S-Bond® Active Soldering of High Purity Fused Silica for Optical Devices

S-Bond®  Ultrasonic Active Soldering of Silica

Ronald Smith    S-Bond Technologies Inc., Hatfield, PA,

Lawrence W. Shacklette, Michael R. Lange, James C. Beachboard,
Harris Corp., Melbourne, FL

and Donna L. Gerrity     E&S Consulting Inc., St. Augustine, FL

Packaging of optical devices often requires the need for creating strong bonds between metal and silica. The most convenient and cost effective approach would be to directly solder to both silica and metal without requiring pre-metallization of the silica. Soldering to oxides and oxidized surfaces has been accomplished with various solders containing metals with strong affinity for oxygen, known as “active solders”.

S-Bond Technologies worked with Harris Corporation to understand S-Bond® 140 active solders, based upon a tin-bismuth eutectic with activating additives of cerium, gallium, and titanium, to produce seals between metals and silica. Titanium and cerium are energetically capable of competing for the oxygen in silica, and are therefore capable of reducing or forming mixed oxides with silica under appropriate conditions. The bond between such an “activated” solder and high purity fused silica (HPFS) has been characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Two variations of solder produced by S-Bond Technologies, S-Bond®140 and S-Bond®140 M1 were bonded to silica using a fluxless ultrasonic technique.

Figure 1 illustrates the ultrasonic soldering process where the resultant cavitation of the molten solder layer continually disrupts the oxides forming on the molten solder surface, enable the active elements to be in direct contact with the base materials own oxide surface, in this case silica, SiO2.

Figure 1. Illustration of ultrasonic soldering process with active solders.

To compare the influence of the active elements on the strength of the soldered bond silica to metal interfaces when using non-activated solders and active solders, a simple overlap soldered coupon ( ~ 1” x 1” ) was used on a compression lap shear test. Figure 2 illustrates the soldered specimen shear test configuration.

Figure 2. Illustration of compression lap shear test configuration.

Table 1 summarizes the lap shear strengths of the active S-Bond® 140 M1 and compares its shear strength to hose of typical non-active element solders.

Table 1. Compression Lap Shear Test Results

The results in Table 1 show that the active S-Bond® 140M1 solders far exceed the shear strengths of non-active elements solders.

To characterize how the active elements are increasing the bond silica-metal joint strengths, Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS] was used to characterize the bond interface. TOF-SIMS measured the distribution of the various S-Bond® elements as a function of depth through the interface. The results show that the activating elements (Ti, Ce, Ga) concentrate at the interface and that their oxides form the interfacial layer between the high purity silica (HPS) and the bulk solder.

The efficacy of these additives was established by demonstrating that the block shear strength of the bond to HFPS was increased by 7 times through the addition of the Ti, Ce and Ga reactive metals to the base Sn-Bi solder.

The resultant data from the investigation showed a significant increase in the concentration of all of the “active” elements present in S-Bond® 140 M1 within a 220 nm interfacial zone between the solder and HPFS.

Figure 3. Charts of Element concentrations made from S-Bond® 140M1 joints between Silica and metal using Time of Flight – Secondary-ion mass spectrometry [TOF-SIMS].

In addition to this accumulation of “active” elements, the quantitative concentration of O was higher in the interfacial region than in areas away from the interface in the solder bulk.  These data support the formation of mixed oxides at the interface play a significant role in adhesion.  The data also support the notion that the interface comprises an oxide to oxide bond, that is, a silica to active metal oxide bond. All three active elements present in the solder seem to participate relatively equally in this bond formation.

Figure 4. Illustration of mixed oxide bond interface at S-Bond® 140M1 solder to silica surface.

It cannot be necessarily concluded that each active element (Ti, Ce or Ga) has the same contribution to bond strength, or whether having an intermetallic mixed oxide offers an advantage over a simple oxide of a rare earth or titanium.  Based on concentration alone, it appears that the role played by all three metals is essentially the same.  The thickness of the oxide layer (220 nm) and the observation of an interface layer with mixed oxides supports the model depicted in Figure 4.

The active elements accumulate at the interface because this is the available reaction site due to the presence of the substrate oxide (silica) and potential free oxygen. Once the oxidation reaction occurs, the active metal becomes bound at the interface, and thus accumulates there. The mechanism for movement of the “active” elements to the interfacial region against an apparent concentration gradient is presumably due to mechanical forces, but could also be aided by thermal convection.  The ultrasonic energy applied to the system is believed to play a key role in the observed movement of “active” elements to the interfacial region and possibly to an enhanced O level in this same region.  Ultrasonic or any other form of mechanical agitation can establish a mixing of the solder that would bring active metal to the interface.

REFERENCES

[1]          Nagono, K., Nomaki, K., and Saoyama, Y., US Pat. 3,949,118.

[2]          Ramirez, A.G., Mavoori, H. and Jin, S., “Bonding nature of rare-earth-containing lead-free solders”, Appl. Phys. Lett. 80, 3 21, 398-400: and US Pat. 6,306,516.

[3]          Tomáš Skála, Nataliya Tsud, Kevin C. Prince and Vladimír Matolín, “Bimetallic bonding and mixed oxide formation in the Ga–Pd–CeO2 system”, J. Appl. Phys. 110, 043726 (2011).

