Gold to Gold Thermosonic Flip-Chip Bonding
L. K. Cheah, Y. M. Tan, J. Wei and C. K. Wong
Electronics Packaging Group, Gintic Institute of Manufacturing Technology,
71 Nanyang Drive, Singapore 638075
Tel: 65-7938540 Fax: 65-7922779 E-mail: v.sg
Abstract
东莞下雪The aim of the project is to establish a thermosonic flip-chip asmbly process to replace the anisotropic conductive adhesive films (ACFs) asmbly method. The thermosonic bonding technology has the following advantages over the existing ACFs mounting method: (i) metallurgical joining is more reliable than conductive particles and adhesive joining, (ii) proce ss cycle time can be re duce d from ve ral minute s to le ss than 10 conds, and (iii) lowe r manufacturing cost per unit. In this paper, the process development of gold bumped flip-chip bonded on substrate with diffe re nt gold finishing pads is de scribe d. The rmosonic bonding te st was conducte d using te st chips with diffe re nt I/O counts and bump ge ome try. The as mble d dice and boards we re subje cte d to die she ar te st to
determine the bond strength. SEM was ud to determine the bonding interface. Thermal cycling and thermal shock tests were performed on the optimized asmbly process. It is obrved that the die shear force can range between 32.16 and 52.20 gram per bump for gold bumps bonded on thick-film and thin-film substrates respectively. Cross- ctions we re carrie d out to inspe ct the de gre e of de formation on the gold bumps afte r the rmosonic bonding. Coplanarity issue is addresd and parallelism adjustment is crucial for good bonding.
Key Words: Lead-Free, Fluxless process, Flip-chip and Thermosonic Bonding
Introduction
Flip-chip asmbly is an attractive solution for high performance and miniaturized microelectronics packaging. A well-established process of flip-chip asmbly is bad on lead-tin solder, which requires flux to remove oxide during asmbly. While it offers high yield and reliable connections, soldering requires complex process and sometimes involves materials with potential hazard to the environment. Alternatives to lead-tin soldering, such as fluxless lead-free asmbly process, have been the focus of much rearch and development in recent years. The fluxless techniques include pressure contacts or thermocompression bonding enhanced by conductive adhesives, thermosonic bonding and bonding by fusible metals.
Anisotropic conductive adhesive films (ACFs) em promising as interconnect materials for COB asmblies becau of veral advantages such as low-temperature asmbly, high density interconnection, low cost, and fluxless bonding, which eliminates the need of cleaning. However, there is concern that the interconnection resistance with ACFs may be too high. Yet, reliability, high frequency properties, and yield are required in applications of ACFs.
Thermosonic flip-chip bonding is an emerging, solderless technology for area-array connections [1,2]. The thermosonic approach is ud to join ICs with gold bumps to gold plated pads on substrate. It offers a range of features superior to soldering and ACFs counterparts, i.e., simplifies the processing and asmbly steps, reduces the levels of asmbly temperature, loading pressure and bonding time, and increas current carrying capacity.
Thermosonic bonding was previously applied mainly for wire bonding. This bonding method is a combination of ultrasonic and thermocompression welding that optimizes the best qualities of each for microelectronics usage. Thermocompression welding usually requires interfacial temperature of the order of >300°C. This temperature can damage some die-attach plastics, packaging materials, and laminates, as well as some nsitive chips. However, in thermosonic welding, the interface temperature can be much lower, typically between 100 to 150°C, which avoids such problems. The u
宝宝智慧ltrasonic energy helps disbur contaminates during the early part of
the bonding cycle and helps mature the weld in combination with the thermal energy. Objective
The aim of the project is to establish a thermosonic flip-chip asmbly process to replace the ACFs asmbly method. The thermosonic bonding technology has the following advantages over the existing ACFs mounting method: (i) metallugical joining is more reliable than conductive particles and adhesive joining, (ii) process cycle time can be reduced from veral minutes to less than 10 conds, and (iii) lower manufacturing cost per unit. Once fully developed, the thermosonic flip chip bonding process can be implemented with conventional equipment and technology such as gold wire bonder, thin or thick film substrate technology, flip-chip bonder and underfill dispenr.
