unavoidable and copper being the natural choice due to its proven application in IC packages. Not only has the demand of copper increases over the years to replace gold, the size of the wire also sees a decrease from over 25 microns in diameter to less 20 microns as package design requires tens and hundreds of wires to be bonded in a small landscape layout. However, copper being not malleable material compared with gold and work hardens when force is applied to it, it post a challenge to user of the wire to avoid damage to the sensitive structure of the IC bond pad when switched from gold to copper. Many questions arise on how hard the copper wire and the subsequent solidified

* Corresponding author. Tel.: +65 6571 7810; fax: +65 6571 7793 E-mail address: [email protected]

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium [Advanced Structural and Functional Materials for Protection] – ICMAT. doi: 10.1016/j.proeng.2013.11.029 Johnny Yeung and Loke Chee Keong / Procedia Engineering 75 ( 2014 ) 134 – 139 135 molten ball when it was required to thermosonically bonded onto the IC pad. As the dimension of the wire and the ball are in the range of less than 50 microns, conventional hardness testers like Rockwell, Brinell and even normal Vickers cannot be used due to the respective size of indenter or the load used. With the size of a bonding wire in micron range, the load required to measure the hardness has to reduce significantly in the range of less than 50 gram force and a hardness tester of specially capability to apply load in such a range is necessary.

Bonding wires are made from casting into millimeter diameter sized wire followed by progressive multi-die drawing and annealing to achieve final diameter of 50 micron or below. As copper will work harden, annealing at high temperature might be needed during the course of drawing, depends on the material characteristics. However, an annealing process at the final diameter must be carried out to induce recrystallization and softening of the wire before it is being put to use. Industry users are most interested in knowing the hardness of the copper wire for ultrasonic wedge bonding and also of re- melted wire that forms a ball for thermosonic ball bonding that will allow best bondability and least or no damage to the device. Hence, to determine the hardness of the wire, they have to be measured at the exact size and configuration in order to reflect the correct property.

2. Sample Preparation and Measurement

Bonding wires of 99.99% (4N) purity are wound onto U-shape frame made with leadframe material and cold mounted with epoxy resin by arranging the wires facing the bottom of the mount (Fig. 1), before subjecting to fine grinding and polishing to achieve scratch-free finish. The free air ball is obtained using a special program in the wire bonder to melt the tip of the wire (Fig. 2) and then bond the other end of the wire to a leadframe die paddle to form an array of FABs (Fig. 1). The FABs are pressed down before cold mounting, again with the wire faced down for subsequent grinding and polishing.

Wire cross-section FAB cross-section Figure 1. Wire and FAB sample preparation prior to grinding and polishing. Figure 2. Free air ball (FAB) formation in a bonding wire.

x > 10x Indenter Indenter

Wire Wire FAB Cold mount resin (relatively soft)

Possible air gap due to resin shrinkage upon curing

Figure 3. Constrain rule for hardness measurement with bonding wire which is not of constant thickness both in the wire and in FAB. Indentation must be done at the centre and in the FAB, although thicker cross-section is enabled due to the ball size of usually 2X wire diameter, care must be taken not to put the indentation too close to the edge.

Hardness test of the wire and FAB was done with a Fischerscope H100C Vickers microhardness tester using a minimum test load of 10 mN (1 gram) to 50 mN for 20 μm and 300 mN for 80 μm wire. Hardness value was determined by measuring the indentation’s diagonal lengths in the SEM and uses the average length of the two diagonals to calculate the Vickers hardness value according to the following equation:-

HV = F/A ≈ (0.1891F)/d2 (1)

136 Johnny Yeung and Loke Chee Keong / Procedia Engineering 75 ( 2014 ) 134 – 139

where F is the test load in Newton and A is contact area of indentation, d is the average diagonal length of the indentation in mm [2]. The contact area can be calculated from the geometry of the indenter and the depth of penetration to the material [3]. With the known tip angle of 136° for a Vickers indenter and the measurable diagonal length, the contact area can be calculated and the HV value can be simplified as shown in equation (1). During hardness test, there are criteria on how close adjacent indentations can be applied [4-5]. Referring to Figure 3 on the proximity of adjacent indentation and to the edge of sample, the indentation depth should not be more than 1/10 of the thickness of the sample and adjacent indentation must be greater than or equal to 3 time the length of the diagonals. It must also not be too close to the sample edge and must be at least 2.5 times the length of the diagonal [4]. In actual testing of the bonding wire, the criteria will post further constrain due to the cylindrical shape of the wire and the FAB. The thickness of the sample changes from edge to the centre due to the curvature of the wire shape. There is also complication of possible air gap between the wire and the cold mount resin upon curing and when a higher load is applied, the sample may not be stable enough for proper test.

