[time-nuts] Wenzel Oscillator Repair

[Joseph M Gwinn]

Chuck,


time-nuts-bounces at febo.com wrote on 01/21/2009 12:41:44 PM:

> Hi Joe,
> 
> When you preheat to 105C, you don't need to travel as far to
> reach the melting point of the solder as you do from room
> temperature (25C).  That makes a big difference in how much
> power you need to couple into the joint to make it melt.
> 
> If you go straight to the torch, and use the brute force
> supply of energy to make the joint melt quickly, you will
> overshoot the temperature every time.  A couple of seconds
> too long, and you can make quite a mess.
> 
> I've done it both ways, with the torch, you are moving very
> quickly, and making hasty yanks and grabs to try and remove the
> cover as quickly as possible.  With preheat you can move
> leisurely, and use a small iron, or hot air gun, to provide
> the additional calories needed to melt the solder.

I've used many a torch, and the trick is to keep the flame far enough from 
the work, so the temperature of the air hitting the work is reasonable. 
But it does take practice to get this right.  The danger is often 
overheating terminals and the like, not the big areas of metal.

But I do have two hot-air guns as well, glorified hair dryers, one for 
stripping paint (1000 F?), the other for shrinking tubing (I'll have to 
measure the temperature).  Need to be sure that the gun can melt solder 
fast enough.

 
> Preheat is best by a long shot.  Try it and I am sure you
> will agree.

Can't say that I ever bothered, but I don't disagree.  There is certainly 
nothing to lose if one preheats.


As for epoxy-glued covers, I have a war story.  In the 1960s, my Father 
worked at RCA in a project that was building various components of the 
moon lander.  The electronics was built in "cordwoods" which were a pair 
of two-sided glass epoxy circuit boards with axial-lead components strung 
between the two boards.  These boards were about 25mm apart, and the 
bottom board was potted in alumina-loaded epoxy.  The epoxy-potted side of 
the cordwood was glued to an aluminum cold plate with alumina-loaded 
urethane glue.  The alumina was for thermal conductivity.  This assembly 
had to work in a vacuum, so convection was unlikely.

The problem was that if a module in the center failed, one had to remove 
(and usually destroy) all the modules from the closest edge to the bad 
module, using a hot knife to cut the urethane.  Then, one of the 
technicians had a brilliant idea: Use a paperclip bent into a hook, some 
stiff wire bent into a bridge, and a rubber band to put a steady pull on 
the bad module, put the whole affair in the 180 F oven, and go to coffee. 
On return, the module would be free, dangling on the rubber band.  The 
glue had crept under the combination of heat and steady stress until it 
let go.  This saved an ungodly amount of money.  (My Father's role was to 
do the mechanical analysis to prove that this method wouldn't damage any 
of the modules, removed or adjacent.) 

Many epoxies will also creep, especially when hot.

Joe

 
> -Chuck Harris
> 
> Joseph M Gwinn wrote:
> > Chuck,
> > 
> > 
> > time-nuts-bounces at febo.com wrote on 01/21/2009 11:34:41 AM:
> > 
> >> Hi Joe,
> >>
> >> Nope, Magnus is talking about foam insulated hermetically sealed
> >> ocxo's.
> >>
> >> The point you are missing is the preheater is only set for a
> temperature
> >> that the foam, etc. can take on a continuous basis... such as +105C.
> >> This preheat reduces the amount of additional heat that must be added
> >> to make the solder melt.  The net result is usually so nice that you
> >> cannot even tell the foam has even been heated.
> > 
> > That the foam must handle the oven temperature is a good 
> point.  Certainly 
> > some preheat can help, but typical 63-37 eutectic solder 
> melts at 183 C, 
> > and for reasonable speed one must heat the metal at least 50 
> C hotter, and 
> > soldering irons are run at 310 C (590 F, 600 F being 
> typical).  The solder 
> > used on the cans I've seen looks more like 60-40 radio 
> solder, so higher 
> > temperatures may be needed.  It's less damaging to go a bit 
> high than a 
> > bit low, because if one is high, the can cover still comes 
> off quickly. 
> > And the faster this is done, the better.
> > 
> > One preheats circuit boards to about 115C before soldering, if only to 

> > ensure that all moisture is driven off.  So 105C is in the range.
> > 
> > Joe
> > 
> > 
> > 
> >> -Chuck Harris
> >>> I think we are talking about different things.  For gettingchips off 

> > a 
> >>> big multilayer board, preheat plus hot air is a standard way to go, 
> > but we 
> >>> are talking about how to unsolder a steel can with foam insulation 
> > within. 
> >>>  Slow heating to near soldering temperature is likely to 
> yield a heap 
> > of 
> >>> goo. 
> >>>
> >>> The point of the torch method is to heat the can's solder seam up 
> > *fast*, 
> >>> so the solder melts before the foam.
> >>>
> >>> Joe
> >>>
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