Sparkfun recommends the skillet method for a number of reasons, but after trying it, I decided to switch to a convection oven and am loving it. Below is my rationale and some pictures.
Here's a picture of my starting setup. I found that my 1200W electric range burner would actually heat up too fast if I only used a single aluminum plate. The All-Round strapping on the sides provides handles that I use with work gloves to remove the top aluminum plate once the boards are ready to be removed.
Long story short, the skillet method is fine for doing a single board at a time, but is super frustrating when trying to do multiple boards all at once. There are a number of reasons for this:
- the heating element on a skillet / hot plate isn't uniform over the heating surface which means you will have hot spots and either need to move the board around very carefully once parts are reflowing, or wait until all the boards have reflowed and risk overheating/burning some of the PCBs/components. I've tried fixing this with almost an inch of aluminum to help spread out the heat, but it didn't help enough.
- the PCBs are not all perfectly flat! I really didn't expect this to be so bad, but some of my small 2x2" PCBs are warped enough that about only half of the board is touching the hot plate at any one time. This means that the other half is being heated by convection which is much slower than conduction and again you either have to press the board down during reflow and risk parts shuffling or wait longer and hope the other hotter boards don't burn. I once accidentally bumped a capacitor while using a screwdriver to hold down a warped PCB and the cap went flying across the board and soldered to a resistor.
- if you are using lead free solder paste, you really don't have much time before the fiberglass in the PCB turns into an extremely toxic smelling black goo! I only did that once, and I can still remember the smell vividly. On the plus side, now my nose can tell me when the bottom of my PCBs are starting to yellow/brown and I have to remove heat before letting the nasty black goo out. Lead free solder paste melts at a higher temperature than the standard toxic leaded solder so you only have about 30-60 seconds after hitting your reflow temperature for everything to solder properly before you must start cooling down. That's not a lot of time to inspect and shuffle around multiple boards all at once. Remember that the bottom of the PCB is sitting on the hot plate and will always be hotter than the top of the PCB where the solder is.
- the conduction of heat through the PCB is variable depending on where ground planes, traces, and vias are. Some parts designed to dissipate lots of heat during normal operation will actually heat up very quickly during hot plate soldering while other parts may have to wait for the fiberglass to conduct heat from the bottom of the PCB to the top
- Using a bunch of aluminum to help with hot spots also makes it harder to control temperature as you have this extra thermal mass which results in a kind of thermal inertia. Depending on how much thermal mass you have below the PCB, your board may continue heating up for 10+ seconds after you turn the skillet off.
- Measuring the temperature off the hotplate is a bit tricky too. You'll need some high temperature tape to hold down the thermocouple probe to the heating surface. You can't simply push the probe into the plate hard enough to ensure good contact because the probe wires are very malleable at high temperatures and just fold over. Also, you aren't measuring the part that counts - the top of the PCB that has the solder paste on it.
- Electric skillets tend to run the heating element around the edges and are all non-stick Teflon coated. The Teflon coating is bad because the heating profile for lead free solder paste requires reaching a max of 249°C relatively quickly and Teflon starts to off gas at 287°C. When the surface of your PCB hits 249°C, the actual heating element may be considerably higher because it is pushing heat through the aluminum plate heat spreader... I have no interest in Teflon fumes killing my imaginary pet parrot. He used to be Clint Eastwood's imaginary parrot so naturally he's a jerk that sits on my shoulder and uses colorful derogatory terms to insult my soldering skills, but he doesn't deserve to die.
- For the reasons above, I chose to use a portable electric burner thinking I would get better heat distribution with a tighter spiral heating element, but the damn element isn't level so I wrapped it up in some aluminum foil and pressed it as flat as I could. Not an ideal solution.
I've recently switched to using a convection oven and loving it! I'll blog about that another time though.
Here's another view where you can see that I have a vacuum system sucking the fumes away from the cooking PCBs. Although lead free solder isn't toxic, the fumes it releases while soldering are actually worse than the old leaded solder. Ventilation is always important.
Recap: the hotplate / skillet method works fine for cooking a single PCB at a time, even if you are using lead free solder paste. I don't recommend it for cooking multiple boards at a time though.