[4]          A.R. Lobato, S. Lanfredi, J.F. Carvalho, A.C. Hernandes, “Synthesis, Crystal Growth and Characterization of g-Phase Bismuth Titanium Oxide with Gallium”, Mat. Res. vol.3 n.3 São Carlos July 2000.

This investigation has shown how effective active element solders, such as S-Bond 140M1 are in bonding metals to silica (SiO2) surfaces. If you have applications requiring the bonding and sealing of fused silica or related glasses, please Contact Us… and we can assist in meeting your need.

Epoxy Bond vs. Solder Bond Applications

Bond assembly can be done via 1) mechanical attachment, 2) adhesive bonding of which epoxy bonding is one form of adhesive, 3) soldering bonding using lower melting filler metals (< 450˚C), 4) brazing using filler metals melting above 450˚C and 5) welding such as resistance welding bonding, ultrasonic welding and friction weld bonding that uses locally melted parent metal.

Bonding is done for a variety of technical reasons a) mechanical attachment, b) thermal contact, c) electrical contact d) gas or liquid seal, or e) any of all combinations of a – d. The choice of bonding method will then depend on the intrinsic properties of the bonding filler materials (hermetic, electrical conductance, thermal conductance, thermal coefficient of expansion, adhesive bond strength related to the intrinsic fillers’ mechanical properties and their adhesive and cohesive strengths…

With all these variables and design considerations how does one choose? The three main guiding principles are:

1. Cost of filler and Cost of bonding processes

2. Performance in Service (based on the properties of the bond and bonding materials)

3. Compatibility with Manufacturing Sequence.

To compare epoxy bonds to solder bonds one has to ask the purpose of the bond… Is strictly a mechanical bond ? Is cost a large factor? If cost drives the choice then many times epoxy is the bonding material of choice. Epoxies are generally low cost thermosetting polymers, that are mixed chemicals which are thermally or UV cured to achieve hardness and adherence. Epoxy by far is the lower cost material over solder metal fillers and thus if low cost is the driving aim of the bond, then epoxy will be the bonding material selected.
Figures 1-2 illustrate typical epoxy bonded applications


When bonds have to be thermally conductive or electrically conductive solders are usually the bonding material of choice. Solder are metal fillers melt below 450˚C are normally alloys of Sn, Ag, Pb, In, or Bi with the Pb-free alloys being preferred for environmental reasons. As metals, these materials are intrinsically 10 – 100 time more conductive than epoxy bonds. In recent year epoxy bonds have been filled with aluminum or silver particles to increase the epoxy bond filler conductivity to values of 3 – 5 W/m-K from 0.5 – 2 W/m-K. When compared to solder bond metals with conductivities of 40 – 400 W-m-K, one can see for thermal bonded components that solder bonding would be preferred. S-Bond Technologies makes active solder alloys that bond to metals, ceramics, glass and their combinations without the need for flux or plating and are many times selected over epoxy bonds for their improved thermal characteristics.
Figures 3 – 4 illustrate typical solder bond applications.

Figures 5-7 show the solder bond process being used to make a heat exchanger.


Bonding for electrical resistance or conductance will many times determine the choice of epoxy bonds over solder or active solders. If the bond joint has to provide electrical isolation, then epoxy has much higher dielectric strength and resistivity, hence are excellent at isolating electrical components from their base materials. However, if the bond has to be electrically conductive solder bonds are preferred.

Bonding for seals are a mixed choice… in the short term epoxy seals can perform and create a sufficient seal for liquids and many gases. However, in applications for long term use epoxy bonds are permeable to certain gases and moisture and are not used in seals that require high hermetic seal integrity. Metals are impervious to moisture and gases thus solder bonds are the preferred bonding materials for high integrity hermetic seals.

Epoxy bonds are “permanent” and less resistant to thermal cycle and temperatures as well as UV exposures (can degrade with time). Solders on the other hand being metallic can be remelted repeatedly to renew or rework the bond. Additionally, as metals, solders are resistant to cracking being ductile and tough and are not susceptible to UV degradation.

Finally, the issue of compatibility with manufacturing sequences and the choice of solder bond vs. epoxy one has to select the bonding materials that will suit not only cost but the sequence of manufacturing operations. The bond has to have the properties that will take the exposure to all the assemblies operations. Bonding is many times completed after machining and fabrication but before plating or coating. If an electrical package the bonding has to be done in a compatible sequence with the electrical soldering operations. For example if a printed circuit board needs to bonded to a heat sink solder bonding the circuit board has to be done a temperatures below the solder reflow temperature on the circuit, for example below 200˚C or one has to epoxy bond the circuit board with a thermally filled epoxy. Compared to solder bonding, epoxy bonds can be less expensive in a manufacturing operation with no need for heating and reflowing solders. On the other hands solders “cure” as soon as the heating is off, while epoxy bonds need “setting time” to cure, which in some high volume applications provide some problems. When electroplating a full assembly, the bonded parts need to be bonded electrically hence solder bonding is the choice, while if powder coating, the epoxy bond may be the bond of choice.

This blog discussed how the choice of epoxy bond vs. solder bond is determined by a host of factors that need to be considered. We hope the discussion has been useful.

If you need help in making the choice of epoxy bond vs. solder please contact us, we can offer the proper counsel for making the right choice and we also offer alternative