Trace Oxygen Measuring Systems for Reflow and Wave Solder Machines

Most manufacturers and users of inerted reflow and wave soldering ovens use trace O2 analysers to either measure the oxygen impurity or even with a feedback loop control the O2 impurity.

There are two major technologies used in these types of applications, namely Zirconia, ceramic, solid state and Fuel Cell. There are advantages and disadvantages with both types of oxygen analyser.

Many manufacturers only produce one type of oxygen analysis equipment, however, Systech Illinois manufacture both types and with over 22 years of experience have sold more than seven hundred trace oxygen detection solutions.

Zirconia based oxygen analysis instruments

How do they work?
This type of oxygen analysis instrument utilises a Zirconia cell to detect oxygen concentrations. The Zirconia cell is an electrochemical galvanic cell employing a high temperature ceramic sensor containing stabilised zirconium oxide.

The instrument is a self contained unit with the Zirconia cell mounted in a high temperature controlled furnace. The necessary electronics process the signal from the detection cell and display it directly via a digital display as oxygen concentration over the range 0.01vpm to 100%.

Advantages of Zirconia based oxygen analysis instruments:

• The response time is very fast, and much faster than the Fuel Cell approach. There is usually no problem with the speed of gas measurement.


• The sensor is non-depleting; in other words it will last the lifetime of the gas analysis instrument.

Disadvantages of Zirconia based oxygen analysis instruments:

• In the presence of hydrocarbons the oxygen that the instrument is trying to measure is catalytically converted to CO2 or other elements and therefore measures oxygen concentrations much lower than expected.


• In order to overcome this, gas analysis instrumentation manufacturers use a special filter canister to absorb these volatiles/hydrocarbons. This increases the time response and running costs and inevitably because the filter has to be replaced often, operators do not carry out these tasks, with breakdown of the measurement.


• Some manufacturers have what claim to be non-catalytic sensors, which do not work effectively in such high hydrocarbon conditions that exist.

Fuel Cell based oxygen analysis instruments

How do they work?
The cell performs essentially as a battery and consists of two electrodes in contact with a liquid or semi-solid electrolyte. The electrodes are connected through an external electronic metering circuit. Oxygen comes in contact with the negatively charged cathode (usually silver) where the first of two reactions take place. Oxygen is reduced (electron consumption) to the negatively charged hydroxyl species anode (usually lead) where the second reaction takes place. The hydroxyl ions react with the lead which is subsequently oxidised (release of electrons) to lead oxide. The use (reduction) and release (oxidation) of electrons results in a flow of electrons proportional to the oxygen concentration and is measured by the generation of an electric current.

Advantages:

• Inexpensive to replace, without interference from other gases (except CO2, which is not encountered in soldering oven applications).

Disadvantages:

• Slow to respond to low ppm O2 when exposed to high % O2 levels. Consequently the O2 level in the oven can be actually lower than the reading of the analyser, therefore can sometimes show the operator non ideal conditions.


• Systech Illinois solve this problem by employing their RACE™ technology. This employs a second fuel cell which protects the first cell from being exposed to high concentrations of O2.


• The guarded cell never sees high O2 levels and therefore responds very quickly, just as quickly as a Zirconia based system at the normal O2 levels seen by soldering ovens.


• Systech Illinois have patents on this technology and therefore it is not available from any other manufacturer.

Which is best -Zirconia or Fuel Cell?

As we make both types of cell, we don't see one as better than the other, as both have their strengths. Some manufacturers have to use Zirconia because the Fuel Cell is too slow. However Zirconia needs the use of expensive filters that normally do not get replaced, rendering the installation useless.

We find that most of our customers use our patented RACE™ Fuel cell based system, exclusive to Systech Illinois, but not all. This oxygen measurement technology employs a second fuel cell which protects the first cell from being exposed to high concentrations of O2. The guarded cell never sees high O2 levels and therefore responds very quickly, just as quickly as a Zirconia based system at the normal O2 levels seen by soldering ovens.

This is the fastest oxygen detection solution available in the marketplace. With RACE™ technology there's no need to use a special filter canister to absorb the volatiles/hydrocarbons. This gives far better response times and much lower running costs. Filters do not have to be replaced therefore operators do not have to remember this task, providing continuous accurate measurement.

Bryan Cummings is the Managing Director for Systech Illinois, which specialises in online, portable and laboratory gas analysis equipment. With 25 years as one of the leading experts in gas and moisture measurement, Systech Illinois’ application and technology knowledge provides efficient gas analysis solutions in many industry sectors worldwide.