Self-Assembling Adhesive Bonding by Using Fusible Alloy Paste for Microelectronics Packaging

Self-Assembling Adhesive Bonding by Using Fusible Alloy Paste for Microelectronics Packaging

Kiyokazu Yasuda

Nagoya University, Graduate School of Engineering, Furo-cho, Chikusa, Nagoya 464-8603, Japan (Received September 9, 2011: Corrected September 15, 2011: Accepted September 20, 2011)

Abstract: In the modern packaging technologies highly condensed metal interconnects are typically formed by high- cost processes. These methods inevitably require the precise controls of mutually dependant process parameters, which usually cause the difficulty of the change in the layout design for interconnects of chip to-chip, or chip-to-substrate. In order to overcome these problems, the unique concept and methodology of self-assembly even in micro-meter scale were developed. In this report we focus on the factors which influenced the self-formed bumps by analyzing the phenomenon experimentally. In case of RMA flux, homogenous pattern was obtained in both plain surface and cross-section surface observation. By using RA flux, the phenomena were accelerated although the self-formtion results was inhomogenous.

With ussage of moderate RA flux, reaction rate of the self-formation was accelerated with homogeneous pattern.

Keywords: self-formation bump, SnBi solder, resin, wetting, adhesive, surface mount technology

1. Introduction

Currently a number of IC chips are implemented on the printed circuit board as array connection packages like Ball Grid Array (BGA), Chip Size Package (CSP), or bare dies by the flip-chip method.

In these packaging methods solder bumps or anisotropic conductive adhesives have been widely used. There are, however, problems of cost ineffectiveness and long process flows for solder bump methods, a problem of low reliability for anisotropic conductive adhesives.

So far the new bonding process based on self-formation mechanism has been proposed as a packaging method.

As shown in Fig. 1, molten solder fillers are coalesced by heating in the resin supplied between a device and a substrate. It utilizes the difference in wettability of the substrate surface and of the electrode land surface. Fillers are guided onto the micro electrodes. Finally electrical conduction paths between vertical electrodes can be implemented by self-assembling manner. In this method, re- design of the mask would be unnecessary even if the design of electrode pattern was changed, and unlike the electrically-conductive adhesive, high reliability can be expected since the conduction paths are joined by metallurgical bonds between the metal electrodes. In the self-formed packaging phenomena occurring between a device and a board precede by the principle of surface

tension induced.

The major subjects for implementing the self-formation process are minimization of, bubbles, defects such as open joints and bridges, and assembly time required.

The purpose of this study is to clarify the guidelines for the bumping and joining based on the self-formation method, and to solve the above subjects by concentrating on the process factors thorough the analyzing the temporal phenomena for using hybrids of Sn-Bi solder fillers and silicone resin.

2. Experimental

The low melting metal filler of Sn-58Bi (wt %) eutectic alloy and liquid silicone resin (TSF451-50) were used.

Physical properties of Sn-58Bi and liquid silicone were shown in Table 1 and Table 2 respectively. The two active agents with different characteristics, RMA flux and RA flux (I and II), were added.

The heating temperature of samples (130, 150, and 160

C) and the filler volume fraction (5, 10, and 20%) were varied according to the physical properties of Sn-58Bi and liquid silicone. For the PCB substrate, FR-4 with copper lands (circular type: 0.3 mm in diameter and square type:

0.3×0.3 mm patterns, and spacing: 0.1, 0.2, 0.3, 0.4, 0.5 mm) was adopted. By this type of substrate it is possible to observe the effect of the shape of the land and pitch in one board.

†

Corresponding author

E-mail: [email protected]

54 Kiyokazu Yasuda

Paste samples were made by mixing the Sn-Bi filler metal, liquid silicone, and flux. The amount of these materials was adjusted to the volume fraction of filler metal.

After mixing these materials, the hybrid paste was dispensed on a FR-4 substrate. A transparent glass slides were used instead of the chips in order to observe the bump formation from the top side. Then the sample was heated by a hot plate.

