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PICList Thread
'[OT] SMT soldering'
1997\09\06@092935 by Andrew Russell Morris

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I like [OT} discussions on the piclist, if they are things that other
PIC'ers would find interesting and don't become excessive. I get a lot of
valuable information that way. I don't have time to subscribe to other
lists. I am currently working on a project for a client which is my first
PIC project and I'm doing well with it. I think an [OT] discussion on the
piclist helped me solve a problem that is indirectly related to PIC's. My
project is currently in a breadboard state, wired with through-hole parts
on perfboard. The pre-production prototypes and the production units will
be SMT (Surface Mount Technology). I wondered how I would solder those tiny
parts onto the circuit board as that would be very difficult for me.
Through reading an [OT] thread on the piclist, I plan, when I get that far,
to buy a toaster oven, a temperature controller and a syringe of solder
paste.

One question: If I end up having to solder parts onto both sides of the
board, how do I keep the ones on the bottom from falling off? Are all SMT
parts the same height, so that the oven tray will hold them in place until
they cool off? Any suggestions are welcome.

1997\09\06@102956 by John Payson

picon face
> One question: If I end up having to solder parts onto both sides of the
> board, how do I keep the ones on the bottom from falling off? Are all SMT
> parts the same height, so that the oven tray will hold them in place until
> they cool off? Any suggestions are welcome.

Glue [or other similar adhesive].  Actually it's recommended for the parts
on both sides of the board, since otherwise parts might move slightly when
the board is put into or removed from the oven.

1997\09\06@131616 by Mike Keitz

picon face
On Sat, 6 Sep 1997 10:40:54 -0400 Sean Breheny <spam_OUTshb7TakeThisOuTspamCORNELL.EDU> writes:
>At 09:29 AM 9/6/97 -0400, you wrote:
>> The pre-production prototypes and the production units will
>>be SMT (Surface Mount Technology). I wondered how I would solder
>those tiny
>>parts onto the circuit board as that would be very difficult for me.
>>Through reading an [OT] thread on the piclist, I plan, when I get
>that far,
>>to buy a toaster oven, a temperature controller and a syringe of
>solder
>>paste.

If your hands are steady and your eyesight reasonably acute, you can
solder typical "coarse" SMT parts such as SOICs, 1206 and 0805 chip
parts, SOT-23 transistors, etc. one at a time by hand using conventional
soldering techniques.  I haven't tried it and probably would not
reccomend it for parts with lead spacing smaller than .04" like QFPs,
etc.

The minimum and usually adequate equipment includes: a pencil soldering
iron with a reasonably small tip (1/16 or 3/32 chisel), a roll of
small-diameter wire solder, a pair of tweezers, and some desoldering
braid.  Liquid rosin flux and a magnifying glass are also useful
sometimes.  If the board or parts have become a little corroded, apply
liquid flux to all the pads so they will take solder readily.

For chip components, first put a little blob of solder on one of the
pads.  Pick up the component with tweezers and slide it in position while
remelting the blob of solder.  Then solder the other end down.  Don't use
too much solder, manufacturers advise that getting solder up over the top
of a chip component can make it unreliabale.  If there are a lot of chip
components all facing the same direction, you can streamline the process,
first blob one end of each component, then place the components, then
turn the board around and solder the other ends.  With a little practice,
SMT resistors and capacitors can be installed a lot faster than thru-hole
ones.

To remove chip components, the best technique is to use two soldering
irons to melt both ends at once and lift the component away, Clean all
the solder from one end using solder braid, leaving the other end
"blobbed" to install the replacement component.  Advice is not to reuse
parts that have been removed.  You'll probably lose them in the carpet
anyway.

The technique for SOT transistors and other 3-legged beasties is the same
as chip components, only there are two small pads on one side.  Blob the
collector pad and tack the part down, then carefully solder the other 2.
Use only a small amount of solder so they don't bridge.  If they do end
up bridged, remove the excess with solder braid.