3. Results and Discussion

3.1. Hard As Drawn 80 μm Copper Wire Hardness

The hardness of a metal can be measured with different test load. To understand the effect of test load on hardness, a range of load was used to see the effect and trend. Figure 4 shows the load displacement curve and respective hardness value of an 80 μm diameter hard as drawn (HAD) 4N purity copper wire tested with a load from 5 mN to 300 mN at the centre of a longitudinally cross-section wire. The measured Vickers hardness (HV) value decreases as the load increases. There is more variation of HV value at very low load possibly due to measurement accuracy variation of the small indentation as well as stability of the test load. The HV value decreases in a somewhat hyperbolic manner and tends to stabilize at higher loading of 150 mN and beyond. However, at this test load, the depth of indentation is more than 2 μm. With a perfectly cross-sectioned 80 μm wire at its diameter, the maximum thickness is 40 μm. Running a normality test of the data (probability plot) for each test load, all showed P value higher than 0.05 (Table 1) indicating that the data falls into normal distribution. To select the suitable test load for this wire diameter, and consider the criteria and dimension constrains in hardness test, a test load of up to 200 mN is possible. The indentation depth at this load is about 2.6 μm. If the indentation landed at 3/4 of a way across the 40 μm diameter, the thickness of the wire is calculated to be just over 26 μm. This is just sufficient to meet the criteria. However, in order to minimize fluctuations and possible air gap between the wire and epoxy resin in the cold mount, a lower load is preferred.

Load vs. Displacement - 3.15mil Hard As Drawn (HAD) Wire 3.15mil Hard As Drawn (HAD) Wire Hardness 190 300 Variable 5mN 10mN 180 250 20mN 30mN 40mN 170 70mN 200 100mN 150mN 160 200mN 150 300mN 150 Load (mN) 100 140

50 130

Hardness, (10 HV Sec Creep Time) 0 120 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 5mN 10mN 20mN 30mN 40mN 70mN 100mN 150mN 200mN 300mN Loading (mN) Displacement (μm)

(a) (b) Johnny Yeung and Loke Chee Keong / Procedia Engineering 75 ( 2014 ) 134 – 139 137

Probability Plot of 5, 10, 20, 30, 40, 70, 100, 150, 200, 300 Normal - 95% CI Variable 99 5 10 20 95 30 90 40 70 80 100 70 150 200 60 50 300 40 Mean StDev N AD P

Percent 30 171.7 9.719 19 0.385 0.358 20 166.5 6.791 18 0.283 0.593 158.6 4.160 20 0.320 0.510 10 153.8 3.072 20 0.459 0.235 151.1 2.394 20 0.156 0.944 5 143.9 3.669 20 0.370 0.390 139.6 3.244 20 0.505 0.179 1 130.4 3.716 20 0.618 0.093 129.1 4.164 20 0.268 0.646 210200190180170160150140130120 Data 127.0 2.684 20 0.497 0.187

(c) (d)

Figure 4. (a) Load displacement curve of different test loads used. (b) Minitab box plot of test load on Vickers hardness of a 3.18 mil (80 μm) HAD 4N Cu showing decreasing trend with increasing test load. (c) Plotting with regular scale, the hardness appears to stabilize at and beyond 150 mN. (d) Probability plot of all loads showing P values higher than 0.05 indicating data fall into normal distribution