After eliminating excess residue by the ultrasonic cleaning, the bump pattern formed was evaluated by an optical microscope observation of the top surface and the molded cross section. Effect of heating temperature, volume fraction of filler metals, and the activity of fluxes on the process were investigated by observing the temporal behavior of fillers by using a microscope with a CCD camera.

3. Results and Discussion

3.1. Shape of self-assembled micro-bumps

According to Fig. 2, in case of the volume fraction of filler of 5%, high ratio of self-assembled results was obtained for the short pitch lands. The self formed bumps were obtained and overall results were good when the filler volume fraction of 10%. On the other hand, in the case the filler volume fraction of 20%, the size of the bumps was generally unstable and many bridges were observed between adjacent lands in the narrowest part of the pitch interval.

Fig. 1. Process of self-assembling adhesive bonding by fusible alloy paste.

Table 1. Physical properties of metal filler.

Solder fillers Sn-58Bi (Eutectic)

Density (g/mm

) 8.6

Melting point (

C) 139

Mean diameter ( µm) 42

Table 2. Physical properties of silicone.

Silicone Liquid resin

Density (g/mm

) 0.960

Kinetic viscosity (mm

/s) 50 Chemical formula -[(CH

)

SiO]

Fig. 2. Optical photographs of self- formed bump pattern

T = 160

C, CRA flux.

The optical micro photographs of the top view and the cross-section of the bumps were shown in Fig. 3. No large difference in shape between 150 and 160

C of the heating temperature was observed in the top views, but significant difference in bump height was found in the cross-sections.

Bump height (90 µm) is low for 160

C compared to that (160 µm) of 150

C.

The viscosity of the molten liquid silicone is low at high heating temperature (160

C) and surface tension of Sn-Bi solder is low. Those factors were thought to be the causes of the differences in the bump height.

Bump formation results with varying the kind of flux were shown in Fig. 4. The homogeneous bumps were formed as RMA flux with weak activity was used (Fig.

4(a)), but inhomogeneous bumps were formed with RA (I) active flux (Fig.4(b)). The shape of those bumps was asymmetric in the cross-section. The cause of this was believed to be the excessive reduction capability of flux, and the driving force by the bubbling action to the molten metal, and the heterogeneity of subsequent melting and coalescence.

In comparison, RA flux has an advantage of rapid reaction, but there was a disadvantage of the inhomogeneity and variation in shape of the formed bumps due to the excessive flux activity. As for no flux condition, the coalescence of bumps was inhibited due to the oxide film, bumps were not formed at all.

From the above mentioned results, the flux is a key factor which influences the self-formation of bumps, even if it is

added by small amounts to the liquid silicone. Also, it was found that holding temperature above the melting point of filler metal and active range of the flux was required condition. In addition, the factors affecting the bump height was heating temperature, maintaining the gap is desirable for eliminating this variation. The impact of type of flux and filler volume fraction on the variation of bump height is small. Though the effect of flux is large for the results of the self-formation bumps, homogeneous bumps formation was not observed in case of the active resin with strong driving force. It is desirable by using mild flux with low-foaming additive.

3.2. Effect of process parameters on the self-assembling time

In the case of substrate temperature of 160

C, with the active agent of the RMA flux, the effect of filler Sn-Bi volume fraction on the self-formation phenomena was investigated. The cumulative time of the phenomena was shown in Fig. 5. In all conditions, completing time of the self-formation phenomena from the start of bubble generation was approximately 290s to 360s. It is because the progress of the reaction is mild due to the usage of RMA flux. Compared with the condition of less filler volume fraction of 5%, for other two conditions over 10%, complet- ing time of the self-formation bumps was approximately 50s faster.

Fig. 3. Cross-sectional photographs of self- formed bump pattern,

V

= 10%, RMA flux. Fig. 4. Cross-sectional photographs of self- formed bump pattern,

T = 150

C, CRA flux(I).

56 Kiyokazu Yasuda

In case of the condition of high filler content, due to the short distance between the adjacent filler particles, contacts and coalescences were much frequent, so the rapid progress was achieved.

