Before you begin...
Always remember that lead is poisonous. Short term exposure to lead
can result in death or serious medical problems. Always use your reflow
oven with adequate ventilation, and avoid skin exposure to the solder paste.
Most of my recent designs have used TSSOP / LQFP packages. These are very
fine lead devices! While a hot-air pencil and paste syringe will work for
minor touch-up and chip replacement, it is not easy to use when you have
several boards to build, or have fine lead devices on them!
An Article on Sparkfun was very timely
, it has enabled me to use a higher
percentage of SMD devices in recent board designs. There are two main
problems with prototyping with surface mount devices:
Getting a consistent and proper dispensing of the paste.
Applying even heat across the board to avoid parts "walking", "torque-ing"
The use of the mylar stencils and an inexpensive toaster oven has solved
these problems for me. Now my only problem seems to be in getting the PCB
house to turn the boards around fast enough and to get parts shipped in a
timely manner (UPS / FedEx, shame on you! The United States Post Office
ships faster than you with their Priority Mail service!).
TopI don't like fry pans!
I suggest that you don't even bother with this technique. I am convinced
that the electric fry pan method is nothing more than a convection heat
method to melt the solder with some conductive heating of the PCB to
act as the preheat. From what I remember about my test with a 5inch
X 8inch board, on the electric stove with a large aluminum slab
to place the PCB on, is the smell of charred fiberglass resin!
Use the toaster oven technique, it is so much easier and you can control the
heat more precisely than with an electric skillet. Also, if you notice, the
boards that are shown to be used on a skillet are relatively small boards?
I believe this is why they are getting convection heat to melt the paste.
TopStart by locking the PCB down
I use same thickness junk board stock to lock the PCB down.
Plop the PCB down onto the melamine shelving work surface,
then arrange the other boards around it to make a continuous flat surface.
Then tape these board edges to the melamine so that the whole thing is anchored
I'll explain those codiment dishes in a moment. ;-)
TopPosition the stencil
This is the most critical step of this whole process. The stencil must
be positioned correctly so that the paste is applied directly onto the pads!
In the some toaster oven tutorials, they suggest using your hand to pin the stencil
down. While this would work for SOP devices, TSSOP devices need a higher
degree of accuracy when placing the stencil, so, I tape it into place.
Refer to the photo of locking the pcb down.
I place a small strip of tape on two opposing sides of the stencil before I
begin the placement process. Also note, those tape strips are not placed on
the sides of the stencil which is the general direction of my blade travel?
Don't tack it down yet, just have the tape there
for when you are ready to do it. As to the codiment dishes, heh, I have
found that the mylar sheet will tend to stick to your fingertips and it is
quite difficult to remove your hands from the sheet without jarring it.
Those codiment dishes don't weight very much and tend to anchor the sheet
in place while you adjust it.
With good lighting, and while looking through the magnifier lamp,
position the stencil so that the cutouts are over the pads. Choose
two ICs at opposite ends of the board to do this. The TSSOP or LQFP
(fine lead packages), if present on your board, are the most critical
to get the alignment "right".
When you are satisfied that the stencil is correctly aligned, touch the tape
to tack it into place, just touch the tape for now. Remove the codiment
dishes, firmly press the tape down, and RECHECK(!) the alignment. The stencil
may have shifted slightly while doing all this. I use a 2.5X eye loupe
held under the magnifier lamp as a sort of microscope.
Make sure that the stencil is reasonably taut across the surface of the
You don't need it tight, just taut. This is so that when you sqeegee
the paste on, the stencil doesn't walk on you while the slack is taken
out from the pressure of the blade.
TopApply the paste
Refer to the photos below. Start the operation by placing a blob of
paste near one corner of the board. Don't worry if, at first, you
start with too little paste, you can add more.
Next, using the knife or squeegee, wipe a thick layer of paste across
the entire stencil. We are going for coverage here, just enough paste
to ensure all cutouts have some. Wipe the paste diagonally across the
stencil, don't wipe it across the vertical or horizontal axis of the PCB.
When applying the paste, angle the knife somewhere about 30..45 degrees,
this will tend to "pack" the paste into the stencil cutouts.
