Reflow oven vs even hot plate

Question

I've recently been searching for a simple, easy yet cheap way to reflow SMDs on a PCB board as I have never tried it before and always wanted to create a small and compact circuit. I then found a crowd funding campaign (whom I will not currently name because my point isn't to advertise) of a product where instead of a soldering oven, it was a hot plate. However in general, hot plates aren't the best for reflowing as the heating is uneven unlike this one where it claims to have an even heating surface (also because it's quite small).

If such product exists being a hot plate that evenly heats up the PCB, how would that be anyway advantageous over a soldering oven? One way I can think of is being able to re-shift the components if they move during the reflow. Any problems or disadvantages?

In addition, Is it safe to put components at 219 degrees Celsius when their max storage temperature is 150 degrees celsius? Looking at the reflow process, for 75 seconds, the oven or the PCB reach over the max (and the hot plate crowd funding product follows this exact same profile). And when looking for a PCB, what aspects should I be looking for to ensure it is reflow friendly? I'm guessing most PCBs are designed for this.

Answer 1

DISCLAIMER: I have backed the campaign in question, but have no other affiliation with the campaign or its creator.

The biggest issue with the r***R, as with all surface reflow tools, is that double-sided loads are essentially impossible to create. Flipping the board over means that it will be held away from the heating surface, resulting in at best uneven heating of the board.

A reflow oven uses either infrared radiation or air convection to heat the board instead, which means that suspending the board by its edges does not impede the reflow process (and in some cases may be required), making double-sided loads possible. It also allows for a mixture of SMT and TH devices to be reflowed (insomuch as TH devices can be).

The flip side is that with a surface reflow tool heat can be transferred to and from the board much more quickly than via air convection, which allows tighter adherence to JEDEC reflow profiles.

In addition, the exposed nature of a surface reflow tool means that not only can parts be manipulated during reflow as you note, but also during rework after the board may have been in service for some time. This of course requires tighter thermal control than has previously been available using more... primitive tools.

As for storage, this refers to long-term thermal profiles, a month or longer. The few minutes during reflow or even rework have been factored into the construction of most devices from reputable suppliers.

Answer 2

I use an old Sears Kenmore 4-element toaster oven for reflow, with a thermocouple probe into a digital multimeter for temperature monitoring. Heat to 150C to activate the solder paste flux (Kester EP256), heat to 190-195 for 90 seconds for solder reflow, turn off heat and let it cool down slow with the door cracked a little until below 180C, then open the door more.

We do double sided also, using a couple of bare boards to hold the components off the tray for the 2nd cycle, sometimes with kapton tape if we're worried about them falling off.

We can do two 100mm x 100mm boards at a time, and more if the boards are smaller.

Answer 3

Regarding the temperature, thats certainly not a problem. The components are made for that. Of course you cannot reflow solder plastic headers (there might be some that can withstand that).

Plate vs. oven, I would recommend to go for the oven because the heat comes from all sides and if you need to rework something you can still do it with a solder iron. You probably dont want to work on such a heating plate while it is hot (in full operation). Maybe the advantage of the plate is that it is more conveniently storable. Furthermore the oven can probably more precisely follow a heating pattern than the plate can.

Would be nice to see the specs of the plate and compare it to an oven.

What comes also to my mind are double side populated pcbs. On a plate I can only imagine that they get too hot on one side in order to get them hot enough on the other side. Or you have to do the process twice, once for each side.