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.

Considerations for Choosing the Best PCB Cleaning Solvent

The printed circuit board (PCB) assembly process can result in residues from the solder and the flux, and these can cause current leakage without proper control and monitoring. In some cases, even a no-clean flux may need to be cleaned to prevent issues. You can save time and effort by using a specially formulated PCB cleaning solution, a solvent that will make the clean-up process easier and fuss-free. Here are some things to consider when choosing the right cleaning agent for your applications: 

• High-quality cleaning solvents are designed for electronic assembly manufacturing applications, such as PCB and stencil cleaning. However, you can also find a non-solvent-based PCB cleaner that is semi-aqueous or aqueous. 

• If you choose a solvent-based cleaner, be sure to look into its formulation. A good PCB board cleaner is made of organic solvents for non-aqueous applications. It effectively removes most kinds of uncured SMT adhesives and raw solder paste residues, and it is effective on organic contaminants and residues of post-soldering flux.

• Make sure that the PCB cleaning solution evaporates well and leaves no distinctive or strong odors. Likewise, consider a cleaning solvent that will not require you to rinse it off. That way, you can save time and move on to the next stage of your manufacturing process.

• Look for a PCB cleaner that is non-corrosive, so it will be compatible with most PWA and stencil materials. Moreover, consider a formulation that can minimize the build-up of solids, so you can reduce the need for equipment maintenance.

• Go for a PCB board cleaner that is non-flammable and non-hazardous. A good cleaner does not contain any glycol ethers or amines to keep your workers safe, and no VOCs, so you do not have to apply for relevant permits.