We recommend tinning the pads if you’ll be point-soldering with an iron, since it helps the solder wet out faster, though it’s not strictly necessary. If you’re reflow soldering components instead, you can skip this step entirely and just reflow your components directly to the ink.
That said, reflow soldering on a bare pad of conductive ink is a bit different than point-soldering. The reason for this is that conductive inks, unlike bulk metal, contain an organic phase. This organic phase is mostly a binder resin which is added to conductive inks to give them mechanical stability. The cliff’s notes is that you need to have a component on the paste to have good wetting. If you’re interested as to why, read on!
As you know, solder really only likes to bond to metal, and really dislikes sticking to anything else. When you point solder with an iron, you’re feeding a large volume of molten solder directly onto the pad while keeping it in place, which allows the molten solder to anchor directly to the metal and wet properly. On the macro-scale, the effect of the organic phase of the ink is relatively minor.
However, when you reflow solder with solder paste, you’re dealing with a bunch of small solder particles, which are individually much more concerned with the micro-scale surface. As the paste melts, there is a tug-of-war happening, with 3 competing factions: the cohesion of the solder particles to each other, the wetting/binding of solder to the metal part of the ink, and non-wetting of solder to the organic part of the ink. The organic phase has a larger effect at this scale, and the solder tends ball up in islands instead of distributing evenly over the pad. If you were to look at a pad that has gone through this process under a microscope, you’d see threads of solder winding around the pad surface where solder has bonded, but failed to ‘convince’ the rest of the solder to stick around. If you’re really interested, I can try to dig up some pictures.
When you put a component down on the paste, the component lead does a similar job as the soldering iron - it allows the solder paste to wet a friendly surface (the tinned component lead), then take its time anchoring to the metal in the pad, which gives you a stable electrical and mechanical connection.
In practice, all you really need to think about is following the recommended procedures:
- For point soldering, use the included solder alloy, flux, and heat to 180-210C. Avoid too much pressure on the pad.
- For reflow soldering, put down components on the solder paste, and use the automatic reflow profile.
I hope that this has been helpful. Let me know if you have any other questions!