Homebrew PCBs

I have recently decided I need to utilize SMT resistors on one of my 
designs as there simply isn't room for regular thru hole resistors. 
From reading recent posts it seems as if I would simply use solder 
paste to glue/hold the SMT res. in place then heat it to melt the 
solder. Can I do this with a soldering iron or do I need an expensive 
hot air type machine? What stops the solder paste from bridging the 2 
pads beneath the SMT Resistor? Any suggestions would be appreciated.

>From: "aussiedude36" <aussiedude@...>
>Reply-To: Homebrew_PCBs@yahoogroups.com
>To: Homebrew_PCBs@yahoogroups.com
>Subject: [Homebrew_PCBs] Soldering SMT resistors
>Date: Mon, 01 Sep 2003 16:42:44 -0000
>
>I have recently decided I need to utilize SMT resistors on one of my
>designs as there simply isn't room for regular thru hole resistors.
>From reading recent posts it seems as if I would simply use solder
>paste to glue/hold the SMT res. in place then heat it to melt the
>solder. Can I do this with a soldering iron or do I need an expensive
>hot air type machine? What stops the solder paste from bridging the 2
>pads beneath the SMT Resistor? Any suggestions would be appreciated.

If you apply the correct amount of paste, it won't cause any shorting.

I have used sokler paste with a Weller Pyropen, but it's easier to use thin 
solder with a fine-tipped soldering iron. Lightly tin one of the pads, 
position the device and tack down the end on the tinned pad. Solder the 
other end to its pad, then solder the first end properly. A good pair of 
tweezers is the best way to hold and position the parts. you'll probably 
need some magnification.

73, Leon
--
Leon Heller, G1HSM Tel: +44 1424 423947
Email:leon_heller@...
My web page: http://www.geocities.com/leon_heller

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In article <bivsu4+9hlp@...>,
   aussiedude36 <aussiedude@...> wrote:
> I have recently decided I need to utilize SMT resistors on one of my
> designs as there simply isn't room for regular thru hole resistors.
> From reading recent posts it seems as if I would simply use solder
> paste to glue/hold the SMT res. in place then heat it to melt the
> solder. Can I do this with a soldering iron or do I need an expensive
> hot air type machine? What stops the solder paste from bridging the 2
> pads beneath the SMT Resistor? Any suggestions would be appreciated.

I've tried solder paste but haven't been terribly happy with it. The paste
actually contains the solder in tiny beads, rather like small ball
bearings and the "ball bearing" effect made it quite difficult to hold the
component exactly where I wanted it. This may have been a property of the
particular paste I used, I don't know, but the beads were clearly visible
through the microscope I was using. I know you can get pastes which
specify very small particle sizes, which may have been better, but they
are (to me) prohibitively expensive.

Even though the paste was supplied in a small syringe I found very precise
application to be difficult, both in terms of quantity and geographically,
requiring careful cleaning up afterwards.

Commercially I believe they use a UV curing adhesive to hold the
components in place before soldering.

My own technique is to "tin" the pads and then apply flux from a flux pen
dispenser. Holding the component in position with a dental probe I then
"tack" it into place at one end before soldering the other end properly.

I use an Antex 15W iron with a 1mm tip and, against conventional wisdom, I
touch the tip with solder before applying to the joint to be "tacked".

As far as solder bridging the gap between pads, components are coated with
a substance that repels solder to make this less likely; though it is
possible if you apply too much solder and heat.


Stuart.

>

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|__||__)/ __/  \|\ ||_   |   /
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___________________________/ stuartwinsor@...

101 uses for a Pentium: No1 - A slow cooker.

This is what I've found to work.

Get yourself an extra soldering iron fitted with a very fine tip. 
  Using some 0.5 mm diameter solder, tin one of the pads so its 
loaded with a small bead of solder, if both the pads are already 
beaded then use solder wick to remove solder from one of the 
pads. The idea is to have only one pad loaded with a bead of solder.

Get some good quality tweezes and hold the SMD chip in position 
and with your other hand touch the pad with the soldering iron, 
and let the SMD chip fall into place. Then you are free to solder 
the other pad. Use thin solder and a clean soldering tip so you 
can control how much solder the pads receive.

