solarwind wrote:
> * 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?

It can, which is why there are such things as soldering temperature profiles
(temperature as a function of time).  Professional reflow equipment is not
cheap since it implements these profiles, ususally by moving the board thru
a series of "zones" at preset temperatures.

> * 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?

Yes.

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

Popcorn.

> * I've also heard of reflow soldering "profiles". What exactly are
> these profiles?

Temperature as a function of time.  They are designed to get the solder to
flow with minimal heat stress to the components.

> Is it a requirement that I adhere to them for homebrew
> reflow soldering projects?

Only if you want the parts to live.  There is some wiggle room to be sure,
but the more you deviate, the higher the probability of dead parts.

> * How do I apply solder paste on pads?

In real production, this is done with a stencil.  The stencil is usually
thin steel of a specific thickness.  The solder paste is sortof squeegied
accross the stencil so that it gets wiped off the stencil but blobs remain
in the voids formed by the holes in the stencil.  The stencil is then lifted
off the board and just the blobs of solder paste remain.

For one-off home use, you'd probably apply the paste manually with a
syringe, and then expect to fix a few shorts afterwards.

> * Where to buy solder paste,

Buying small quantities is tricky.  It also has a limited shelf life.

For one-off home construction, you are probably better off just using
regular solder and a fine tip iron for fine pitch parts.


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