Lesson 3: SMT and Thru Hole Soldering
Sam DeBruin
February 15, 2012
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Introduction
The third lesson is a guide to soldering surface mount and thru hole parts. The purpose of
this lesson is to introduce students to the concepts of surface mount and thru hole soldering
on a small, easy to assemble board. The skills and techniques acquired here can be applied
to any board of any size and with any number of components. In some cases, however, hand
assembly becomes overly difficult. This applies especially when the design contains Ball Grid
Array (BGA) parts. In these cases, companies exist to do assembly for a fee.
This lesson uses a demo board produced specially for the Student Projects Lab. It also
employs a custom stencil for applying solder paste. While these stencils can be produced
professionally, the University has the resources to produce these stencils at little or no cost
to the PCB designer. Consult Lesson 2: Eagle Layout and Gerber File Generation for
information on producing custom solder stencils.
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High Level Overview
The order of assembling the boards is important in this case. For the surface mount parts,
use a putty-like substance called solder paste. Solder paste, when heated and cooled, turns
into solid solder and connects the IC to its PCB pads. Additionally, when the solder paste
becomes warm, it has surface tension like water. This will cause the solder paste to pull
away from non metallic areas of the board, naturally removing shorts between unconnected
nets.
The heating process is achieved in the SPLAB through a hot plate. The surface mount
soldering must be done before thru hole soldering because thru hole soldering creates imperfections on the underside of the board that will prevent good thermal conduction between
hot plate and board.
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Solder Stencil
The first step in assembling the board is to apply solder paste. In the SPLAB, use the
stencil provided. This stencil will make the process of applying solder paste faster and more
effective.
Align the stencil with the pads on the board. Tape it down, taking special care to keep the
pads aligned with the holes in the stencil. Use enough tape that the stencil can be tight to
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the board but not so much that it is difficult to remove.
Use the solder syringe to apply a small amount of solder paste near the top of the board.
Carefully scrape this solder paste over the holes in the stencil using the putty knife. Take
care to keep the stencil from warping and allowing excess solder paste to accumulate.
Once application of solder paste is complete, carefully remove the tape and peel the stencil
away from the board.
You should now have solder paste on all surface mount pads but not on the holes. Ask your
SPLAB instructor if you are unsure.
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Add Surface Mount Components
Using the reference names on the board as a guide, place the surface mount parts on the
board. The resistors have no orientation but take care to orient the LEDs such that their
negative terminal is oriented toward GND.
GND on the LEDs is denoted by the green band on the underside of the LED. When placing
the LEDs, these green bands should be oriented inward (green bands on the LEDs close
together).
Likewise, make sure the decoder IC is placed in the correct orientation.
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Reflow
Once all the surface mount components have been placed, carefully take the board over to
the hot plate. At this stage, the solder paste has some adhesive qualities such that the
components should not fall off under normal circumstances. However, aggressive movements
or upending of the board will cause parts or misalign or fall off.
Place the board on the hot place. During this process, fumes will come off the board visible
as white or grey smoke. If possible, don’t inhale any of these fumes and make sure the area
is well ventilated with a fume extractor.
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During this process, the dull grey paste will become liquid and eventually metallically shiny.
Pay attention to all areas of the board, especially areas with larger amounts of paste. Once
all areas have transformed, use a forceps to move the board to a cooling area. Be careful, at
this stage the board will be very hot. Leave it to cool for 1-5min.
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Thru Hole
Once the board has cooled, the final stage is to add thru hole components. In this case, the
thru hole components are just a few 2-pin headers for inputting and extracting data.
To solder a thru hole pin, hold the iron at the pin for a few seconds or until the pin is hot.
Once warm, touch gently with solder. Do not touch the solder to the iron but heat the solder
by the heat of the pin. You should see the solder quickly get ‘sucked’ into the hole. Once
this happens, apply enough extra solder to fill all gaps.
To solder a header, place the short end through the appropriate holes. It will be difficult to
hold the board, header, soldering iron, and solder all at once so first we will put solder on
the connection and straighten the header out later. Make sure to only solder one pin at this
point as straightening becomes much more difficult after multiple pins have been soldered.
Once one pin of the header is soldered, hold the board and header between your thumb and
forefinger. Position your hand so you can look down the header and evaluate its straightness.
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Note: your forefinger should be touching the unsoldered pin as the soldered one is about to
get very hot.
Use the iron to re-heat the pin’s solder. With the iron still touching the solder, use your
finger to straighten the header. Once straight, make sure to remove the iron and let the
solder cool before releasing with your finger.
Evaluate the straightness again. If necessary, repeat the previous step.
Solder the other pin of the header and repeat for all other headers on the board.
The board is now ready to be tested!
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