Reflowing two sides

[Falcon69]

So, I am working on a small project, which in order to get it to fit into the package /box i've designed for it, it appears I may need to solder components to both sides.

I have a toaster reflow oven I've made, which works well.

My questions in, how is this done to solder both sides? 

Do you reflow one side first, the one side with the lightest components?  Then place components and reflow the other side?

Does heating up the solder and then reheating (on the second reflow process) make the solder bad? Or, will parts fall off the bottom side on the second pass?

I'm currently watching videos on this, but I'm still worried about the durability of the solder if it goes through two passes.  I've read, (with people reflowing video cards to fix errors) that reflowing again degrades the solder, and errors on the circuit board start happening again after several months later.

[Howardlong]

Do you reflow one side first, the one side with the lightest components?  Then place components and reflow the other side?

Does heating up the solder and then reheating (on the second reflow process) make the solder bad? Or, will parts fall off the bottom side on the second pass?

That's the basic idea, design your board with the heavy parts on one side, and reflow that side last. That works reasonably well, the light parts won't just fall off. I have heard of stories where different eutectic mixes are used with highly managed reflow profiles, but you're wasting your time on this if you're using a toaster oven!

[Falcon69]

why am i wasting my time using a toaster oven? It works well, have a PID controlling the temps and with a data logger on it to check profiles, etc.

[Howardlong]

why am i wasting my time using a toaster oven? It works well, have a PID controlling the temps and with a data logger on it to check profiles, etc.

No you're not wasting your time using a toaster oven doing this, I've done the very same thing myself.

You would be wasting your time if you started trying to use different melting point solders with a toaster oven though, that was the point I was trying to make, sorry for the misunderstanding.

Personally, I've never had much luck with PID or any other controllers on toaster ovens. The profiles never seem to be consistent and the thermal mass in conjunction with the element power never seems to be able to approach recommended profiles, so I just switch them on and look through the window, monitoring time and temperature. But the main feedback loop is seeing the solder melt, give it another ten seconds or so, then switch off and open the door. Seems to work a treat.

[IconicPCB]

I have had to assemble double sided boards without component adhesive.

To protct the solder side with lighter components i fabricated a heat shield whihc holds the PCB .
The whole thing... shield and PCB then go into reflow oven.

Solder side does not reflow and the outcome is a satisfactory PCB assembly.

[Falcon69]

what was heatshield material you used?

[IconicPCB]

Blank FR4

[tszaboo]

0603 0805 but even 1206 tend to be small enough that it will just stay there with surface tension. Re-heating is only an issue if your initial soldering wasnt good. Look out for tombstoning or fallen components. They are easily find, since they will rattle in your oven.

[wraper]

The part don't need to be small. It just needs to be light enough so can be held by surface tension. For example, BGA chips won't fall down because they cave big total solder pad area compared to their overall size.

[Falcon69]

thank you, I will give it a go.  any other advice, other then placing a blank piece of fr4 under the flip side (side already reflowed) before attempting this?

[Howardlong]

thank you, I will give it a go.  any other advice, other then placing a blank piece of fr4 under the flip side (side already reflowed) before attempting this?

One of those areas of engineering where you need a bit of bravado, then once you've done it a few times you'll wonder what all the fuss was about.

As with all things in this art, always be prepared for failure, learn from it, and continue to perfect your process. Oh, and unless you have a very good memory write down what worked and what didn't so that next time you come to do it a few months later you don't have to re-invent the wheel.