Desoldering is a pinch. Use two soldering irons on each pad, the 
SMD chip usually lifts off when you raise both soldering irons, 
and will stick to one of the tips. I give the iron a quick tap on 
a piece of wood to flick the SMD chip off.

note:
Rosin flux in ethanol is also very handy for SMD work.


aussiedude36 wrote:

> I have recently decided I need to utilize SMT resistors on one of my 
> designs as there simply isn't room for regular thru hole resistors. 
>>From reading recent posts it seems as if I would simply use solder 
> paste to glue/hold the SMT res. in place then heat it to melt the 
> solder. Can I do this with a soldering iron or do I need an expensive 
> hot air type machine? What stops the solder paste from bridging the 2 
> pads beneath the SMT Resistor? Any suggestions would be appreciated.
>  
> 
> 
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>

I've found that a very fast and reliable way to solder SMT 
resistors is as follows:

a)  Make a simple hold-down, e.g.,  from a block of wood 
and a bent piece of wire.  The wire is attached at one end and
bent so that the other end presses firmly on the top surface 
of the wood:

  
       _______    Wire
      /       \    hoop
     /         \
    /          |
    |          |
    |          |
 +--+---------------------+
 |                        |
 |     Wood block         |
 |                        |
 +------------------------+

(The wire can be pushed into a hole, or screwed
to side or top of the the block.  Or to your bench...
It's best if the wire has a flat end (use a file) rather
than a point.)

b) Now place the circuit board and the resistor under
the free end of the wire, using the wire to hold the
resistor exactly where you want it:

                | |       <-- Wire end
                | |
                |_|
              +-----+     <-- Resistor
              +-----+
            ====   ====   <-- copper pads
      +------------------
      |////////////////// <-- board
      +------------------
  XXXXXXXXXXXXXXXXXXXXXXX <-- wood surface
  XXXXXXXXXXXXXXXXXXXXXXX

c) Now you have both hands free to apply
solder and soldering iron in the usual way.  A fine
the (roughly the dimensions of the resistor/pad at the
tip) is necessary.
 
Mike Cowlishaw

On Wed, 3 Sep 2003 08:26:48 +0100, Mike Cowlishaw <mfcowli@...> 
wrote:

> I've found that a very fast and reliable way to solder SMT resistors is 
> as follows:
>
> _______    Wire
> /       \    hoop
> /         \
> /          |
> |          |
> |          |
> +--+---------------------+
> |                        |
> |     Wood block         |
> |                        |
> +------------------------+
>

very nice idea. i will try it.

another thing i often make:

when i have to do rapairs on smd ressitors etc.
and can't find appropriate tools to hold them (i know they must be 
somewhere ;-) )
i often use a piece of wire, more often a 1/4 watt wired resistor and 
solder one end to the smd part.
(needs not to be a good joint, just glued).
then you have a handle on the smd part, which you use to hold it while 
soldering the other pad.
(maybe tin the other smd parts end or pad)
when the other pad is solid just remove the "handle" and solder it.

you have to take care that you don't pull off the metal on the resistor 
with your handle.. but not too difficult..

this is a bit more work but you can place it very precise.


i never use solder paste, it is too expensive.
i worked with it one time, using a pneumatic dispenser.
it had a footswitch and a syringe like dispenser on a hose.

but you are right, bad or too much paste causes the parts to "drift away" 
in the oven.
i don't like it because it is so expensive.
any ideas how to make the paste at home?
i heared by electrolytical plating you can make very fine powder if you 
shake the electrode during plating?
would this work with solder?
anyways the dispensers are too expensive and i can solder well with the 
wire.. no need for paste.

i once saw a soldering iron where the tin was applied automaticly through 
the tip.
it was used to solder d-sub connectors (one hand holds the iron, the other 
the wire, plug fixed in holder.
footswitch for solder start/stop.
i think only solder was dispensed, flux was put on the wire/plug i guess.
that interested me very much to build, it would be a great help to have one 
hand free.
but i have no idea how to make this.

you can't transport the solder liquid, the whole hose would need to be hot.
you can't transport powder or wire too... it would melt at the tip and 
block it or not?
i really wonder how this is possible... please discribe if you know...


st

> aussiedude36 wrote:
> > I have recently decided I need to utilize SMT resistors on one of 
my 
> > designs as there simply isn't room for regular thru hole 
resistors. 
> >>From reading recent posts it seems as if I would simply use 
solder 
> > paste to glue/hold the SMT res. in place then heat it to melt the 
> > solder. Can I do this with a soldering iron or do I need an 
expensive 
> > hot air type machine? What stops the solder paste from bridging 
the 2 
> > pads beneath the SMT Resistor? Any suggestions would be 
appreciated.


Look in the Links section, there's a link to the Seattle Robotics 
newsletter with a method of SMT soldering using a toaster oven. I've 
never done it myself. The surface tension of the liquid solder pulls 
it together and away from non-wetted surfaces.

> > Be sure to visit the group home and check for new Bookmarks and 
files:
> > http://groups.yahoo.com/group/Homebrew_PCBs

Steve

A friend who's worked in the industry for a long time told me he'd 
hired an immigrant asian woman to assemble SMT boards for him, based 
on her reputation for speed. He said she used a small paintbrush to 
put solder paste on each pad, then a bamboo stick (like those 
potsticker skewers from the grocery store) with a bit of beeswax on 
the end of it to pick up and hold the parts in place. Then a touch 
with a small soldering iron.

Steve Greenfield