TopBackground

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" or "tombstone-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!).

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 a preheater. 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.

TopMy reflow setup

Here is my setup:

Oster 1500 Watt Toaster Oven, model #6293.

large aluminum slab, about 8" x 8" X 0.250 thick.

Madell 150mm Steel Stencil Squeegee.

500 gram jar of Kester Easy Profile 256 paste.

a roll of paper tape.

tweezers, dental picks, etc.

3x magnifier lamp.

2foot X 2foot X 3/4inch Melamine Shelving.

The squeegee and oven are important. Find yourself a good squeegee, you maybe could use a spackling knife. I've tried some other ways of applying the paste, but I like the stainless steel squeegee. The squeegee has a nice flexible blade that gives a more even pressure across the blade edge.

As to the oven, I picked mine up at Target for $79. There were a number of different models of ovens, but this one has three things going for it:

Four elements, two on the top and two on the bottom.

Convection fan to avoid "hot spots" in the oven.

Temperature dial calibrated in Fahrenheit and Celsius degrees.

The mylar stencils may be ordered from jan@pololu.com. You generate an RS274-X gerber of the cream layer, then send that to Jan. Jan will then send you a quote and a pdf of your gerber to check. Once you PayPal them the amount, they will laser cut the stencil and ship it to you. They have a variety of shipping options.

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 down.

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 PCB. 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 the stencil.

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 remain moist. 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!