3.2. Hard As Drawn 20 μm Copper Wire Hardness

The 80 μm HAD drawn wire was drawn further down to 20μm and the hardness of the wire was measured at different test load to see if similar effect is observed. Figure 5 shows the load-displacement curve and box plot of the hardness results from 5 mN to 40 mN. Again the effect of lower hardness registered with increasing test load has been observed. There is a sudden drop in the hardness beyond 15 mN of which the root cause was not clear. As the dimension of the wire is small with a hemispherical cross-section along the wire, the stability of hardness registered depends very much on the surface stress as a result of grinding and polishing as well as the rigidity of the wire sitting in the cold mount epoxy where no air gap should be present. Should there be variation in the condition of the sample, the measurement accuracy might be affected. Nevertheless, the trend of hardness with test load is obvious. The indentation depth ranges from 0.3 μm to about 1.4 μm at this load range. The material thickness at 3/4 way across the diameter of the wire is calculated to be just over 13 μm and so test load of 30 mN with the indentation depth just over 1 μm can be used. Normality check of the entire test load data showed at 15 mN, the P value (Table 1) is less than 0.05 indicated that at the data of this test load do not fall into normal distribution and hence the data cannot be trusted. Still, too high a test load would violate test rule while too low a load could lead to instability. Therefore, a test load of 10 to 15 mN should be used for this wire diameter.

HAD 0.8mil 4N Cu Wire Hardness HAD 0.8mil 4N Cu Wire Hardness

140 140

130 130

120 120 Average 110 110

100 100 Hardness, HV (10 Sec Dwell Time) Dwell Sec (10 HV Hardness,

90 90 5 10 15 20 30 40 0 10 20 30 40 Loading (mN) Loading (mN)

(a) (b) (c)

Figure 5. (a) Load displacement curve of HAD 0.8 mil (20 μm) 4N Cu wire. (b) & (c) Minitab box plot and line plot of test load on Vickers hardness showing decreasing trend with increasing test load. The hardness appears to drop suddenly beyond 15 mN load and continue to decrease with load.

138 Johnny Yeung and Loke Chee Keong / Procedia Engineering 75 ( 2014 ) 134 – 139

3.3. Annealed 20 μm Copper Wire and FAB Hardness

The hardness of the 20 μm annealed copper wire also showed decreasing trend as HAD one and the hardness tend to show stabilization at 15 mN load and beyond (Fig. 6). At 15 mN, the indentation depth reached about 0.9 μm, close to the limit of test rule. However, if the indentation landed again 3/4 way to the edge of the wire, there is only about 6.6 μm thick material, and therefore a lower load of 10 mN must be used. At 10 mN load, the indentation depth is about 0.6 μm and this allows more flexibility in location of indentation in the wire. However, normality check showed the P-value of this test load is less than 0.05 (Table 1), again, indicating the data does not fall into normal distribution. Great care must be employed in conducting the test at this load to ensure consistency.

Load vs. Displacement - 0.8mil Annealed 4N Cu Wire 0.8mil Annealed 4N Cu Wire Hardness 30 Variable 5mN 120 10mN 25 15mN 20mN 110 25mN 20 30mN 100 15 90 Load (mN) 10 80 5

Time) Creep sec (10 HV Hardness 70 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 5mN 10mN 15mN 20mN 25mN 30mN Displacement (μm) Loading (mN)

(a) (b)

Figure 6. (a) Load displacement curve of annealed 0.8 mil (20 μm) 4N Cu wire. (b) Minitab box plot of test load on Vickers hardness showing similar decreasing trend with increasing test load as HAD wire. The hardness appears to stabilize at and beyond 15 mN load.

The FAB hardness can be measure by cross-sectioning the ball parallel to the wire where it was formed or perpendicular to the wire direction. Figure 7 shows images of cross-sectioned FABs, longitudinally and transversely, with indentation mark from various test loads and their respective FAB hardness. Both cases show decreasing hardness with test load as seen in wire hardness. For longitudinal cross-sectioned FAB, hardness decreases from an average of about 95HV at 10 mN test load to just above 60HV at 40 mN. The hardness appeared to start stabilizing from 30mN onwards. For transversly cross- sectioned FAB, the hardness tested under the same loads differs significantly and dropped from 80HV to 50HV across the same range. There is 16.6% difference in hardness obtained at 10 mN test load between logitudinal and transverse cross- section The lowest difference is about 8% at 20 mN test load. Viewing from the cross-sectioned image, the grain morphology between the two cross-sections are different with large, columnar grains appeared at longitudinal direction. The grains seen in transvers direction is just a slice of the large columnar grain seen in perpendicualr direction. The reason why a lower hardness was registered with transverse cross-sectioend FAB is the fact that the indentation will mostly landed onto the bulk of a large single grain and reflects the characteristic of a single crystal’s hardness. In the longitudinal cross- section, the indentation could land onto a grain boundary not far below the surface. The presence of grain boundary, which has high dislocation density will respond with higher hardness. P values for both test directions are shown in Table 1 and indicate all data are in normal distribution.

