REDUCE THE COST OF REFLOW
FURNACE APPLICATIONS
A LOW OXYGEN (O 2 ) CONCENTRATION IN REFLOW FURNACES DELIVERS A HIGHER
QUALITY PRODUCT, REDUCES DEFECTS, AND LOWERS MAINTENANCE COSTS,
POTENTIALLY SAVING OPERATORS THOUSANDS OF DOLLARS.
WHY MEASURE O 2
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 O 2 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 O 2 in the reflow furnace can
compromise the wetting properties of the solder as it flows, causing
poor solder joints, so the O 2 level in the furnace must be monitored.
THE DRAWBACKS OF TRADITIONAL SENSORS EXCEPTIONALLY ACCURATE O 2 ANALYSIS
Galvanic sensors have traditionally
been used to monitor O 2 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. Using Servomex’s non-depleting
Coulometric sensor, the
SERVOPRO MonoExact DF150E
provides an effective solution
for reflow O 2 analysis that avoids
the problems associated with
galvanic or Zirconia sensors for
this application.
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.
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 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.
As the cell depletes, measurements
become less accurate. For galvanic
sensors, low expectancy is a
function of usage – they eventually
Lead Anode (Depleting)
PbO
OH-
Silver Cathode
Membrane
KOH
Electrolyte
O 2 (Dissolved)
O 2 (Gas Phase)
GALVANIC SENSOR
P12
Gas
Sample
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 O 2
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 O 2 is reacted
with hydrocarbons that are given
off by the flux in the furnace.
This reacted O 2 is not being
measured, so the sensor gives
false readings that result in poor
quality welding.
ZIRCONIA OXIDE SENSOR
The fast response offered by
the Coulometric sensor gives an
immediate indication of changes
in O 2 , regardless of whether they
are caused by an ambient air
leak or incorrect nitrogen (N 2 )
inerting levels.
See pages 04-05: for a description
of the Coulometric process
See pages 10-11: for more about
the MonoExact DF150E
SERVOPRO
MonoExact DF150E
TRACE OXYGEN
ANALYZER
KEY BENEFITS OF THE
COULOMETRIC SENSOR
NO periodic replacement
NO reaction with the sample
NO false low readings
NO loss of measurement sensitivity
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