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LEAD ATTACHMENT
1. Although the DT-470/471/670/671 SD sensor package is a two-lead device, measurements should be preferably made
using a four-wire configuration to avoid uncertainties associated with lead resistance.
Two-lead measurement scheme – The leads used to measure the voltage are also the current carrying leads. The
resultant voltage measured at the instrument is the sum of the temperature sensor voltage and the voltage drop across
the two leads. See Figure 1A.
Four-
lead measurement scheme – The current is confined to one pair of current leads with the sensor voltage measured
across the voltage leads. See Figure 1B.
2. Lead Polarity: When viewed with the base down (the base is the largest flat surface) and the leads towards the observer,
the positive lead (anode) is on the right and the negative (cathode) is on the left.
3. Strip the insulation from the connecting wires by delicately scraping with a razor blade, fine sand paper, or steel wool.
Phosphor-bronze or Manganin wire, in sizes 32 or 36 AWG, is commonly used as the connecting lead wire. These wires
have low thermal conductivity and high resistivity which help minimize the heat flow through the leads. Typical wire
insulation is Polyvinal Formal (Formvar™) or Polyimide (ML). Formvar™ insulation has better mechanical properties such
as abrasion resistance and flexibility. Polyimide insulation has better resistance to chemical solvents and burnout.
4. Prepare the connecting wire ends with a RMA (rosin mildly active) soldering flux, tin them with a minimal amount of 60%
Sn 40% Pb solder. Use a low wattage soldering iron which will not exceed 200 °C.
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lean off residual flux with rosin residue remover. The sensor leads can be prepared in an identical manner.
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oin one sensor lead with two of the connector wires. Apply the soldering iron to the connector wire above the joint area
until the solders melt, then remove the iron. Repeat for the other set of connector wires and the other sensor lead. Heat
sinking the SD sensor with a flat jaw alligator clip is good practice to eliminate heat build up at the sensor element.
7. A
void putting stress on the device leads and leave enough slack to allow for the thermal contractions that occur during
cooling which could fracture a solder joint or lead. Some epoxies and shrink tubing can put enough stress on lead wires to
break them.
HEAT SINKING/THERMAL ANCHORING
1. Since the area being measured is read through the base of the sensor, heat flow through the connecting leads can create
an offset between the sensor chip and the true sample temperature. Thermal anchoring of the connecting wires is
necessary to assure that the sensor and the leads are at the same temperature as the sample.
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onnecting wires should be thermally anchored at several temperatures between room temperature and cryogenic
temperatures to guarantee that heat is not being conducted through the leads to the sensing element. Two different size
copper bobbins are available from Lake Shore for heat sinking connecting leads: P/N 9007-900 and 9007-901.
3. I
f connecting wires have a thin insulation such as Formvar™ or Polyimide, a simple thermal anchor can be made by
winding the wires around a copper post, bobbin, or other thermal mass. A minimum of five wraps around the thermal mass
should provide sufficient thermal anchoring. However, if space permits, additional wraps are recommended for good
measure. To maintain good electrical isolation over many thermal cycles, it is good practice to first varnish a single layer of
cigarette paper to the anchored area then wrap the wire around the paper and bond in place with a thin layer of IMI 7031
Varnish. Formvar wiring insulation has a tendency to craze with the application of IMI varnish. If used, the wires cannot be
disturbed until the varnish is fully cured and all solvents have evaporated (typically 12
– 24 hours).
4. A final thermal anchor at the sample itself is good practice to ensure thermal equilibrium between the sample and temperature
sensor.