For ICs, blob one corner pad and tack the part down so the other pins
line up.  Double-check that it is the right IC and in the right
orientation.  Solder the other corner lead down.  Using an absolute
minimum of solder, solder the rest of the leads.  It may appear that the
soldering iron tip is too large.  Actually it is OK to touch and melt
more than one pin at a time as long as there isn't enough solder around
to bridge them.  The important thing to control is the quantity of
solder.  Clean the iron tip frequently on a sponge so it doesn't
accumulate a blob of solder.  Think of the soldering iron as a source of
heat, not a device that applies solder.

Solder paste helps to speed up the process by making it easy to apply a
small controlled amount of solder.  Apply a thin line of solder paste
under each row of pins before setting the chip down, then mash away with
the soldering iron to melt it 2 or 3 pins at a time.  Buy solder paste in
small quantities.  It is perishable and will become useless after about 6
months.

PLCC chips are similar, but a lot harder to keep the solder from bridging
under the chip.  After soldering, test all adjacent pins with an ohmmeter
and if any shorts are found use desolder braid to pull some of the excess
out from under the chip.

Removal of ICs is difficult.  Start by using solder braid to remove as
much solder as possible.  If the IC is under 20 pins it may be possible
to use two soldering irons and braid to distribute the heat to melt all
the pins free at once.  If not, heat each pin and use a dental pick or
large sewing needle to bend it slightly so it is clear of the board.
When the part is off, clean the remaining little bumps of solder off the
pads.

There is a kit inclucing a bismuth alloy to form a low melting mess of
all the solder, so it will stay melted on all the pins and the IC can be
removed.  I haven't tried it.
{Quote hidden}

Commercial SMT assembly uses a metal mask to screen solder paste onto the
pads to be soldered (Solder paste is a mixture of fine particles of
solder and flux).  Then the parts are set onto the board with their leads
pushed down into the solder paste.  The reflow oven applies a dry heat to
melt the solder paste and cause it to reflow onto the parts.  The flux is
washed off.

>        If it is going to keep the solder moving, then how?
>I am just generally interested in how this thing will work. Also, if
>there
>will be through hole components on the boars also, do they have to be
>soldered after the SMT or can they also be soldered at the same time
>as the
>SMT?

This is another technique, the SMT parts are glued in place to the solder
side of the board, the thru-hole parts placed, and the board goes through
a conventional soldering machine.  The components do indeed "sit in a
pool of molten solder."  They are designed to withstand it.

1997\09\07@112336 by Eric van Es

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Mike Keitz wrote:
>
> Removal of ICs is difficult.  Start by using solder braid to remove as
> much solder as possible.  If the IC is under 20 pins it may be possible
> to use two soldering irons and braid to distribute the heat to melt all
> the pins free at once.  If not, heat each pin and use a dental pick or
> large sewing needle to bend it slightly so it is clear of the board.
> When the part is off, clean the remaining little bumps of solder off the
> pads.

Good advice Mike! I actually enjoyed reading a rather long mail!

I once spoke to a guy who had to desolder and solder smt componenets
without the special equipment for the componenets. Apparently you get a
soldering iron tip that fit over an ic, ie one for 20, 40 pin etc.
Only very expensive....

He ended up using a scalpel to lift the IC's from their pads. It sounded
like it worked pretty good.

Cheers!
--
Eric van Es               | Cape Town, South Africa
.....vanesKILLspamspam@spam@ilink.nis.za | http://www.nis.za/~vanes
LOOKING FOR TEMPORARY / HOLIDAY ACCOMODATION?
http://www.nis.za/~vanes/accom.htm

1997\09\07@193930 by fastfwd

face
flavicon
face
Mike Keitz wrote:

> > Removal of ICs is difficult.  Start by using solder braid to remove as
> > much solder as possible.  If the IC is under 20 pins it may be possible
> > to use two soldering irons and braid to distribute the heat to melt all
> > the pins free at once.  If not, heat each pin and use a dental pick or
> > large sewing needle to bend it slightly so it is clear of the board.
> > When the part is off, clean the remaining little bumps of solder off the
> > pads.

