Piezoelectric Materials

Soldering to silver electrodes

Soldering electrical wires to the screen-printed silver electrodes make excellent and time-stable connections. Processes are tightly controlled during manufacture to do everything possible to supply parts with high quality electrodes. The thickness, solderability and bond strength of electrodes are routinely tested. The thickness of the screen-printed silver electrode is in the range of 3µm up to 10µm.

Occasionally there can be problems with wetting the solder on the silver surface and soldering can then be difficult. This phenomenon is mainly caused by a reaction between sulphuric molecules in the atmosphere with the silver surface with the formation of a silver sulphide layer on the surface of the part. The formation and thickness of this layer is influenced by several factors such as age, pH, humidity, etc.

In order to overcome such problems, it is good practice to gently clean the surface of the electrodes on the part before soldering.
A glass brush (RS 514-868) or steel wool is very useful for this operation. Figure 1 (left) shows various PZT components with fired on silver electrodes.

When soldering is performed it is important that the temperature does not exceed the Curie point of the material, since this will immediately depolarize the piezoelectric phase. For normal PZT types, use soldering temperatures between 240 and 300°C.

Furthermore, silver is soluble in the solder, and if the solder time is too long the electrode will completely dissolve in the solder. In order to increase the possible solder time, use a solder with silver content of 2-4%. Even if the possible soldering time is increased with this type of solder, the solder time must not exceed 2-4 seconds (see Figure 2).

Mechanical stressing of the joint after completion must be avoided or at worst kept to the minimum. The positioning of the wire when the connection is made should be as close as possible to the final position. Bending the wire at right angles to its original position will almost certainly break the joint due to the high peeling stress introduced.

It should also be noted that the piezoceramic parts do not only have a high piezoelectric coefficient, but also a significant pyroelectric coefficient. This means that the increase in temperature introduced by the soldering process will generate an electrical charge. This charge can be released as a spark, which, even if completely harmless to humans, can be very unpleasant for the operator.

It is therefore recommended to solder parts in short-circuit conditions.

N.B: If a piezoelectric element is heated to its Curie point, the charges can be disordered and the element becomes completely depolarised. A safe operating temperature would normally be about half way the Curie point.

Connection to Nickel Electrodes

Our Nickel electrodes vary depending upon the deposition method used:

• Chemically plated

• Electroless Nickel

• Evaporated Nickel

For connection, we therefore recommend two different methods:

Evaporated Nickel
Conductive epoxies are typically used to provide a consistent reliable electrical connection.

Electroless Nickel
Materials used: 15W soldering iron, Flux (Superior 30 or equivalent), unleaded solder (if possible), glass brush, Q-Tip and Ethanol. Gold sputtered electrodes are also available.
Procedure: Pre-heat the soldering iron than clean the electrode area using Q-
Tip with Ethanol, or a mild abrasive such as an eraser. Melt some solder on the tip of the soldering iron. Tin the lead with solder and then dip the thinned lead into the flux. Place lead on the electrode area and place the soldering iron with mild pressure on the lead until the solder flows onto the electrode. Hold the lead stationary for approximately 5 seconds to allow solder to solidify.

Key Points to Remember
1. Always keep the tip coated with a thin layer of solder.
2. Use fluxes that are as mild as possible but still provide a strong solder joint.
3. Keep temperature as low as possible while maintaining enough temperature to quickly solder a joint (2 to 3 seconds maximum for electronic soldering).
4. Match the tips size to the work.
5. Use a tip with the shortest reach possible for maximum efficiency.
6. Too high Soldering temperatures and too large temperature gradient such as rapid heating or cooling may cause electrical failures and mechanical damages of the devices.

Lead Free Soldering

Successful soldering results have been obtained using Multicore® 96S flux- cored solder, which has been specially developed to provide a lead-free substitute for tin/lead cored solder wire in all hand soldering operations. Minor adjustments to soldering temperatures will be required, but the resultant solder joint will perform as well as tin/lead solder joints. There are numerous other suppliers of lead-free solders and each Morgan Electro Ceramics site has its own preference. Therefore, it is recommended to contact the specific site for more detailed information on solder recommendations.

