Homebrew PCBs

There is perhaps some confusion about what the term \ufffdsolder(ing) paste\ufffd
means.

 \ufffdSolder paste\ufffd is solder powder in a paste form, generally including some
flux.  It is used for mounting surface-mount parts onto PC boards. 
Ideally it is thixotropic which means it doesn\ufffdt flow when the pressure
applied to it (such as from gravity) is below a certain threshold.  It is
also sticky, so it holds SMT parts onto the PCB before the parts are
heated to melt the paste and create a solder joint.

\ufffdSolderING paste\ufffd generally refers to a type of flux, and doesn\ufffdt contain
solder powder.

Flux: 5 : a substance used to promote fusion (as of metals or minerals);
especially : one (as rosin) applied to surfaces to be joined by soldering,
brazing, or welding to clean and free them from oxide and promote their
union (definition from m-w.com)

There are many types of flux, some more corrosive than others.  Acid flux
is definitely to be avoided for electronics work.  I have used sal
ammoniac paste (ammonium chloride in petroleum grease, brown colored
stuff) for electronics tinkering in high school.  I don\ufffdt recall it being
very corrosive.  However, there are better types of flux available for
electronics use, so I don\ufffdt recommend the sal ammoniac paste, either.

Rosin (resin from pine trees) is the traditional flux used for electronics
work.  There is also water-soluable flux available, however this should be
washed off within a few hours of use.  See
http://www.kester.com/faq_ts.html and http://www.kester.com/flux.html .


Recommendations: use rosin core solder for general electronic use.  Use
rosin core solder or solder paste for surface mount parts.

Bare copper oxidizes and becomes hard to solder.  To improve solderability,

-	solder your parts to the board soon after the board is made and cleaned
-	dip the PCB in etchant for a few seconds and then rinse thoroughly to
brighten up the copper before stuffing parts and soldering
-	clean the copper with an abrasive cleaning pad (some people on this list
have warned against using steel wool because it leaves iron particles in
the copper which then rust)
-	if you use photosensitive boards, leave the photosensitive mask on until
you want to solder to the board.  Some people have reported that
photosensitive mask material can be left on the board, and need not be
removed before soldering.
-	Tin plate the board immediately after etching, using for example Tinnit
electroless tin plating solution:
http://jameco.com/cgi-bin/ncommerce3/ExecMacro/jameco/searchResult.d2w/report?sort=BKW&search=tinnit
-	Apply rosin-based flux to the areas to be soldered.  I\ufffdve avoided that
because it would be a hassle to clean up, or would be messy if left on the
board.

Cheers,
- Jan

-----Original Message-----
From: Stuart Winsor [mailto:stuartwinsor@...]
Sent: Sunday, August 31, 2003 7:23 AM
To: Homebrew_PCBs@yahoogroups.com
Subject: Re: [Homebrew_PCBs] but is "soldering paste" ok to use?

In article <22.3d3bf382.2c819b2e@...>,
   <JanRwl@...> wrote:
> "Solder paste" is a dark brown CORROSIVE substance one might use for,
> say, roofing or plumbing with copper pipes.

Perhaps it should be mentioned that there are solder pastes specifically
for electronic work. See the example given.

http://uk.farnell.com/jsp/search/searchresultstwo.jsp?action=0&ImgDisp=Y&QText=Solder+paste>>

If you go to that company's home page there seem to be lot of options for
selecting your country so maybe you can buy from them.

For all my work I now use "Low mwlting point solder" also available from
the above, who are a major supplier of electronic components of all sorts.

Interestingly they have a kit for a CNC PCB drill that is made of wood!

Stuart.
No connection with the above company other than as a satisfied customer.

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