No-Clean Solder Paste: What Does “No-Clean” Actually Mean?

You are likely to come across no-clean solder paste when choosing materials for your soldering and PCB assembly needs. Its purpose is to eliminate the need to clean the printed circuit boards after the soldering process, but with some products still requiring you to clean the board, you might be wondering what ‘no-clean’ actually means. In some cases, the no-clean variant will still require you to do some cleaning. A manufacturer may have adjusted their assembly lines to use this type of solder paste, then clean only the boards that require cleaning. So what does ‘no-clean’ mean then?

In a no-clean solder paste, ‘no-clean’ actually means solder paste with minimal residue. After reflow, flux will still leave a small amount of resinous but non-corrosive residue around or on the solder joints. The residue may vary in color, usually appearing amber or transparent, depending on the flux resin used and the reflow temperature. The amount of residue will depend on the amount of the activators, gelling agents, and the solid resins in the flux. So, the less solids content it has, the less flux residue on the board. Contemporary no-clean solder pastes contain around 50 to 70 percent of solids.

‘No-clean’ may also pertain to the solder paste leaving a clean look, making it as good as a water-soluble paste. But why remove residue from a no-clean solder paste from a PCB? One reason is to minimize problems during in-circuit testing, as the test pins are unable to go through the flux and establish and electrical connection with test pads. Moreover, residue could build up gradually on the pins and cause issues with test accuracy and maintenance. However, you do not have to worry about those problems when you use high-quality no-clean solder paste with recent formulations.