solarwind wrote:
>I recently read an article on the Sparkfun website regarding reflow
> soldering for surface mount components that can be done at home on any
> form of hot plate (stove top frying pan, skillet, etc...).
>
> I have several questions regarding this (that were unanswered in the
> article, and for which I was unable to find answers on the Internet).
>
> * I thought heat damages components. I was consistently cautioned not
> to overheat components during hand soldering. If the idea is to heat
> up the solder just enough to melt (and maybe a bit more so as not to
> form a "cold soldered" joint), then wouldn't reflow soldering do the
> same damage to components?

No, unless the temperature is too high.


> * One of my DIP chips came with a notice saying that it should be
> "cooked" at 125 degrees Celsius prior to reflow soldering. I would
> never reflow solder a DIP chip, but this is interesting. Why do they
> have such requirements? Is it because of any possibility of trapped
> moisture in the chip that could rapidly expand in high heat and damage
> internals?

Correct.


> What's the worst case for not obeying this notice?

Silicon popcorn. But in most cases, nothing bad will happen.


> * I've also heard of reflow soldering "profiles". What exactly are
> these profiles? Is it a requirement that I adhere to them for homebrew
> reflow soldering projects?

Profile is basically a temp vs time graph, that shows you what the 
temperature should be at a certain point in time. What it boils down to, is 
you need to preheat the board at a lower temperature (preheat), then quickly 
raise the temperature so the solder paste melts (ramp-up), then cool the 
board, but not too quickly (ramp-down).

A quick giggle brought this up (see graph on page 3): 
http://www.actel.com/documents/Solder_Reflow_LeadFree.pdf

Notice that this is for lead-free solder.


> * How do I apply solder paste on pads? Do I just squeeze it out of the
> tube and rub it consistently over the pads so that all of it is
> connected? Does a very precise amount need to be applied discretely on
> each individual pad? How is this done?

Take a small cyringe, carefully file off the sharp end to make the end flat, 
then use it to dispense solder paste. See this page for info on application 
techniques:

http://www.seattlerobotics.org/encoder/200006/oven_art.htm

You don't need to be too precise, you just need to dispense the right 
amount. Reflowing is self-correcting, to a point.


> * Where to buy solder paste, are there different types, what brand,
> what type? Should I use flux? Should I pre-flux the pads or apply flux
> and then solder paste?

This is the hard part. :)


> * I've also heard that it is wise to calibrate or know where your
> skillet/stove reaches a certain temperature. What temperature is good
> for reflow soldering? How do I measure this temperature?

See graph; you can wing it, or use a thermocouple.

The temperature isn't that critical for hobby stuff. Preheat the 
oven/skillet, then quickly raise the temperature and wait until all the 
solder paste melts.


> * How long should the PCB be on the skillet/frying pan during the
> reflow soldering process?

See graph.


> I know these are a lot of questions, but I am very interested in this
> cost-effective do-it-yourself technique to solder surface mount and
> fine pitch components on PCBs with ease.

You're on the right track. Baking is way easier than using a soldering iron, 
or a heat gun.

Vitaliy 

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