Solder paste is the glue that holds together modern consumer electronics, binding surface mount electronic components to circuit boards and providing electrical and thermal connections in the process. But have you ever really looked at it?



(Above, matte-grey solder paste selectively stencilled onto one of the RoboGames 2013 medals that we wrote about in April.)

A little review: Solder paste comes in a tub or tube, and is normally kept refrigerated to preserve its shelf life. Its color is that of grey goo. Its consistency is that of cake frosting, and it spreads well with a palette or frosting knife. Perhaps coincidentally, some varieties also smell like cake frosting. (Ours in particular has a strong amaretto scent. Not sure why that’s a good idea.)

After finishing a batch of boards, there may a bit of frosting solder paste left on the tip of our palette knife; Let’s take a look at it under our microscope:





As we zoom in further, you can start to see that it’s not truly a uniform grey buttercream frosting, but rather a grainy sort of mixture.

Finally, when you zoom in far enough, you can see what we’re really looking at: untold millions* of shiny little balls of solder.

Yes, shiny. When you really get down to it, the tiny bits of solder there really do look like the regular wire solder that you might use at your workbench. Only, they’re neatly round, and suspended in a liquid solder flux. After stenciling and placing the components, the circuit boards will be baked in a reflow oven until all those solder balls melt and fuse together, becoming roughly indistinguishable from solder that started out in a wire.

This is “type 3? solder, which allows solder balls of up to 45 ?m in size, where 99% of those particles are below 38 ?m. This is a good type of paste for basic surface mount work (the kind where you can still see the leads) but finer-mesh solder is needed for working with finer-pitch electronic components.

*The weight of a 38 ?m solder ball is somewhere around 210 ng, so given that 90% of the weight of solder paste is from the metal (not the flux), we’re looking at roughly a billion solder balls per 250 g tub, to give an order-of-magnitude estimate.

So what about back on the circuit board? Sometimes, if you know what to look for, you’ll see the little solder balls, particularly if you’re inspecting a board under the microscope before reflow.

You can just make out the solder balls in this photo. You can also see the thickness of the stenciled paste, which was laid down with the help of a 4-mil laser-cut stainless steel stencil. The little ceramic chip capacitor there is 0603-sized (60 mils by 30 mils) for scale.

Even in larger areas of the circuit board, paste is still three-dimensional. From the side, under a microscope (or sufficiently good macro lens), you’ll see little piles of solder balls, not just a 2D uniform grey.

A little final perspective: you can see how big the solder balls are here at the corner of an ATtiny2313A-SU microcontroller, which has an SOIC-20 package with 50 mil pitch. The legs press into the sticky paste, holding the chip in place until reflow.