My procedure for SMT soldering¶
It turns out that it is quite easy to make solder masks on a laser cutter, and low-temperature lead-free solder paste practically eliminates the risk of overheating components. Here is what I do:
Design your layout and have your board manufactured. Do yourself and the environment a favor and pay the little extra for a lead-free finish.
Export the paste mask as an svg file with 0.005” (0.125 mm) shrinkage. This file can be loaded into Corel Draw for laser cutting.
Cut the paste mask out of overhead transparency. I use Apollo Laser Printer Transparency Film (#VCG7060E) placed on top of a sheet of aluminum. The film is 0.002” (0.050 mm) thick; the thickness of the aluminum is not important. Cut the outlines of the pads in vector mode. On my 75-W ULS laser cutter, I use 25% power, 100% speed for the pads, cutting three times. Then I cut the board outline with 65% power, 100% speed, once. This gives a nice flat surface on the back with slight ridges on the front. Check for hanging chads.
Tape the mask to the board. I use packing tape, but it is not critical. Use a stereomicroscope or a magnifier to ensure precise alignment. (A pair of +2.00 to +2.50 reading glasses makes for a remarkably effective head-mounted magnifier.)
Squeeze solder paste into the pads. The paste I have been using is Sn 42%/Bi 57%/Ag 1%, melting point 137 °C. It is described as “No-clean flux, 87% metal (20–38 microns).” The package expires in 12 months, so don’t buy too much at once. I squeeze it out of the syringe onto one side of the layout, then use a metal spatula or a plastic ruler to distribute it into the holes.
Remove the mask. This has to be done very carefully so as not to smudge the pads.
Preheat the board. I have a really cheap reflow workstation. The thermostat of the IR preheater is broken. No matter, use a thermometer to check that the temperature at the location of your board is around 100 °C. Preheating a PCB takes 2–3 minutes. Don’t worry about overheating: 100 °C is a profoundly safe temperature for all electronic components I’ve ever worked with. (Some plastic LED lenses may get a little soft, but they have never actually melted.)
Melt the solder. My reflow workstation comes with a mini heat gun. I set it to 175 °C and melt all the solder by moving the heat gun slowly over the board. You can see the paste turn shiny. Components may shift a little at this time; usually they settle very nicely to center over their pads. Be careful not to blow your parts away with too much air flow!
Cool the board. Remove the board from the heat using a pair of pliers (careful: it’s hot!) and watch the solder solidify under a dissection scope.
Clean up. It is very important to clean the metal spatula (and the mask if you plan to use it again).
A note on temperature¶
It might seem that 137 °C is a crazy low melting point, but it is actually not unreasonable: For instance, the max. operating temperature of Luxeon LEDs is 135 °C (at the diode junction). Thus, the solder should never melt under normal operating conditions. In my experience, soldering through-hole components onto the same board with “regular” solder (tin-silver-copper, see below) doesn’t transfer enough heat to melt these connections either.
Notes on solder types¶
Most solder formulations melt at a much higher temperature than the one I use, which makes it much more difficult to avoid damage to electronic components: Old fashioned leaded solder melts around 183 °C and other lead-free formulations tend to melt at even higher temperatures. For reference, here are some common formulations. All of the information in this section was taken from https://en.wikipedia.org/wiki/Solder.
The formulation I use:¶
Sn42/Bi57/Ag1
Melts at 137–139 °C. Patented by Motorola. Wiki says “Good thermal fatigue performance.”
Other lead-free formulations:¶
The tin-silver-copper (Sn-Ag-Cu or “SAC”) family
These typically have 3–4% silver, 0.5–0.7% copper, lots of tin, and have melting points around 217–220 °C. That requires pretty high soldering temperatures, which is obviously a challenge in a relatively poorly controlled environment. Sn-Ag-Cu-Mn formulations have a slightly lower melting point (211–215 °C) but still well above Sn-Pb (next section).
In97/Ag3
Melts at 143 °C. Reportedly used for cryogenic applications and photonic devices.
In100 (or In99)
Melts at 157 °C. Used in low-temperature physics and for soldering gold. Bonds to aluminum.
Sn88/In8.0/Ag3.5/Bi0.5
Melts at 197–208 °C, closer to the Sn/Pb types. Patented by Panasonic.
Leaded solder types:¶
60/40 Sn/Pb
Melts around 188 °C.
63/37 Sn/Pb
Melts at exactly 183 °C, the lowest of all tin-lead allows.
Of course, you don’t want to use these unless you have a really good reason.