10mN 15mN 20mN 30mN 40mN FAB Hardness Comparison between Longitudinal and Transverse 110

100

90

80

70 FA B Ha r dne s s HV 60 50 40 10-L 10_T 15_L 15_T 20_L 20_T 30_L 30_T 40_L 40_T

(a) (b) Figure 7. (a) Image of hardness indentation from different test load on longitudinal and transversely cross-section FAB. (b) Minitab box plot of test load on Vickers hardness of the FAB showing decreasing trend with load and higher HV for longitudinally cross-sectioned balls. Johnny Yeung and Loke Chee Keong / Procedia Engineering 75 ( 2014 ) 134 – 139 139

Table 1. Normal probability plot’s P-value of various test loads used in copper wire and FAB hardness test

Test Load (mN) 5 10 15 20 25 30 40 70 100 150 200 300 80µm HAD 0.358 0.593 N/A 0.510 N/A 0.235 0.944 0.390 0.179 0.093 0.646 0.187 20 µm HAD 0.259 0.806 0.044 0.361 N/A 0.323 0.128 N/A N/A N/A N/A N/A 20 µm Annealed 0.204 0.017 0.856 0.455 0.885 N/A N/A N/A N/A N/A N/A N/A FAB (longitudinal) N/A 0.201 0.906 0.839 N/A 0.351 0.296 N/A N/A N/A N/A N/A FAB (transverse) N/A 0.491 0.827 0.798 N/A 0.815 0.662 N/A N/A N/A N/A N/A

4. Conclusions

Measuring Vickers hardness of copper bonding wire is a challenge due to the physical size of the sample and the preparation method applied which would lead to variations. Only very low test load of 15 mN or less can be applied to the 20 μm diameter sample. Even the larger diameter of HAD wire at 80 μm, the load used should be less than 200 mN, consider the test criteria and constrains according to standard. It was found that in all tests conducted, the hardness values decrease with increasing test load. The rate of decrease in hardness will slow down significantly when higher loads are applied. For the 20 μm wire, much lower test load of 15 mN or 10 mN must be used to achieve good statistical results. The hardness of FAB depends on direction of measurement. Hardness at transverse cross-section has hardness value of 8% to 16.6% lower than in longitudinal direction. This is likely due to indentation on large grain than on shallower grain with adjacent grain’s boundary beneath it. Therefore, for 20 μm wire, test load still needs to be kept at 15 mN or 10 mN. Since the hardness of wire and FAB is dependent on test load, the HV value convention of stating test load and dwell time must be use when quoting hardness value.

Acknowledgements

The authors would like to thank our former industrial attachment student from School of Materials Science and Engineering, NTU, Jessica Oh Si Jia, for the support of sample preparation and conduct testing and measurement of the wires and FABs.

References

[1] G.G. Harman, Wire Bonding in Microelectronics: Materials, Processes, Reliability, and Yield, McGraw Hill, NY, 1997 [2] Instrumented Indentation, N. P. Laboratory May, 2007 [3] D.I.h. G. Michalzik, "Determination of the Hardness and Other Characteristic Materials Parameters Using the Instrumented Indentation Test Using Hardness Testing Equipment from Fischer Instrumentation," Helmut-Fischer instrumented indentation test, pp. 1-15, 2004. [4] ISO 6507 – 1: 2005 Metallic Materials – Vickers hardness Test – Part 1: Test Method [5] ASTM E384: Standard Test Method for Microindentation hardness of Materials