Jim,
I'm with you in that most folks find it easier to work with leaded
components on a SMT PC board -- until things start going bad. So, I'll take
the counterpoint on this one...
I learned the hard way that using leaded components on a surface mount
pad/trace can lead to permanent PCB damage. For example, if making the K3
AF mod with the leaded resistors, one needs to avoid the temptation of
moving the part after heat is applied and removed. Some folks (like your's
truly), will be tempted to press the resistor against the PCB after
soldering to ensure adaquate clearance from surrounding hardware.
The surface area and bonding strength of the copper cladding relative to the
glass board is extremely small. A small leaded component can take on the
characteristics of a lever. A small amount of pressure on the parts yields
pressures too great for the PCB pad. At some point, the breaking strength
of the bonding is exceeded and the solder pad and/or trace lifts from the
glass.
When I first started working with SMT parts and began testing various leaded
resistors of differing value, I did what I always do: I began to straighten
the leaded component for visual aesthetics. Generally, this isn't a problem
with through-hole boards. Why? The lead passing through the hole exhibits
little, if any pressure on the surrounding pad or trace when the component
is flexed. The pressure is built-up against the hole sides and not the
surface of the PCB. By contrast, flexing of the leaded part on the surface
of the PCB will almost always result in permanent PCB damage.
Probably the moral of my story is that if leaded components are used on a
SMT surface, do not flex the part thereafter!
Paul, W9AC
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