Vectronics VEC-1500K - Manual (Page 22)

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1. A more massive tip contributes more energy to a solder connection than a
smaller tip (per degree of temperature rise).
2. A hotter tip contributes more energy than a solder connection that a cooler tip
(per unit volume of tip mass).
3. And, finally, a larger and hotter tip contributes a lot more energy than a
smaller cooler tip.
Of course, practical considerations limit how massive or hot a "real world"
soldering iron can get! Irons need to be relatively small and light-weight to work
on today's miniaturized equipment. Also, any time tip temperatures rise above
750-800 F, excessive heat can easily destroy sensitive miniature components and
damage solder pads on PC boards. It all comes down to finding the right
balance.
Selecting Unregulated Irons: The heating element in an iron without
thermostatic control remains powered continuously. Continuous heating means
the iron cannot adjust itself to a very wide range of heat demands. If the iron is
too small for a given job, the tip will cool too rapidly and won't transfer enough
heat to melt solder. If too large, it may overheat sensitive components and
damage PC boards. Ideally, the wattage of the heating element--along with the
size and shape of the tip--will provide a good "fit" with the demands of the job at
hand. The chart below provides guidance for selecting an unregulated iron:
Element Tip Temp. Tip Dia. Application
25-W 625 1/8" Precision, SMD
30-W 700 1/8" Instrument repair
35-W 750 3/16" Light PC board
50-W 800 1/4" PC-board, small connectors
60-W 750 5/16" Hand-wiring, installing connectors
In addition to choosing the right iron, it's important to choose the right tip.
We've noted that tips with greater mass store more energy (that's why irons with
more powerful heating elements come with larger tips installed). A small
diameter tip with a thinly-tapered end has less mass available for storing energy--
and less contact area to transfer heat onto the connection. This could be an
advantage when soldering extremely fine or temperature-sensitive work, but an
extreme disadvantage when soldering heavy parts onto large PC board land
areas. For heavy jobs, a large-diameter tip with a blunt screwdriver shape stores
and delivers energy more efficiently.
Using Thermostatically-Regulated Irons: Thermostatically-regulated irons
and solder stations are more flexible because they can adapt to a wider range of
heat demands than unregulated irons. While unregulated iron elements are