and Eric van Es <PICLISTspamKILLspamMITVMA.MIT.EDU> replied:

> I once spoke to a guy who had to desolder and solder smt componenets
> without the special equipment for the componenets. Apparently you
> get a soldering iron tip that fit over an ic, ie one for 20, 40 pin
> etc. Only very expensive....
>
> He ended up using a scalpel to lift the IC's from their pads. It
> sounded like it worked pretty good.

Guys:

Here's another way that often works better AND doesn't bend the IC
leads (a necessary requirement if, for instance, you're removing a
surface-mount PIC for the purpose of putting it in a programmer and
verifying its contents):

   1.  DON'T use solder braid to "remove as much solder as
       possible".  Instead, ADD solder:  Lay down a thick bridge of
       solder across all the leads on each side of the IC.

   2.  Turn the board upside-down.  Gravity is your friend.

   3.  Heat the leads on one side of the IC a little, then heat the
       leads on the other side.  Switch back and forth between the
       two sides, heating each a little at a time, until both sides
       melt more-or-les simultaneously.

   4.  The IC will drop off the board onto your bench.  The force of
       the impact will knock nearly all the molten solder off the
       IC's leads... The small amount that remains can be removed
       with solder braid.

If the chip is glued to the board, of course, you'll need to break
the adhesive bond... A dental pick or small screwdriver GENTLY wedged
under one side of the chip (AFTER the solder is melted) will snap
the chip free from the board without bending the leads.

-Andy

P.S.  Wear safety glasses.

=== Andrew Warren - .....fastfwdKILLspamspam.....ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499

=== For PICLIST help (including "unsubscribe" instructions),
=== put the single word "help" in the body of a message and
=== send it to: EraseMElistservspam_OUTspamTakeThisOuTmitvma.mit.edu

1997\09\08@010215 by tjaart

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face
Mike Keitz wrote:

> If your hands are steady and your eyesight reasonably acute, you can
> solder typical "coarse" SMT parts such as SOICs, 1206 and 0805 chip
> parts, SOT-23 transistors, etc. one at a time by hand using conventional
> soldering techniques.  I haven't tried it and probably would not
> reccomend it for parts with lead spacing smaller than .04" like QFPs,
> etc.

There is a way... You can solder any SMD IC like this:
1) Solder (sOLDER for mIGUEL) two opposite pins onto the pads to lock
the
  IC into place.
2) Wet all the pins with flux to ensure good joints (not the Jamaican
  type, man).
3) Roll a big blob of solder to and fro accross the pins.
4) Clear all the bridges with solder braid

> The minimum and usually adequate equipment includes: a pencil soldering
> iron with a reasonably small tip (1/16 or 3/32 chisel), a roll of
> small-diameter wire solder, a pair of tweezers, and some desoldering
> braid.  Liquid rosin flux and a magnifying glass are also useful
> sometimes.  If the board or parts have become a little corroded, apply
> liquid flux to all the pads so they will take solder readily.

If you use the above method, you don't even need any special tips. (You
should see mine...)

{Quote hidden}

I fully agree here...

> To remove chip components, the best technique is to use two soldering
> irons to melt both ends at once and lift the component away, Clean all
> the solder from one end using solder braid, leaving the other end
> "blobbed" to install the replacement component.  Advice is not to reuse
> parts that have been removed.  You'll probably lose them in the carpet
> anyway.

And here...

>
> Removal of ICs is difficult.  Start by using solder braid to remove as
> much solder as possible.  If the IC is under 20 pins it may be possible
> to use two soldering irons and braid to distribute the heat to melt all
> the pins free at once.  If not, heat each pin and use a dental pick or
> large sewing needle to bend it slightly so it is clear of the board.
> When the part is off, clean the remaining little bumps of solder off the
> pads.

