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'SMT'
1997\05\10@112057 by Harold M Hallikainen

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       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 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.
       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?

Harold



Harold Hallikainen                               phone/fax/bbs +1 805 541
0201
Hallikainen & Friends, Inc.                web
http://slonet.org/~hhallika
PO Box 4737                                         email
spam_OUThhallikaTakeThisOuTspamslonet.org
San Luis Obispo, CA 93403-4737     email .....hhallikaKILLspamspam@spam@broadcast.net
USA                                                       email
ap621spamKILLspamcleveland.freenet.edu

1997\05\10@125332 by Gerhard Fiedler

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At 11:16 10/05/97 EDT, Harold M Hallikainen wrote:
>        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.

This sounds pretty good -- I'll definitely try it with solder paste.
(Sounds like it spares you a hand :-)

>        Years ago I saw a vapor-phase soldering system at a convention.
>It was a Pyrex jar with condensor tubes around the top.

>        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?

I'm also interested in any small numbers production type tool, like you
mentioned some. If anybody has any suggestions...

Thanks for the replies.

1997\05\10@132434 by Gary Sutcliffe

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face
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.

{Quote hidden}

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
.....ppvvppKILLspamspam.....mixcom.com              PO Box 133 Slinger, WI 53086
http://www.qth.com/w9xt        414-644-9036

1997\05\10@170645 by Clyde Smith-Stubbs

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Thus spake Harold M Hallikainen (EraseMEharoldhallikainenspam_OUTspamTakeThisOuTJUNO.COM):

>         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

I've successfully soldered quite a few boards using a heat gun (nothing special,
just a standard heat-shrink/paint-stripper type gun). This includes soldering
a 100 pin fine pitch (0.5mm pitch!) device on a non solder-masked board. Of
course you get bridges between the pins - but cleaning up with solder wick
(or coax cable braid soaked in spray flux) fixes all that very nicely!

The way I do it is to squeeze solder paste from a syringe onto the pads (just
lay down a stripe), place the components, then heat (on a heat-proof surface)
with the gun, gently at first then once the whole board is hot I concentrate on
one component at a time until the solder melts, give it about 5 seconds fully
melted (the gun is about 3" above the board at this stage) then move to the
next component. IT takes about 60 seconds total for a 4" square board, and
providing you don't overheat, this is within specs for soldering.

Be sure not to move the board until it has cooled down (or the chips will fall
off - I know!) Then inspect with a magnifying glass, use solder wick to remove
bridges (small ones can be removed just by drawing the iron tip between pins)
and resolder any pins that did not get soldered.

Be sure NOT to get solder paste underneath the component - it can lead to some
very
hard-to-find faults, because it does not get melted, and can short out pads.

I actually find this easier than hand-soldering lots of through-hole pins. But
for
any number of boards, it's better to get them automatically done.

Have fun!

--
Clyde Smith-Stubbs    | HI-TECH Software,       | Voice: +61 7 3354 2411
clydespamspam_OUThtsoft.com      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.htsoft.com | QLD, 4051, AUSTRALIA.   |
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