[Elecraft] K3 Audio Upgrade Mod

[Paul Christensen]

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