If you are going to scrap the PCB, there is a fast and safe (for the IC)
way. Turn the board upside down (IC facing down). Heat the top of the
PCB with a really hot heat gun and watch your undamaged drop onto your
desk. Gasp and run to open all the windows (PCB's are generally not made
to smell good at high temperatures.)

--
Friendly Regards

Tjaart van der Walt
tjaartspamspam_OUTwasp.co.za
________________________________________________________
|        WASP International   http://wasp.co.za          |
|   R&D Engineer : GSM peripheral services development   |
|Vehicle tracking | Telemetry systems | GSM data transfer|
|Voice : +27-(0)11-622-8686  |  Fax : +27-(0)11-622-8973 |
|             WGS-84 : 26010.52'S 28006.19'E             |
|________________________________________________________|

1997\09\08@011638 by tjaart

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face
Andrew Russell Morris wrote:
>
> One question: If I end up having to solder parts onto both sides of the
> board, how do I keep the ones on the bottom from falling off? Are all SMT
> parts the same height, so that the oven tray will hold them in place until
> they cool off? Any suggestions are welcome.

Hey, those are two questions! ;)

The one side of the PCB is laid out to contain all the through-hole
components, and most of the SMD's. This side is soldered by a process
called
reflow. This is basically an oven with a conveyor belt running through
it (a
bit like some high volume Pizza ovens).

The rest of the SMD components (*NOT* through holes) are arranged (and
glued)
on the other side in rows so their leads are lined up. The board is held
with
this side facing down over a solder bath. A wave is created in the bath
and
this runs under the board soldering the components as it travels from
one
side to the other (See why you need glue?).

This is called wave soldering.

Never put though hole components on both sides of the board even if your
friendly manufacturer assures you it is possible.

Try to keep your component count as low a possible by using, for
instance,
resistor arrays in stead of discrete resistors. These arrays are more
expensive than the discretes, but the component cost is small in
relation
to the placement costs (per component).

Try to avoid using through hole components. The placement costs are high
and you pay per pin, where with SMD's you pay per component.

Use the smallest components your manufacturer can handle. The higher
cost
is paid for by the savings on PCB and enclosure costs. You'll see that
these last two often-overlooked aspects often consistute more than half
the total cost.

--
Friendly Regards

Tjaart van der Walt
@spam@tjaartKILLspamspamwasp.co.za
________________________________________________________
|        WASP International   http://wasp.co.za          |
|   R&D Engineer : GSM peripheral services development   |
|Vehicle tracking | Telemetry systems | GSM data transfer|
|Voice : +27-(0)11-622-8686  |  Fax : +27-(0)11-622-8973 |
|             WGS-84 : 26010.52'S 28006.19'E             |
|________________________________________________________|

1997\09\08@035112 by Peter Homann

picon face
Andrew Russell Morris wrote:
>
> One question: If I end up having to solder parts onto both sides of the
> board, how do I keep the ones on the bottom from falling off? Are all SMT
> parts the same height, so that the oven tray will hold them in place until
> they cool off? Any suggestions are welcome.


You'll need to support the pcb along the edges so that the bottom of
the pcb is not in contact with a tray, etc. The surface tension of the
solder paste will hold the components on the bottom side of the pcb,
unless thet are very heavy. In that case it will be necessary to use
component glue for those parts. Hint, put all the large components on
the top side of the pcb.

When using a toaster oven, ensure that the temperature of the oven is less
than 80 degs C, before placing the pcb in. Then, turn on the oven, with the
temperature of the controller set to, say 209 degs C. When the temp is
reached, the controller will switch off the heating element. Open the door
and let is cool for a while before removing.

Note: the toaster should be a "Fan Forced" one so that the temperature is
consistent throughout the oven.

I hope this helps.