Experiment Details
Die Design
Thermosonic flip-chip asmbly evaluation was conducted using FA10 full array dice [3] as shown in Figure 1. The FA10 is a daisy chain test die for u in evaluating asmbly techniques, board continuity, temperature cycle and life test evaluation. The FA10 is a 4.98 mm per side device which c
ontains a full array of 10 mil pitch pads across the entire surface of the die. Contact pads were made of aluminum. The pull-off gold bumps were realized using a Panasonic wire-bonding machine with 1 mil (25 µm) gold wire as shown in Figure 2. This resulted in approximately spherical bump of 75 and 50 µm in diameter and height respectively. This wire bumping method is ideal for getting bumps onto individual chips. Once the wafer is diced it becomes difficult to bump by other process. Three types of bumping configurations were designed with 32, 48 and 68 I/Os. Figure 1 shows the FA10 die with 32 I/Os.孙楠楠
Substrate Design
To investigate the thermosonic bonding process, the test substrates were fabricated with dimension of 100 mm x 100 mm and 0.4 mm thick. The design provides three different bonding configurations with eight bonding sites each to create a continuos daisy chain. By measuring the resistance between the two ends of the daisy chain, any missing bond can be detected. Figure 3 shows the daisy chain on the substrate to fit the die with 32 I/Os.To investigate the effect of different pads finishing, two types of gold surface were prepared:
i)Printed gold paste fired on 96.5% alumina
substrate. The thickness of the gold layer is about 10 µm
站长软文
ii)Sputtered Au / TiW on 99.5% alumina substrate.
The thickness of the gold layer is about 1 µm. A multi-targets unbalanced magnetron sputtering system was ud to produce the Au / TiW layers continuously without breaking the vacuum to
ensure good adhesion of the films
Figure 1. FA10 full array die with 32 pull-off
bumps
Figure 2.
Pull-off bump
Thermosonic Asmbly
In this project, a Panasonic flip-chip bonder with an ultrasonic tool was ud to perform the evaluatio
n. Prior to the thermosonic bonding, co-planarity of the die and the substrate must be carefully aligned to achieve good bonding. The bonding procedure begins with the substrate sitting on a heated stage. A vacuum holds the substrate in place. The temperature of the substrate is maintained at 150 °C. The chip is held by the bonding tool with vacuum and is brought into contact with the substrate. After the bonding force has reached a certain level, ultrasonic vibration is applied through the ultrasonic tool for a predetermined length of time to complete the process. Thermosonic bonding with loading pressure from 40 to 100 g/bump bonded on different pad materials
were studied.
Figure 3. Daisy chain pattern on substrate for 32
I/Os die
Process and Reliability Study
After the flip-chip was bonded to the substrate, the asmbled parts were subjected to die shear test to obtain qualitative value for bond strength. The die shear test was performed on the parts asmbled without underfilling.
To asss the reliability of the thermosonic bonding process, the dice and substrates were asmbled with lected process parameters. Underfill was dispend after the bonding cycle and was cured for 2 hours at 150°C. The parts were subjected to cross-ction inspection and open/short test to determine the deformation of the gold bump and the connectivity respectively. Electrical resistance was recorded for each part. The parts were subjected to the following tests for reliability asssment:i)Thermal cycling test was performed using
Heraeus HT7012 S2 2 zone air to air thermal shock chamber with temperature tting from –
55 to 125 °C
ii)For moisture nsitivity test, Napco (Model 8100-TD Test Chamber) pressure cooker tester
(15 psig in 100% RH water) was ud Experimental Results
Die shear test was carried out on the flip-chip dice, bonded with different bonding pressures, on pads with printed gold paste fired on 96.5% alumina substrate. The shear force increas from 8.40 to 32.16 g/bump as the bonding pressure increas from 40 to 75 g/bump and decreas to 23.49 g/bump as the bonding pressure increas to 100 g/bump. An optimum bonding pressure of 75 g/bump was obrved with highest shear strength. Figure 4 shows the plot of the die shear strength versus bonding pressure.
