S-Bond® Joining for Thick Film Heaters

“Thick film” heaters are built from conductive, resistive, or dielectric circuit elements that are deposited via screen printing materials (~0.0005″ thick) onto substrates. Thick film are made from inks made by mixing ceramics and metals (known as cermets) to make the resistors and conductors.

Typically the metallic materials are silver, gold, platinum, palladium, ruthenium and related alloys. Glass-ceramic inks are also used for dielectrics between encapsulants covering the circuit layers. Substrates include steel, stainless steel, alumina and more aluminum nitride.

The challenge is to make reliable solder connections that perform at thick film heater temperatures (200 – 350°C) and eliminate more expensive mechanical connections, as shown in Figure 1 – 3 . S-Bond Technologies has developed and patented active solder S-Bond® 400. S-Bond 400 Zn-Al-Ag-Ce alloy that can, with mechanical activation, bond electrical leads without plating and/or flux, to many of the various commercial heater elements as shown in Figures 4 – 5.

Figure 4 illustrates S-Bond 400 active solder joints that were produced by ultrasonically soldering “pre-tinned” copper wire leads to pads on heater substrates. The process consists of first stripping the high temperature insulation back from the wire lead, heating the leads to 450°C with hot air, adding S-Bond 400 solder to the tip of an ultrasonically activated solder iron, and tinning the wire lead. After tinning the leads, the thick heater substrate is heated and the ultrasonic soldering iron tip presses the S-Bond 400 tinned lead to the thick film pads on the heater.

The S-Bond 400 joints permit the heaters to operate at higher temperatures with a direct solder joint, not a mechanical connection and produces stronger connections than conventional solder joints since the Zn-Al-Ag-Ce alloys do not soften until well above 300°C. Additionally the flux free joining process eliminates the contamination that fluxes can cause and also eliminate the need to post solder clean.

If you are interested in evaluating how active solders can be a solution for you, Contact Us.

Ultrasonic Soldering and Active Solders

Conventional soldering processes use chemical fluxes to remove the oxides from the molten filler metal and the underlying base metal surfaces that are being solder joined. In these processes, once the oxides are removed from the molten filler and the base surfaces, the molten filler wets and form a metallurgical bond. The problem with chemical fluxes is that they are corrosive chemicals and in any flux residue on the surface there is the potential of corrosion which is a long term reliability problem. Ultrasonic soldering is process solution that removes the need for corrosive chemical fluxes.

Figure 1 illustrates the ultrasonic soldering process mechanism employed a heated solder probe tip which is vibrated at frequencies from 20 – 60 kHz. Focused acoustic power in the specially designed tips initiates cavitation in the molten solder which remove oxides at the surface of the molten filler metal and at the surfaces of metals being joined. The molten solder acts as the acoustic transfer medium for the ultrasonic energy and as cavitation ( micro-bubbles) burst on all surfaces the action cleans the surfaces and exposes oxide-free metal that is immediately wetted by the surrounding molten filler metal.

Figure 2 illustrates a typical commercially available ultrasonic soldering iron. One should note that ultrasonic soldering is not ultrasonic welding. Ultrasonic welding (bonding) employs higher pressure shear forces and does not have molten metal phase and is a high frequency shear force micro-friction welding process.

Ultrasonic soldering is very well suited as a mechanical activation process for fluxless soldering with S-Bond active solders. S-Bond solders rely on reactive elements such as titanium (Ti) and rare earth elements such as Cerium (Ce) to enable direct wetting of metals, ceramics and glass without the need for chemical fluxes or plating. However, active solders are not “self” wetting since their own oxides that form on melting create a barrier to the reactive elements in the molten solder. The cavitation from an ultrasonic solder iron tips disrupt these oxide films that form on the molten solder. Figure 3 illustrates how an ultrasonically activated heated tip enables the active solder fillers to wet and adhere directly glass. The same process enables active solders to wet and bond to works on all metals and ceramics.

Ultrasonic soldering tips can be narrow and suited for soldering narrow lines and/or point soldering as shown in Figure 4, or larger heated ultrasonic horns (Figure 5) can be used to activate a much larger area to deposit an active solder layer as the first step in soldering two surfaces together with an active solder.

Ultrasonic soldering of active solder is an excellent high volume production process. Figure 6 illustrates a wire fed robot operated soldering iron that can be used in spot, line or large area ultrasonic activation of active solders.

Ultrasonic soldering is growing in application with the ever increasing use of dissimilar materials and where many soldered assemblies need to eliminate the use of corrosive and contaminating fluxes. Also ultrasonic soldering and active solder permit the precise placement of solder filler metal.

