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Date:	Thu, 23 Mar 2017 08:13:50 -0700 (PDT)
From:	"Frank Miles (fpm AT u DOT washington DOT edu) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To:	"John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject:	Re: [geda-user] handsolder QFN packages
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On Wed, 22 Mar 2017, John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com] wrote:

> On 03/22/2017 04:15 PM, Erich Heinzle (a1039181 AT gmail DOT com) [via 
> geda-user AT delorie DOT com] wrote:
>> The key features are a ~3mm thick plate to spread the heat, having the
>> 800W hot plate starting at ambient remain on for 250 seconds, then
>> turned off, then lifting off the board with the jig at about 280
>> seconds, to get a really good fit to the recommended JEDEC heating
>> curve.
>> 
>> http://vk5hse.blogspot.com/2011/05/finishing-off-sdr-cube-transceiver.html
>> 
>> being able to lift the board without jostling at 280 seconds avoids
>> the problem of thermal inertia of the hot plate continuing to heat the
>> board longer than necessary.
>
>
> Thanks, I'm going to try this with my hotplates too.  Not sure which one will 
> give the best thermal
> rise-time, so I'll try both.  Do you know your hotplate wattage and mass? 
> One of mine is cast iron top,
> one aluminum.

Wattage can be important if you're trying to match the manufacturer's temperature
cycle ideal.  I experimented with varying sheet/plate thicknesses, trying to
make the heat as uniform as possible over its surface while keeping the thermal
mass low enough that the heater could warm things up quickly enough.  Think that
was a 0.8 - 1kW heater, which turned out to be kinda marginal (given the weak
thermal path from heater -- cast iron top -- top plate -- circuit board).  Think
I ended up with a ~0.1" thick plate.

If the thermal mass of your aluminum top is low enough it would probably be the
better choice.  While aluminum has a higher specific heat (absorbing more energy)
its conductivity is disproportionately greater.

 	-Frank

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