Figure 4.Shear force at different bonding
pressure
The asmbled dice with different loading pressures were subjected to cross-ction inspection. Figures 5 to 8 show the deformation of gold bump under bonding pressures range from 25 g to 100 g. Stand-off height is about 25 µm for samples bonded above 50 g. The gold to gold diffusion was obrved on the thermosonic bonded interface.
Figure 5.
25 g loading pressure
Figure 6.
50 g loading pressure一周趣事周记
Figure 7.
75 g loading pressure
Figure 8. 100 g loading pressure
Gold bumped flip-chip is bonded on two different gold surface finishing boards. It is noted that the shear force for flip-chips bonded on the thin film sputtered gold surface is greater than the thick film printed gold surface finishing for bonding pressure of 75 and 100 g/bump as shown in Table 1. A plot for shear force versus displacement is shown in Figure 9for flip-chip bonded with 75 g/bump. The better performance of the thin film sputtered gold finish substrate can be attributed to smoother surface morphology. The uniformity and the thickness of the sputtered thin film are relatively easy to control compared to the printed thick film technology.Table 1.
Die shear strength on flip-chips bonded on different surface finishing pads ID
Bonding pressure (g/bump)Thick film printed gold (g/bump)Thin film sputtered gold
(g/bump)
1
7532.1652.202
10023.4946.53
different gold surfaces
Flip-chips with different I/Os were bonded and underfilled bad on the matrix shown in Table 2.The bonding pressure was 75 g/bump.Table 2.Bonding configuration
ID I/O Substrate Amount 132Printed gold paste 16248Printed gold paste 16368Printed gold paste
16432Sputtered gold 16548Sputtered gold 16668
cad怎么倒角
Sputtered gold
英语一分值
16
Unlike gold wire bonding, co-planarity is important to achieve good gold to gold thermosonic flip-chip bonding. Parallelism and co-planarity adjustment of the ultrasonic tool with respect to the substrate must
be achieved to obtain good gold to gold diffusion bonding. Figures 10 and 11 show the effect of typical co-planarity issues that affect asmbly yield.
Figure 10.
机遇英文Misalignment may cau diffusion
bonding on side A but cold joint on side B due to insufficient pressure. Most of the I/Os on side A will pass the test but tho on side B will be opened
Figure 11. Misalignment may cau short circuit
on side A and possible diffusion
bonding on side B due to overpressure.One or more I/Os on side A will be shorted and most of the I/Os on side B will pass the open/short test The asmbled boards were subjected to open/short test to determine the connectivity of the daisy chain between the flip-chip and the substrate. All the asmbled chips pasd the test. Electrical resistances
measured for each asmbled die ranged from 3.5 to 5.5Ω. Figure 10 shows the typical asmbled boards.As a first step of investigating reliability, the asmbled boards were pasd through the reliability tests described. The thermal cycle test was stopped after 500 cycles and the electrical resistance was unchanged after the cycles. This was followed by pressure cooker test for 24 hours. The electrical resistance remained unchanged. The results demonstrated that good gold to gold diffusion bonding has been achieved.
Figure 12.
Thermosonic flip-chip asmbled boards with eight bonding sites for three different daisy chain configurations
Conclusion
A thermosonic flip-chip bonding process using conventional equipment has been successfully developed,
i)Gold wire bonder to form pull-off bump on
silicon die with gold wire bondable aluminum pads
ii)Thick or thin film ceramic substrate
iii)Conventional flip-chip bonder with optional
ultrasonic tool to provide alignment, heated stage, thermocompression loading, ultrasonic power and controllable duration to perform the flip-chip thermosonic flip-chip asmbly
iv)Conventional dispensing system for underfilling
dispensing The thermosonic flip-chip bonding process is proven to be uful for die with dimension up to 5 x 5 mm and up to 68 I/Os. The following steps have been taken to verify the reliability of the process,
