At 11:16 AM 5/10/97 EDT, you wrote:

In my *real* job I work for a contract electronics manufacturing company. We
do a lot of SMT work.

>        So far for prototyping SMT, I've used solder paste and TRIED to
>dab a little on each pad, then drop the part into position.  I then hold
>the part in position with the point of an Xacto knife and heat up one
>pin.  Once that is secured, all the other pins can be heated with a fine
>point iron.

For small quantity hand assembly I prefer to use a small soldering iron and
a curved tweezers.  It takes a bit of practice but after a while you can
really fly on it.  When you consider the time it takes to bend the leads and
cut them off, its probably faster to use SMT over leaded resistors and small
caps.  ICs are definitely require more care with SOIC type packages.  Solder
bridges will result if you do not use extreme care, but I do it all the
time. HINT: Don't drink a lot of coffee when you are doing this!!

Since solder paste stencils cost about $350, we don't usually use them for
prototypes unless there are a lot of parts (> 300-400).  If we do use a mask
we apply the paste, hand place the parts and run it through the IR oven.  I
have placed paste with a pump dispenser, but don't think it is worth the
trouble.

By hand, I found it is really useful to use some water soluble solder flux.
We use that by the gallon and I use a small dropper bottle.  The flux pens
Digi-Key sells for about $3 also work pretty well and are not as messy.  Be
sure you stop and wash off the board every hour or so, or the flux can cause
corrosion.

For chip parts put some flux over the pads and place the part. While still
holding it with the tweezers I tack solder one end. Then I solder the other
lead then finish the first.  For ICs I hold the part in place and tack
solder a couple of pins.  Then I double check that all the pins are lined up
right before soldering them.

Everyone I know who does SMT hand soldering uses a slightly different
method.  Some people like to hold the part down with a tooth pick.
Experiment a bit and do what works for you.

>        For production, I'm wondering about having our PC house (down the
>street) silkscreen on solder paste.  I wonder how long we can store the
>boards before the paste dries up.

If you put it in the refridgerator you can generally get by over night, but
thats about it.

>        Years ago I saw a vapor-phase soldering system at a convention.
>It was a Pyrex jar with condensor tubes around the top.  You heat fluid
>in the bottom and run water through the condensor.  The fluid vaporizes
>and stays below the condensor.  Loaded boards are lowered into the vapor
>for a few seconds, soldering all connections.  I haven't seen the system
>since then, but it looked pretty neat.
>        I also wonder about using a low flow temperature controlled heat
>gun for soldering.  High flow blows the parts off the board.
>Nontemperature controlled melts parts.  Anyone have any experience with
>this?
>        Finally, how about a temperature controlled heat gun as a mini
>environmental chamber?
>

To properly solder the whole board at once you need to go through a complex
heat profile to make sure the board and parts heat and cool at the right
temperature and right rates.  Each board is different depending on the size,
types and sizes of components, etc.  Setting that up is quite an art.
Unless you have a few hundred thousand dollars to spend on the proper
equipment I suggest that you either solder them by hand or find someone else
to do it for you.  There are also SMT repair units available for a few $K,
but you are still doing one part at a time.

Hope this helps!

- Gary

------------------------------------------------------------------------
Gary Sutcliffe,  W9XT          Unified Microsystems
ppvvpp@mixcom.com              PO Box 133 Slinger, WI 53086
http://www.qth.com/w9xt        414-644-9036