Peter.
--
Peter Homann   email: KILLspampeterhKILLspamspamadacel.com.au       Work : +61 3 9596-2991
Adacel Pty Ltd                                   Fax  : +61 3 9596-2960
250 Bay St, Brighton 3186, VIC, AUSTRALIA      Mobile :     014 025-925
http://www.adacel.com.au     Australian Software Engineering Excellence

1997\09\08@231901 by Andrew Russell Morris

picon face
>
>When using a toaster oven, ensure that the temperature of the oven is less
>than 80 degs C, before placing the pcb in. Then, turn on the oven, with the
>temperature of the controller set to, say 209 degs C. When the temp is
>reached, the controller will switch off the heating element. Open the door
>and let is cool for a while before removing.
>

Do you turn the oven off and open the door as soon as 209 degrees is
reached, or sometime afterward. I know the the controller will switch off
the heating element at 209 degrees, but it will come back on when the
temperature drops a little, and keep turning on and off, maintaining
temperature at the set point until I turn it off.

I don't want to appear stupid. I think you mean as soon as the controller
switches the heater off for the first time, but I just want to be sure I
understand what you meant. :-)

1997\09\08@234438 by Peter Homann

picon face
Andrew Russell Morris wrote:
>
> >
> >When using a toaster oven, ensure that the temperature of the oven is less
> >than 80 degs C, before placing the pcb in. Then, turn on the oven, with the
> >temperature of the controller set to, say 209 degs C. When the temp is
> >reached, the controller will switch off the heating element. Open the door
> >and let is cool for a while before removing.
> >
>
> Do you turn the oven off and open the door as soon as 209 degrees is
> reached, or sometime afterward. I know the the controller will switch off
> the heating element at 209 degrees, but it will come back on when the
> temperature drops a little, and keep turning on and off, maintaining
> temperature at the set point until I turn it off.
>
> I don't want to appear stupid. I think you mean as soon as the controller
> switches the heater off for the first time, but I just want to be sure I
> understand what you meant. :-)


To clarify the above, the oven must be below 50 degs C (120 F) before the
heating cycle is started. This is necessary so that the solder paste can
pass through the point needed to drive of solder paste solvents before
the melting point is reached.

The oven should be switch off when the temperature reaches 219 degs C
(426 F) NOT 209 degs C as above, but the fans should continue to run.
Once the set-point is reached the heater must remain off to allow the
cooling cycle to commence.


I hope this is clearer.

Peter.
--
Peter Homann   email: RemoveMEpeterhTakeThisOuTspamadacel.com.au       Work : +61 3 9596-2991
Adacel Pty Ltd                                   Fax  : +61 3 9596-2960
250 Bay St, Brighton 3186, VIC, AUSTRALIA      Mobile :     014 025-925
http://www.adacel.com.au     Australian Software Engineering Excellence

1997\09\10@181223 by Martin McCormick

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face
Tjaart van der Walt writes:
>desk. Gasp and run to open all the windows (PCB's are generally not made
>to smell good at high temperatures.)

       If you have a smoke alarm in the room, find another place.  I set
ours off doing that to a conventional board with a paint stripper gun.  A
respirator that keeps the smoke directly out of your face is probably a
good thing to use, also.  That vapor can't possibly do any good for one's
lungs and liver.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1997\09\11@102104 by lilel

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face
>Martin interjected thqat:
>  Tjaart van der Walt writes:
> >desk. Gasp and run to open all the windows (PCB's are generally not made
> >to smell good at high temperatures.)
>
>         If you have a smoke alarm in the room, find another place.
>         I set
> ours off doing that to a conventional board with a paint stripper
> gun.  A respirator that keeps the smoke directly out of your face is
> probably a good thing to use, also.  That vapor can't possibly do
> any good for one's lungs and liver.

Solder smoke has definitely been linked to occupational asthma.  Do
not breathe it if you can help it.  Install a little fan on your
bench to keep it out of your face, get one of those air cleaners for
your bench.  Whatever you do, don't end up like me, puffing away on
inhalers for the rest of your days.


Best Regards,

Lawrence Lile

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