Electronic – Reflow soldering issues


I've been using a halogen oven with a home made controller as a simple reflow oven. The controller uses a thermocouple to measure and control the temperature profile.

The problem I have is that I'm getting inconsistent results depending on the component. As a test I made a simple board with 8 resisters and 8 LEDS. In this case I've put a little solder on the pad using a soldering iron, positioned the components on the pads and then re-heated in the oven. I cleaned the board with liquid flux before applying the solder and again after applying solder (In one case – I had similar results when I didn't do this second clean)

The resistors have all come out fine. They are positioned well and the solder looks good. The LEDS however all come out terrible. They all just about have proper electrical contact so the board "works" but they are very poor indeed.

The components are all about the same size and have similar contacts. There was no difference in the preparation. The LEDS are very slightly thicker I suppose. They are within 1cm on the board and I don't believe would have significantly different temperatures.

I'm wondering if anyone has any explanation or suggestions about how to improve? Maybe I need to increase the temperature? Or hold the temperature a little longer?

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Best Answer

If you must use solder instead of solder paste then you really need to ensure that there is plenty of flux on and around the pads when you reflow. Without the flux the solder won't flow into the components properly.

Solder contains flux in the core, but that burns off as you melt it onto the pad leaving just (almost) pure solder behind. You need flux for the solder to flow onto the metal of the component.

Solder paste is made from millions of little tiny beads of solder suspended in flux. If you're going to reflow you really should be using paste. It's not expensive (well, it is, but you use such small amounts a little goes a long way). Personally I use a nice low-temperature one (138°C melting point) with my home made "toaster oven" reflow oven (ChipQuik SMDLTLFP) in a syringe.

Here's an LED I reflowed earlier today using my homemade oven:

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BTW, the pad layout for the LED above is exactly as specified in the datasheet for the LED. Check with the datasheet for your LEDs that the pads you are using are suitable. If they are spaced wrongly (and they look like they may be) then reflowing may be problematical at best.

It's even good for fine-pitched QFN:

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I wouldn't even think about doing those with normal solder without having the board swimming in flux when I reflowed.