Complete Circuit Board Lab & POV Business Card

Step 11 Bake your PCB

Now it is time to bake the PCB! I promise it will be fun.

Programming the Reflow Controller
_______________________________________________________________________

First things first: it is important that the reflow oven is properly configured, or this won't go well. It's all about the set point parameters here. The maximum temperature and time maintained should not be any longer than the most sensitive component can handle, and it is recommended that it actually be 5 degrees lower than that value. In this case, the most sensitive component is, surprisingly the LEDs. The microcontroller can take, based on it's volume, up to 260º C, and the resistors and capacitors have tolerances above that. The LEDs have a recommended max temperature of no more than 225º. The manufacturers were kind enough to provide a reflow slope, so I just used that one.

If your oven is slow to heat up, you may encounter problems near the peak of your reflow ramp. The idea is to bring the temperature up slowly, spike it to "re-flow" the solder, and then drop it off rapidly, but not instantly, to room temperature. So if getting that peak is a problem, you may have to step in and open the door of your oven to  speed of the rate of cooling from that peak temperature. 

Baking the PCB
_______________________________________________________________________

Once you have the oven programmed, place the board in the center of the rack, making sure the thermocouple is near the surface of the PCB so it can get a reading that close to the actual conditions of the solder joints.  Close the door, and start the program. Fire extinguisher nearby, of course.

Wait until the temperature drops to room temp. to take the board out , as it will embrittle the solder joints if they are denied their cooling cycle. 

I should note here that if you do not need to make many, many identical PCBs, you don't really need to go to the trouble of building a reflow oven or screening the solder; it is very possible to solder the parts by hand. You will need a soldering iron with a relatively fine point, however. The trick is to tin on pad on each component, hold it in place with a tweezers (or the vacuum tool) and flow the solder on that joint with the iron. Then it's secure and perfectly aligned to do the other side.
 

Tools & Supplies

• Reflow oven & controller
• solder-screened and populated PCB

Comments

[dustinandrews]

I have been using an electric skillet set to 350-375 and just moving the boards off to a bit of wood as soon as the solder paste all melts. I think as long as your components sink heat at about the same rate that is an easier approach. I did have problems with some boards with large-ish surface mount caps. The LEDs might have melted before the caps were soldered. I solve that problem by using the hot plate in two passes.

Do you think there is a danger with my method of cooling too fast and having some ill effect? It sure is easier.

[sponges]

The ideal reflow lasts peaks only for a few seconds. Take a look at the reflow curve in step 11. Of course there's a danger; theoretically and practically, the lifetime of the components will be compromised. The question is, by how much, and is it worth your time?

If you are making a thousand of something to tight tolerances, then the failure rate and inconsistency are unacceptable, and it's worth the time. If you are making 10 of a thing, and have the time to QA every one, then the skillet is probably easier. If it works, it works.

My setup is probably overkill for most home SMD projects. I'd even say it was overkill for my projects. :) As it's been said before, overkill is a way of life.