No-Clean
Technology Review

Released October 1996

View a summary of the no-clean cleaning technology review findings.

NO-CLEAN APPROACHES DEFINED

No-clean approaches can be defined as any approach to a process that allows a previously required cleaning step to be eliminated. When applicable, no-clean strategies are usually the best possible alternative to a current cleaning process. All of the costs and wastes associated with cleaning are completely eliminated.

The PPRC’s investigation found that published literature about no-clean approaches focused on one specific application: no-clean applications for soldering in electronics assembly. Two different types of approaches have been used to eliminate the cleaning step required after traditional soldering: using no-clean fluxes and, very recently, using fluxless soldering. This article will focus on no-clean fluxes and will briefly introduce fluxless soldering.

Flux Basics
Fluxes are used in electronics manufacturing to promote the wetability required to make a good solder joint. Specifically, soldering flux performs the following functions:

• reacts with or removes oxides and other contamination on the surface to be soldered;
• dissolves the metal salts formed during the reaction with metal oxides;
• protects the surface from reoxidation before Soldering occurs;
• often provides a thermal blanket to spread the heat evenly during soldering; and
• often reduces the interfacial surface tension between the solder and the surface in order to enhance wetting (Ref. 1).

Two types of soldering are most commonly encountered in high-volume electronics assembly processes, and each type uses a different form of flux.

• Wave soldering circulates molten solder from a reservoir to form a standing wave. The bottom surface of the circuit board passes through the crest of the wave, applying solder to all solder-wetable surfaces. For wave soldering, flux in a liquid form is typically applied using a foam fluxer or spray fluxer and then preheated prior to the wave soldering process. Wave soldering is used primarily for electrical components that are connected to a circuit board via leads that pass through holes in the board (called "through-hole components").

• Oven reflow soldering uses solder in a paste form containing both solder flux and solder. The solder paste is usually stencil printed onto a circuit board at appropriate points so that surface mount component leads rest on the paste. The board is then run through an oven, which heats the board and paste enough for the solder to melt (known as "reflow"). Oven reflow methods are typically used for surface mounted components.

In addition to coming in a liquid or paste form, fluxes come in three primary types of formulations, each of which has different requirements for post-solder cleaning.

• Rosin-based fluxes use rosin, a naturally occurring resin found in pine tree sap. Traditional rosin-based fluxes leave residues on the circuit board that are often removed with a cleaning solvent such as Freon or trichloroethylene. Semi-aqueous and saponified aqueous cleaners can also be used to remove residues left by many rosin-based fluxes. Prior to when the Montreal Protocol was signed and concern over health issues related to use of chlorinated solvents became prevalent, rosin-based fluxes were the most commonly used type of flux in the electronics manufacturing industry.

• Water-soluble fluxes have been available for decades and leave a residue behind that must be removed using water. Water-soluble fluxes are among the most active available today, and they are good at removing oxidation and providing a clean surface that will promote solder flow. Issues such as corrosion other failures related to higher humidity must be addressed when using these fluxes. In addition, because of their high activity, including high halide content in some cases, water soluble flux residue can cause sustained corrosion and electrical problems without adequate cleaning.

• No-clean fluxes have been available for almost a decade but have become more widely available, with dozens of products available from vendors, in the 1990s. These fluxes are based on a wide range of chemistries and composition, but all share the characteristics of leaving a minimal and theoretically benign residue on electronic assemblies that does not have to be removed with any post-solder cleaning process. Many of these products contain a much lower solids content than traditional fluxes — 2-3% by weight as opposed to 25-35% by weight. No-clean fluxes are sometimes used in conjunction with an inert atmosphere such as nitrogen to further reduce the potential for oxidation during soldering. No-clean fluxes can be difficult to implement because the "process window" of acceptable operating parameters becomes smaller when using them due to the lower activity of no-clean fluxes (see the next paragraph for an explanation of activity).

A number of industry-standard tests are used to evaluate fluxes and their residues. The tests are used individually or in combination in a number of the evaluations of no-clean fluxes reported on in the literature. Therefore, it is important to be at least introduced to these test methods. These tests are used collectively to categorize fluxes in one of three categories — low, moderate or high activity. In general, the higher the activity of a flux, the more reactive/corrosive the flux and its residue is. The tests are:

• Copper Mirror Test. A drop of flux is placed on one end of a glass slide with a small copper deposit. A drop of water white rosin flux is placed on the other end of the slide. After a holding time and alcohol rinse, the slide is examined to see how much breakthrough of the test area occurred by seeing if white paper is visible through the slide. The more breakthrough, the higher the activity of the flux being tested.

• Halide Content. The presence of the halides chloride, bromide, and fluoride are indicators of flux activity. Qualitative tests are used to see if any of the halides are present. If present, a quantitative test is used to determine the percentage of total halides. The more halides the higher the activity of the flux.

• Corrosion Test. A copper coupon with a small indentation in it is precleaned with acid and ammonium persulfate, and a sample of flux solids and some solder wire are placed in the indentation. The coupon is then heated to allow the solder to reflow, and stored for 10 days at 50° C and 65 percent relative humidity. After the test period, the coupon is inspected for corrosion. The more corrosion, the higher the activity of the flux.

• Surface Insulation Resistance (SIR). SIR measures the resistance to current flow between two surface conductors. The test is often performed using a standardized test coupon containing four comb patterns. A test voltage is passed through the conductors and resistance measurements are taken. The test is pass/fail with a minimum resistance being the criteria. Low-activity fluxes used on a test board must pass the test both if cleaned or if not cleaned. Medium-activity fluxes must pass the test either with or without cleaning. High-activity fluxes must pass the test only when cleaned.

No-clean fluxes are mostly low-activity and sometimes medium-activity. Fluxes that require cleaning are typically medium- or high-activity. Additional background information on soldering and fluxes is readily available. (Ref. 1) Additional background information on no-clean soldering can also be found in a number of sources. (Ref. 1-4)

Technical Issues and No-Clean Approaches
A discussion of the technical feasibility of no-clean approaches and results that indicate visible residues are not necessarily detrimental. Further information that using a no-clean flux does not necessarily eliminate all cleaning in the solder area, and that use of an alternative atmosphere, such as nitrogen, can improve the no-clean process. Lastly, those considering a change to no-clean will require specific testing.

No-Clean Economics
A summary of research that evaluates the conversion to and operational costs of no-clean approaches.

Gaps in Existing No-Clean Research
An analysis of areas that merit further study.

Summary of No-Clean Technology Review Findings

Cleaning-related Projects in the Pollution Prevention Research Projects Database

Other Cleaning-related Internet Sites

No-Clean Bibliography