Reduce the cost of reflow furnace applications

A low oxygen (o2) concentration in reflow furnaces delivers a higher quality product, reduces defects, and lowers maintenance costs, potentially saving operators thousands of dollars.

Why measure o2 in the furnace?

Traditionally, printed circuit boards (PCBs) were cleaned with chlorofluorocarbon (CFC) based compounds. However, with the widespread elimination of CFCs, PCB manufacturers must now create an inert environment in the solder furnace to reduce O2 concentration.

In wave soldering, the PCB is passed over a cylinder of molten solder. In the reflow process, the PCB is pre-pasted with solder, then passed through a pre-heater, allowing the solder to run onto the components and hold them in place. The presence of O2 in the reflow furnace can compromise the wetting properties of the solder as it flows, causing poor solder joints, so the O2 level in the furnace must be monitored.

The drawbacks of traditional sensors

Galvanic sensors have traditionally been used to monitor O2 in reflow furnaces. These have a short lifespan – typically a few months – because they use a lead anode and silver cathode, which are consumed in the measurement process.

As the cell depletes, measurements become less accurate. For galvanic sensors, low expectancy is a function of usage – they eventually read low as the electrode sites are depleted and sensitivity is reduced.

The effect of trace level acid gases – generated by the flux on the PCBs – on the silver cathode means that these analyzers also require frequent recalibration, and regular sensor replacement. Without this, the analyzer will give falsely low readings, resulting in poor quality or scrap product.

A permeable membrane separates the gas sample from the electrolyte, allowing the O2 ions to migrate into solution.
If the sensor is exposed to an over-range condition, such as the solder system hood being opened, the membrane will saturate with oxygen from ambient air. It can take several hours for oxygen to return to parts-per-million (ppm) levels.

Zirconia sensors are not suited to this application, because any reducing gases (those that can be oxidized, such as carbon monoxide, hydrogen, methane and hydrocarbons) significantly interfere with the measurement.

Due to the high temperature of the sensor, the O2 is reacted with hydrocarbons that are given off by the flux in the furnace. This reacted O2 is not being measured, so the sensor gives false readings that result in poor quality welding.

Exceptionally accurate o2 analysis

Using Servomex’s non-depleting Coulometric sensor, the SERVOPRO MonoExact DF150E provides an effective solution for reflow O2 analysis that avoids the problems associated with galvanic or Zirconia sensors for this application.

It uses carbon electrodes which are not affected by chemical changes, and so does not need periodic replacement, nor does it produce false low readings, as there is no loss of measurement sensitivity or reaction with sample gas components.

The fast response offered by the Coulometric sensor gives an immediate indication of changes in O2, regardless of whether they are caused by an ambient air leak or incorrect nitrogen (N2) inerting levels.

Ensuring accuracy with no sensor drifting and no false low readings, it provides highly effective protection against inerted material oxidization. This supports excellent product batch quality and significantly improves nitrogen consumption.

The MonoExact DF150E can be easily integrated into multiple zone sample systems using a manual or automatic sequencing system. This makes it ideal for OEM furnace manufacturers who supply analyzers as an option or as standard for the end-user, heat treatment contractors and on-site or laboratory processes.

Next-generation Coulometric analysis for a range of industrial applications

Using a new and improved digital touchscreen with icon-driven guider user interface (GUI) for easier operation, the MonoExact DF150E combines the reliability of Servomex’s tried and tested Coulometric oxygen sensor in a more user-friendly package.

Built around the latest innovations in software and hardware, including an updated Coulometric digital oxygen sensor, the MonoExact DF150E brings users new operational and maintenance benefits that improve user control and reduce cost of ownership.

For more information or expert advice contact Servomex

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