The process of cleaning electronic assemblies is in a state of transition with the lead-free (Pb-free) movement and trend in the direction of paste/flux chemistries.  The impetus to restrict lead within the US began with the EPA’s proclamation to gradually reduce lead in gasoline starting in 1973. In addition, the Consumer Product Safety Commission began regulation of the manufacturer of paint with greater than 0.06% lead in 1978. Within the electronics industry the Environmental Protection Agency’s (EPA) Toxic Release Inventory implemented a reduction in the reporting thresholds in 1999 for lead from 10,000lbs to the current threshold of 100lbs per year. Outside the US, the European Council and European Parliament started a ban on hazardous materials like lead affective July 1, 2006 as per the Waste Electrical and Electronic Equipment (WEEE) Directive and the Restriction of Hazardous Substances (RoHS) Directive. In Japan, the Japanese Ministry of Industry and Trade had a proclamation that lead usage will be reduced by 67% by 2005. As a result, the shift in PWA manufacturing overseas will and has impacted domestic customers.

Properties of Pb-free solders
Eutectic tin-lead solder (also known as Sn63) consists of 63% tin and 37% lead. In metallurgical terms eutectic refers to the lowest melting combination of these two metals, in this case 183°C. Historically, this ratio of tin- lead solder has dictated the types of materials which could be used in PWB base material and component packages.

In combinations other than that of eutectic, solder does not melt or become liquidus at a specific temperature but has a melting range. As a result, the use of non-eutectic combinations means solder joint formation occurs over a range. The alloys shown in Table 4-1 are being evaluated by many manufacturers for future production applications or are being used in the field currently.

In the case of SAC (tin-silver-copper) Pb-free solders, the surface tension is greater than the standard eutectic Sn63/Pb37 [548 mN/m vs. 481 mN/m respectively,], indicating this Pb-free solder type would not wet as easily.

Many of the Pb-free alloys have a tendency to readily oxidize since tin is often the major component and oxides of tin are more stable than those of lead (i.e. lower heat of formation [∆Hf ]  SnO -69 cal/mol, SnO2 -143cal/mol, PbO -53cal/mol, PbO2 -67cal/mol).  The presence of these stable tin oxides inhibits wetting.  Another trend being observed with commercial customers is the use of Organic Solder Preserve (OSP) as an inexpensive answer to the Pb-free requirements in combination with Pb-free solders and no-clean chemistries.

These factors, in addition to increased use of no-clean chemistries have forced manufacturing engineers to modify their processes in order to obtain the appropriate soldering results with more flux being used in the case of hand soldering. These increased flux loadings and because hand soldering involves isolated heating of the board, more residue can be expected. Such residues can lead to dendrites, electrochemical migration (ECM) or corrosion (Figure 4-1).

The increased reflow conditions of Pb-free solders is another possible source of residues as polymerization of the flux can occur making cleaning difficult.  The accepted guideline, J-STD-004, describes the different flux systems by type and activity. Activity refers to what gives the flux/paste its cleaning action. Historically activity was achieved through addition of halides. Halide residues are usually very corrosive by themselves and their level is important. There are other less aggressive means to achieving activity such as addition of a weak organic acid. This defined activity is broken up into halide activity as a percentage, and 0 or 1 (the absence or presence of halides respectively). The flux residue activity is represented by letters: L=low, M=moderate, H=high. The specific designations are:

• Rosin (RO) or the traditional flux type is based upon tree sap (colophony)
  
  
• Resin (RE) based are synthetic and contain some form of organic polymer as the matrix
  
  
• Organic (OR) are composed of weak organic acids (e.g. formic acid, succinic acid or adipic acid)
  
  
• Inorganic (IN) are synthetic and can be composed of salt-based (e.g. ammonium chloride), alkali (e.g. amines) or mineral acids (e.g. phosphoric acid)

The definition of what a no-clean is does not allow it to fall under one of these categories. The key with the no-cleans is they are designed burn off during soldering with little or no detrimental residue remaining which may or may not be cleaned.

No-clean fluxes historically provide different soldering results as compared to activated fluxes. In addition variation in the make up of such residues can be drastically different from manufacturer to manufacturer (Figure 4-2).

Cleaning trends
Cleaning chemistries have changed since the Montreal Protocol banned halogenated hydrocarbons. Those early chemistries worked very well for the flux residues of the day (activated rosin) by dissolving and washing them off the surface through direct immersion, spraying or vapor degreasing. The number of options available today is less as water is accepted as the “universal solvent”.

Because of the limited solubility of many materials in water, the key to removal is a combination of time, water volume, temperature and efficiency of agitation. Ultimately, water alone may not suffice, thus additives (surfactants) are introduced to reduce surface tension, improve penetration, and induce one or more of the following phenomenon: wetting, emulsification, solubilization, saponification, deflocculation, and sequestration. , When dealing with a cleaning dilemma, the key considerations are:

(1)Understand what you are trying to remove.
(2)Determine the limitations of the assembly.  Are there any temperature, moisture or vibration sensitive parts?
(3)How densely populated is the assembly. Where are problem spots going to be where residue could collect or become trapped?
(4)What are the long term reliability goals?

As the implementation of Pb-Free materials into electronic manufacturing centers continues to increase, the demand for products built with tin-lead materials declines, impacting the availability of tin-lead plated parts. This has required manufacturing centers to find alternative resources to obtain tin-lead components, often from warehouses and inventories where proper storage conditions were not always maintained.  The ability to recover these materials for use will depend upon what surface contaminate is present (dirt or surface oxide). ACI has in the past demonstrated its ability to recover heavily oxidized parts through a process called ROSA (Reduced Oxide Solderability Activation). This recovery technique is expected to find increased demand as the previously mentioned trend takes effect. A more recent example of the EMPF’s quick response to an emergency request involving recovery of urgently needed  replacement parts is shown in Figure 4-3.

Ultimately, the Pb-free transition has begun. While most of the issues with Pb-free are recognized and being addressed, there are remaining issues still present that revolve around logistics and sourcing of parts. There will be instances of companies running both lead and Pb-free processes on the same floor until a complete transition to Pb-free occurs. In addition, there will be customers who maintain tin-lead processes for contractual reasons or exemptions as with the US military. As a result, they will still need support in this arena. The EMPF has recognized this and is making efforts to satisfy those needs.