For more information on ultrasonic soldering and for an evaluation of how ultrasonic soldering can help you application, Contact us.

S-Bond Solders Used to Assemble Piezoelectric Sensors

Piezoelectric sensors stem from the natural effect in certain crystals that generate a voltage when the crystal is deformed. A piezoelectric sensor uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge, as illustrated in Figure 1. Such sensors are used in medical, aerospace, nuclear instrumentation, and as a

pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric sensor.

Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. The ceramic materials include PZT (lead zirconate ceramic) and new single crystal materials such as Lead Magnesium Niobate-Lead Titanate (PMN-PT).

The challenge when making sensors is to make an electrical connection to the crystal and also to bond the crystal to the sensor elements and housing. Many times these piezoelectric crystals are silver or gold coated on their attachment surfaces to which conventional solder joints can be made to make electrical and mechanical connection; however many times these solders introduce fluxes or require plating. Active solders such as S-Bond can, with their reactive elements, directly bond to the crystal surfaces without the need to initially coat the crystal surfaces. Also, S-Bond Technologies range of active solders, bond at temperatures from 115 – 400°C. These active solder alloys permit the bonding of the piezoelectric crystal well below the crystals’ Curie temperature, where these crystals lose their piezoelectric properties.

Figure 2 shows an S-Bond 220 joined PZT crystal, bonded to an aluminum housing. This bonded assembly is part of an ultrasonic actuator.

If you are interested in evaluating how active solders can be a solution for you, Contact Us.

Ultrasonic Assisted Solder “Welding”

S-Bond Technologies have demonstrated a process for making “active solder filler metal” joints and seals on aluminum assemblies. The process is similar to MIG welding processes which use higher melting temperature filler metal wires fed into a moving arc to create a weld fillet. In Ultrasonic Assisted Solder “Welding”, an ultrasonic solder tip is a heat source to melt the solder wire, instead of a welding arc. The heated tip melts S-Bond 220 wire solder which is continually fed to the solder tip, as seen in Figure 1. For aluminum soldering it is suggested the area of the joint be heated to the solder melting temperature using supplemental heat sources to provide a stable molten pool for the ultrasonic activated tip to run.

The solder iron tip is “drawn” through a molten solder bead on the heated aluminum component faying surfaces (190 – 250C) in order to deposit a solder bead that through mechanical activation wets and adheres to the underlying aluminum. This ultrasonic assisted technique enables the active S-Bond solders to directly wet and bond to the underlying aluminum surfaces without the need for an aggressive chemical flux.

Figures 2 and 3 illustrate the Ultrasonic Assisted Solder “Welding” process on a tube enclosure and on butt joint for aluminum sheet. In these applications the component surfaces are heated and the active solders are melted and applied via the heated soldering iron tip to form a molten bead at the joint. The ultrasonic soldering iron tip is submerged in the bead and mechanically disrupts the oxides on the active S-Bond solder. Due to the active elements in S-Bond, the ultrasonic gitation of the solder tip enables the solder to directly wet and adhere to aluminum and many other metals.

Figure 4 shows an actual Ultrasonic Assisted Solder “Welding” process in operation as it forms a joint between two aluminum sheets.

Figure 5 illustrates the mechanism of how an ultrasonically activated heated tip enables the active solder fillers to directly bond to most metals. The figure illustrates how an ultrasonic soldering tip creates cavitation (intense bubbles) in the molten solder which disrupts oxides that have formed on the molten solder and the joint surfaces. The cavitation in effect, cleans and mechanically fluxes the soldering area. The soldering tips are driven by a power source that excites the ultrasonic soldering horn at 12 – 25 W of power and a transformer to resistively heat soldering iron tip. Figure 6 is a picture of typical ultrasonic soldering equipment.

Ultrasonic soldering is very well suited as a mechanical activation process for soldering with fluxless, active solders such as S-Bond. S-Bond solders rely on reactive elements such as titanium (Ti) and rare earth elements such as Cerium (Ce) to enable direct wetting of metals, ceramics and glass without the need for chemical fluxes or plating. Ultrasonic “solder welding” is an extension of this principle that can be used to make joints and seal in aluminum assemblies without

having to use conventional aluminum welding which might distort or “burn though” thinner aluminum structures.Active solders are versatile at joining and sealing many different types of assemblies. Ultrasonic solder welding is another example of how S-Bond Technologies adapts processes to provide bonding solutions.

Contact us to evaluate how ultrasonic solder welding or our other bonding solutions can be used in your applications.