Electronic Engineering Blog

Solder paste under the microscope.

In electronics we regularly use solder paste in the assembly of printed circuit boards (PCBs).
This paste comes in a variety of packages, styles, types and so on. Today I look at one small tub of solder paster under the microscope. Quite literally. 

When assembling PCBs we use solder to make the electrical connection between the components and the tracks on the boards. In the recent USB modules that I designed some of these components are far too small to solder by hand. Take a look at these tiny LED

leds_001You can get an idea of their size in this image of them next to an English 20p coin. They are known as 0603 components and they are 0.8mm x 1.6mm in size.

In order to solder these to a circuit board, we cannot just use a soldering iron and solder wire, we need another approach. This is where solder paste comes in.


tubThis image show a tub of solder paster. In this case it is a 500 Gram tub of lead free solder paste. It has the consistency, roughly speaking, of smooth peanut butter. A tub of about that size is very expensive, costing around about £65. So it’s fortunate that you need very little of it to make PCBs. In the picture the card being used to scoop out some of the paste is a standard credit-card size. These cards make ideal paste spreaders!


stencilIn order to assemble the PCBs, you spread the solder paste onto the PCB using a stencil, like the one in this image, and a spreader. The stencil is a very thing piece of steel or aluminium, or sometimes even plastic, which has been laser cut to have holes wherever solder should be spread on your board.

You use a jig to hold the PCB and the stencil in perfect alignment and then spread solder past across the stencil, depositing it onto the circuit board in exactly the right place, with exactly the right amount. Any excess can be returned to the tub!

Now all you need to do is use a pair of tweezers to place your components onto the solder pads you’ve just deposited on the PCB and put the board into a special reflow oven to melt the solder.  Easy!

The solder paste looks like thick grey peanut butter, so how does it work? Well, lets take a look at some solder paste under a microscope.

001Here I have spread some paste onto s sheet of white paper and looking at it under a microscope we can see that it looks a bit like wet sand. It actually contains millions of tiny spheres of solder held in a micture of flux and resins. The flux and resins help to clean the metal areas to be soldered as well as controlling the heat distribution to the solder and components during the reflow soldering process. They also ensure that the solder sticks only to the metal surfaces and resists sticking to the non-metal surfaces, so that there are no shorts in your circuit.


If we zoom in even closer, we can see the individual balls of solder. The line going across the image is a fine human hair, just to give you a sense of scale.

You can click on any of these images for a larger version.

Bye for now.

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