Wave Soldering

Lead-Free wave soldering can be relatively straightforward but does necessitate a number of changes to the process if good solder joints are to be formed on a consistent basis.

Choosing solders

In Europe the Lead-Free alloys that have been recommended are wave soldering applications are tin-silver-copper (SAC) and tin-copper eutectic, which melt at around 217 and 227°C respectively. The absence of silver in the tin-copper alloy means that it is significantly lower in cost than the SAC alloys. However, wetting balance tests performed on boards with a variety of popular solderable finishes such as silver, nickel-gold and OSP have shown that the tin-copper eutectic alloy offers inferior wetting properties. This means that longer solder wave contact times may be needed in production to achieve hole filling. The higher melting point of tin-copper also requires increased solder temperatures.

For both tin-silver-copper and tin-copper alloys, the typical solder bath temperatures will normally need to be in the region of 245 to 270°C. It is likely that a higher preheat temperature will be required. This may be as much as 160°C for tin-copper wave soldering. It may also be necessary to reduce conveyor speeds in order to achieve better wetting and filling of through-holes. A reduction of around 15% can help but it will negatively impact productivity and, could lead to increased solder bridging due to exhaustion of the flux.

Standard tin-copper eutectic alloy has found wide use in wave soldering, however a modified alloy containing a small addition of nickel has been shown to offer significantly improved properties and performance.

Process and temperature control

A key requirement for the formation of high quality solder joints in wave soldering is the correct combination of flux, heat and solder. The critical variables include flux deposition rate, preheat and solder temperature and solder dwell time. For example, correct heating of the substrate and components is essential to achieve solder joint quality and overall assembly reliability. Tight temperature control is important and too much or too little preheat can cause problems. Accurate profiling of topside temperatures can be used to eliminate secondary reflow by using a process control tool to monitor changes in contact time and temperature. If the components being soldered do not become hot enough to wet in the limited time they are exposed to the solder wave, the result will be poor solderability.

Flux choice

Choice of flux plays an important role in determining the efficiency of the wave soldering operation. It is known that fluxes originally designed for use with tin-lead are generally unable to provide adequate hole filling with Lead-Free alloys without using increased volumes or longer contact times. This means that if fluxes designed for lead based solders are to be used, conveyor speeds will need to be reduced.

Liquid fluxes with higher activities and higher solids contents perform better with Lead-Free solders. Water-washable fluxes, have been demonstrated to give the best hole filling with Lead-Free alloys, even with the more difficult to wet solderable finishes such as OSP. The activators used are potentially corrosive, but they do remove all of the oxides adequately, even with the longer contact times and increased soldering temperatures used with Lead-Free. New formulations of no-clean liquid fluxes have also been specifically developed that use enhanced activator materials. They are designed to be thermally stable at higher preheat and solder pot temperatures and can survive contact with the solder for longer. They are still active as the boards emerge from the wave and thereby help to reduce bridging and assisting hole filling.

The general trend in Lead-Free wave soldering is to the use of VOC-free fluxes which offer a more environmentally friendly operation. VOC-free fluxes generally perform well with Lead-Free alloys and higher solids in the range of 4% or more are reported to be best.

Problems and defects

One of the main concerns about converting to Lead-Free wave soldering is that it can lead to an increase in the incidence of defects and related problems. The reduced wetting properties of Lead-Free solders, coupled with higher temperatures can lead to problems such as insufficient solder, skips, lack of hole filling and grainy joints. The correct choice of flux can have the biggest influence in reducing their occurrence. Other problems that can occur with wave soldering include, solder balling, fillet lifting and non-wetting, pad lifting and joint tearing. As with reflow soldering, some boards may show a tendency to sag. This can be reduced by the use of a central board support but there may be an impact on the board design.

Fillet-lifting has been seen in Lead-Free assemblies and, although it is not considered to be a cause of catastrophic failures, the factors that influence its formation need to be understood and, where possible, avoided.

Changes to equipment

Using Lead-Free alloys or wave soldering could contaminate the solder pot leading to compositional changes in the solder. The use of solders containing high levels of tin can lead to both dissolution of copper into the solder and more rapid attack and corrosion of stainless steel solder pots. Solder pots and other components such as flow duct nozzles and pumps, need to be replaced or coated with a tin resistant material to prevent corrosion and premature failure. Various materials are compatible with the tin-rich Lead-Free solders such as nitrided steel, titanium, cast iron, and various ceramic coatings. Manufacturers of wave soldering machines are increasingly aware of the issues around conversion of their equipment to be Lead-Free compatible and many offer suitable replacement parts made of the appropriate materials. However, for older equipment, the availability of replacement parts may be more of a problem. Cast iron often is used in smaller solder dipping pots. These are not affected by conversion to tin-rich Lead-Free solder alloys.

Increased levels of copper in the solder can change the visual appearance of a joint; however this does not have a significant effect on joint reliability. An increase in the level of copper in a solder will, however, lead to intermetallic formation which is known to increase with higher soldering temperatures. Over time, copper levels may reach as high as 2% and the alloy can exhibit needle-like dendritic formations of the copper-tin intermetallic. With conventional tin-lead solders this intermetallic floats on the top of the solder making its removal reasonably easy. However, with lower density Lead-Free solders, the copper-tin intermetallic's relatively high density means that it sinks to the bottom of the solder pot making removal more difficult.

This dissolution of copper into the solder bath will result in the need to change the solder bath more frequently. Once the copper level rises above ~1.55%, it is often necessary to replace the solder. For tin-silver-copper alloys it is possible to alleviate the problem by topping up the solder bath with a tin-silver alloy in order to adjust the alloy composition. The type of solderable finish applied to the board also influences the rate of dissolution of copper into a solder bath and the use of a barrier layer such as nickel can be beneficial. The presence of nickel as a bath coating helps to reduce copper dissolution but, if it is used as in a nickel-gold solderable finish, the bath will tend to show a build up in gold levels instead.

Inerting

Lead-Free alloys do not wet solderable surfaces as readily as tin-lead eutectic solder and longer dwell times with higher pot temperatures are required. However, as with reflow soldering, the use of a nitrogen atmosphere can significantly improve the wetting process. A nitrogen environment can also enable lower solder pot temperatures to be used without compromising the solderability. The cost of using an inert atmosphere must therefore balanced against the improvements achieved in productivity and yields.

Wave soldering summary

Given that there are many variables and interactions that need to be understood when converting to Lead-Free wave soldering a systematic approach is needed if conversion is to be successful. Some of the key steps in converting to Lead-Free wave soldering are:

• Make sure that the wave soldering unit is properly converted to be Lead-Free compatible - this may well involve dialogue with the manufacturer and the purchase of certain replacement parts.
• Obtain the required Lead-Free components and PCBs with a Lead-Free solderable finish.
• Decide whether to use a SAC alloy, tin-copper eutectic or another proprietary alloy.
• Select a suitable flux chemistry that has been developed for Lead-Free compatibility (VOC-free fluxes are preferred).
• Carry out a series of soldering trials to determine the best operating parameters for maximum wetting and hole filling.
• Convert other soldering processes so that they are compatible with the newly established Lead-Free wave process.
• Lead-Free solders and tin-lead solders should not be allowed to mix. This is even more important with wave soldering because the accidental introduction of tin-lead solder into a Lead-Free wave soldering operation can be an expensive mistake to correct.