Soldering Iron Tip Life

Lead-free solder alloys and their cored wire fluxes are both more aggressive to soldering iron tips than tin lead cored wire. As a result, tip life can be significantly shortened. The tip life will depend to a large extent on the soldering temperature used. As a general rule, a 10°C increase in temperature will halve tip life.

However, manufacturers are currently introducing tips designed for extended life under these conditions. Lead-free is less forgiving and the right tip for the job will go a long way in preventing defects. Choose a solder tip, which has enough heat delivering capacity. Fine point tips cannot be used in all applications and in some cases a tip such as a chisel type is best suited to deliver sufficient heat to the parts to be soldered. Tip life will be reduced with lead-free solders and it is important to choose tips really designed for lead-free soldering. Many tips are only tinned with lead-free solder and the iron plating is no different than traditional soldering tips. High tin solders like to dissolve iron and this reduces tip life.

Type of Soldering Iron

Soldering irons of various types are available. The main differences are in the heat output available and the accuracy of the temperature control. Older types of soldering iron use a method of temperature control that results in large variations in tip temperature. At worst this can result in the solder freezing at the lowest temperature and being too hot for some components at the highest temperature.

Improved results can be achieved with lead-free solders if modern soldering irons with good tip temperature control are used. Modern irons are available with higher power ratings, which itself is beneficial, but an ability to maintain temperature and minimize fluctuation during soldering is more important as this will enable operators to use a lower tip temperature.

Hand Soldering

Figure 4: Lead-free solder joint

For operators used to working with tin-lead solders, hand soldering is often controlled by specifying the soldering iron's tip temperature.

However, as lead-free alloys have much higher melting points than tin- lead formulas, resulting in a significantly narrower process window, a more comprehensive set of parameters must be defined. The key in reducing operator issues and reduced wetting resides in the optimization of the soldering process. To avoid issues use a flux content of 2-3% by weight in the solder wire, use a solder tip temperature of 270-320°C. The main issues encountered with lead-free hand soldering are cold solder joints, poor wetting and de-wetting. These can be avoided.

A step-by step process transition would be as follows:

• Ensure the tips are designed for lead-free
• Ensure the flux content in the wire is a least 2% wt/wt
• Use Lead Free tips with the longest life
• Ensure the parts are easily solderable with the chosen flux
• Avoid prolonged contact times
• Avoid needless reworking of the joint
• Avoid the use of additional liquid flux

The picture above illustrates the difference between typical lead and lead free solder joints. These joints were obtained using a temperature regulated soldering station with a lead and lead-free chisel tip. Different temperatures settings on the solder tip were used to obtain the above joint results. Notice the excessive amount of flux residue for the lead free joint compare with the Sn62 alloy.

Epoxy Bond

In heat sensitive applications it is often necessary to observe particular attention to the heat treatment process for the connection joint. 

Soft piezoelectric materials such as PZT5K1, PZT5H1 or of similar characteristics have a low Curie temperature, hence making it more susceptible to depolarisation. Silver conductive epoxies are a good alternative to traditional soldering techniques. They typically come in the form of a two-part composition; often requiring a mixing ratio of 1:1 which is more suitable in a production environment. To ensure a high strength bond is achieved, thoroughly clean the surface with acetone or equivalent cleaning agent, allowing it to dry before applying the epoxy. Consult the manufacturer’s technical datasheet if curing of the epoxy is required.

Electrode Sputtering

MEC-Bedford offers in-house RF/DC sputtering of Gold/Nichrome, Nichrome, Silver, Aluminium and others upon request that can be supplied at varying film thicknesses from 100Å to 20,000Å with a film uniformity of ±15%.

Film characterisation includes:
1. Thickness measurement via X-Ray Fluorescence (XRF) and/or surface profilometer.
2. Resistivity measurement via four point probe and
3. Adhesion test.

The sputtering is done in a class 1000 clean room environment and adapted for high volume production.