Posted on Tuesday 18th March, 2014

Why clean “No Cleans” – Printed Circuit Board Flux Removal

Today, nearly half of all printed circuit boards (PCBs) are assembled with no clean solder paste. The development of no clean solder paste was initiated in order to eliminate the need for further post-solder cleaning of circuit boards. Yet, many electronics assemblers are still struggling with the process of adequately and effectively removing no clean flux materials.

No clean fluxes are now the most popular fluxes to clean in electronics manufacturing. Most manufacturers have designed and dialed in their assembly lines to run no clean paste. And, they only clean the boards they want to clean, not always the boards they perhaps need to clean. Additionally, the flux residues left behind by no-clean pastes are more difficult to remove from PCBs than other fluxes. Since they are formulated not to be cleaned, the residues from no clean products can be left on the board in unwanted areas with detrimental effects on the PCB. That creates a cleaning challenge if you need to clean off a residue that was not designed to be removed.

No clean flux and solder residue on the board depends on the amount of solids in the material, the type of gelling agents, and various activators in the flux. The fluxes in most of today's no clean pastes contain up to 60 percent solids. The lower the total solids, the fewer residues on the board. No cleans, in theory, contain lower total solids so as not to need removal/cleaning. Following the reflow process, the flux leaves a small amount of residue around the various solder joints.

There are several reasons to remove no clean flux residues from PCBs. Historically, no clean fluxes were developed as tacky resins which inherently coated and stuck to all surfaces. The residue would gradually build up on the test pins. Manufacturers began cleaning no clean fluxes when problems emerged with in-circuit testing. The latest technologies of no cleans are improved and no longer tacky, but still can interfere with signal transmission in most instances.

Also, no clean fluxes can inhibit proper adhesion of conformal coatings. No clean flux residues can absorb moisture in process. In any further curing processes, the release of any moisture can cause the coating to be separated from the board and incomplete conformal adhesion can occur. This can allow corrosive materials, carbon dust build-up, or moisture to penetrate under the assembly and cause corrosion, signal transmission problems, and component failure.

Today, no clean paste manufacturers are aware that end-users are cleaning no cleans from board surfaces. The type of flux chemistry chosen can have significant impact on the cleaning process. In order to effectively clean and remove no clean solder pastes and no clean flux residues, a polar solvent or a solution of water and saponifier is necessary. Most no clean pastes require an organic saponifier for effective flux removal, but some can be cleaned with inorganic saponifiers. No clean fluxes that contain halides leave the most residue and are easier to clean. However, halide-free no clean fluxes produce less residue, yet are more difficult to remove. In general, no clean fluxes can be extremely difficult to clean particularly following any reflow processes as they can be baked on and tough to remove.

It can be vitally important to resolve any issues with no clean technologies and their subsequent cleaning challenges at the forefront of the device design process especially with high-reliability products such as avionics or medical devices. In applications where cleanliness is critical, it is important to consult cleaning experts like ACL for the most effective, board-safe cleaning technology.

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