The condition of the volume fraction of Sn-Bi filler (20%) and the activator (RMA flux) were fixed. Only by changing the heating temperature of the substrate, we observed the effects of heating temperature on the self-formation phenomena. Starting point of each phenomenon was shown in the temperature profile. This is the results by the condition of the substrate temperature of 150

C, filler volume fraction of 20%, RMA flux of 10%. In order to compare the effects of substrate temperature on each phenomenon, the cumulative time was shown as the bar in the vertical axis of Fig. 6.

Micro bubbles generated at “partial bubble generation”

(represented as PB) point. Then bubbles generated throughout the scene at “Full bubble generation” (FB) point. Melting of the filler was started at “Partial melting” (PM) point.

Melting across the scene was observed at “Full melting”

(FM) point. From the results of Fig. 6, under the condition

of substrate temperature of 130

C which does not exceed the eutectic temperature (139

C), Sn-Bi molten filler did not melt, while occurrences of micro bubbles for more than a minute had been observed from the liquid silicone. On the other hand, in the latter two conditions above the eutectic temperature of Sn-Bi, after the occurrence of micro bubbles for a minute, melting and coalescence of Sn-Bi fillers was observed. By comparing the results with the heating condition of 150

C and 160

C, time to completion of the self-formation phenomenon is approximately 50 seconds earlier. This is because a partial melting of the filler and bubbles (PM points) took almost the same time whereas they took the difference in time until the entire melting (FM points).

Sn-Bi is supposed to melt immediately at the eutectic point, in fact, because of the small temperature gradient between a number of fillers and substrate, and the temperature dependence of the active rosin, the oxide film on the surface of filler is considered that there is a difference in removal.

While time up to partial filler melting onset (PM points) was about the same, reaction of the oxide film reduction on Sn-Bi filler surface by the flux in silicone resin was much faster at high temperature heating conditions 160

C.

Moreover, the effect of the presence of flux activator on the self-formation behavior was investigated in the case of substrate temperature (160

C) with filler volume fraction (10%) by optical microscope. For the flux added to the silicone resin, experiments were conducted under the three levels of conditions, namely, without flux, with RMA flux, and with RA flux I. The cumulative time was shown in Fig. 7.

As is clear from these results, the self-formation phenomenon can proceed very quickly by RA flux. While there was not much difference in PM, the time required for FM has been approximately 80 seconds faster, then the time to CS was about 160 seconds and got much faster. These Fig. 5. Effect of volume content of metal filler on self-assembling

time for RMA flux.

Fig. 6. Effect of heating temperature on self-assembling time for RMA flux.

Fig. 7. Effect of flux on self-assembling time at T=160

C,

V

= 10%.

factors, RA flux has very higher activity compared to the RMA flux. That is, it is due to the effect of the halogen, contained in the flux, reducing oxide film of the filler surface. The reaction that occurs when the flux removes the oxide film is

MO+ 2R (COOH) = M + 2RCOO + H

O (1)

Because of the reduction reaction kinetics of the carboxyl group in the resin acids the reaction rate is slow. However, in addition to (1), the reaction when the RA flux removes the oxide film,

MO + RX

= M + RO + X

(2)

The reaction (2) also occurs. The halogen X (= Cl, Br) in this reaction has strong reducing power, significantly reducing the time required to remove the oxide film on the filler surface.

5. Conclusion

Shape of self-assembling Sn-Bi micro bump and its temporal process using the mixed dispersion in silicone was investigated. It was confirmed that the metal bumps were formed selectively on copper lands after filler metal was melted and coalesced in self-assembling matter. The effect of heating temperature on the height of the bumps formed on the micro-copper land is high while the effect of filler volume fraction is small. RA flux (I) enhances the reaction rate significantly although no significant improvement appeared on the bump height formed on copper lands. The bumps were formed homogeneously using the moderately active RA flux (II) even though the reaction rate is slower than RA flux (I),

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