What is No Clean Solder Paste?
No-Clean Solder Paste is a paste that doesn’t require cleaning after the solder reflow process. After the reflow process, most solder pastes leave behind some flux/residue on the board. This can cause issues if the residue is conductive or corrosive. The reside/flux left behind by No-Clean Solder Paste is non-conductive, non-corrosive and won't interfere with the electrical conductivity on the PCB board. So, the residue left on the board after reflow, does not need to be washed off of the circuit board.
No-clean solder paste residues are made with resins and various levels of solid residues like activators, and gelling agents. All these are categorized by the standard IPC J-STD-004B. No-clean solder paste flux residue is acceptable on all 3 classes of PCBs as defined by IPC-610. Most no-clean solder pastes have rosins (RO) as their main ingredient. They typically display low reactivity (L) towards metals, and some contain halide ions (1) while others are halide free (0). No clean solder pastes are formulated with many different strengths. Typical characterization for no-clean solder paste fluxes is ROL1 or ROL0. Therefore, some solder pastes exhibit excellent printability and superior reflow characteristics, while other pastes are developed to perform well for the in-circuit test or pin in paste applications. Certain other no-clean solder pastes give ultra-low voiding performance.
We have seen an increase in usage of No-clean solder pastes in recent times. No clean solder pastes generally outperform water-soluble solder pastes. These solder pastes tend to be less reactive and more environmentally stable than water-soluble solder pastes which result in a longer stencil life, longer abandon time, and longer shelf life. Although the development of no-clean fluxes was done to avoid the need for cleaning PCBs post-reflow, they do require some level of cleaning. Trends like miniaturization, high density, and complexity tend to cause issues if the boards aren’t clean.
PCBs with flux residue, even of the no-clean solder type may face problems such as parasitic leakage, dendrite growth, electrochemical migration, and shorting. Therefore, cleaning the no-clean flux residue is vital for long-term PCB reliability, functionality, and performance.
No-clean fluxes at times can contain salt activators that might react with other chemicals and in the presence of heat, create a whitish residue. Hence, helping to corrode fragile circuitry and enable the growth of dendrites. Thus, simple water can’t clean the flux residue left behind by no-clean solder paste—it requires a solution of saponifier in water.