Don't do this mistake!
Wipe fully across the area of the PCB, then
rotate the knife through 90 degrees. Sort of a swiping motion. Do not
lift the blade! Lifting the blade may cause the stencil to lift from the
surface of the board! Use the swiping motion to dis-engage the knife from
Finally, we scrape the excess paste off the stencil. Angle your blade
somewhat from 90 degrees when you wipe off the paste. This will avoid
damage to the stencil as the laser cutouts have a slight "burr" to then.
You do want to use a steeper angle on the blade when wiping the excess
paste off than you used to apply it.
Use just enough pressure to leave paste in the stencil cutouts (pockets).
Start with some paste
Smear it around
Scrape it clean
Please note that I was using a knife blade here to apply the paste? It did the
job, but not as well as the Madell squeegee. The knife blade appears to be
too rigid to work well, but it did work. With the flexible blade of the
squeegee, you don't have to worry about placing consistent pressure on
both ends of the blade (as you do with the knife). Plus, the knife blade
was a little too small to comfortably maneuver.
TopAnd, pasting is done
Gently and carefully
lift the tape from one side of the stencil, then
"roll" the stencil off the PCB and remove. Place the stencil onto a layer of
paper towels, place another layer of towels on the stencil. Sprinkle some
alcohol onto the towels and wipe / pat the stencil clean of the paste. Then,
lay the stencil onto a dry paper towel.
Peel the stencil off the board
This is the resulting board
Clean the stencil with alcohol.
You may find that the paste looks a little "fuzzy", that it does not have
sharply defined edges. This is fine, that solder paste will flow with the
other paste to become a whole mass. Even if it doesn't, it may become a tiny
ball of solder which is easily brushed off the board. If you are really
concerned about some paste smear, you can now touchup the paste outline
using a dental pick or other probe.
You now have about an hour to put all those tiny parts on the board!
Calm down! I have had one board where it took me an hour and a half to put
the parts on it and it came out fine. The Easy Profile 256 paste has a long
drying time, your relative humidity will vary the time that the paste will
You may want to look at my page for the EagleCAD + Apache parts placement guide?
TopInto the oven!
After placing all the parts onto board, we carefully load it into the toaster
oven so as not to jar the components. Once the door is closed, it is time to
reflow the paste.
Populated board going into the toaster oven
Board ready for reflow
The model of oven that I have, has a timer mode and an "ON" mode. If you
rotate the timer dial to counter clockwise, it will remain on. Or, you
may use the timer, it doesn't matter. In either case, we want both the
upper an lower elements to turn on. With the Oster 6293, placing the mode
switch in the "bake" position will do this. YMMV with your oven.
This is the solder profile for the Easy Profile 256 paste:
As you can see, there are four distinct temperature zones we must follow:
Pre-heat, Soak, Reflow and Cooldown.
From what I have found to be successful is the following, mind you, I don't
have a thermal probe nor a controller for my oven, these were done simply
from the uncalibrated dial of the stock oven:
Pre-heat of 140C for 90 seconds.
Soak at 180C for 90 seconds.
Reflow at 220C for 60 seconds.
About 30 seconds into the 220C thermal, you will see the paste flow. I hold
that temperature for another 30 seconds after the reflow and turn the
oven off. Gently(!) crack open the oven door to spill the heat out.
Then, gradually over the next 60 seconds, fully open the oven door, slide
the grill out.
When the grill is completely out, the board should be cool enough that the
solder has now solidified and you can remove the board for cooldown. I grasp
the board with pliers, gently lift it out, then slide it onto the aluminum slab.
The aluminum slab will soak up the heat from the PCB and it should be safe
to handle in 60 seconds or so.
TopThe finished boards with all parts on them
These are the finished boards with all the parts on them.
I am proud to show these boards to anyone!
TopLead-free Solder Paste
I did get a jar of Lead Free paste and gave that a try. It finally did melt (reflow) in the
toaster oven, but it took more time to do so. I had the temperature turned all the way up on
the oven to get it to melt. The Lead Free paste does specify a higher melt temperature,
I would recommend that you consider delaying using this until the other is no longer available.