piclist 1997\09\06\131616a >
Thread: SMT soldering
www.piclist.com/techref/pcbs.htm?key=smt
picon face BY : Mike Keitz email (remove spam text)



On Sat, 6 Sep 1997 10:40:54 -0400 Sean Breheny <spam_OUTshb7@spam@spamKILLspamCORNELL.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.
<19970906.131340.5150.0.mkeitz@juno.com>

See also: www.piclist.com/techref/pcbs.htm?key=smt
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