Hand Soldering

Hand soldering is widely used for many small run assembly operations, for assemblies with large or delicate components, and also in rework and repair. Hand soldering is usually performed towards the end of the assembly process where the circuit board has a significant intrinsic value so ensuring that the process is properly established for Lead-Free compatibility is essential if costly failures are to be avoided. Operator training and good control of the hand soldering process can have a significant impact in helping to reduce manufacturing costs and enhancing productivity.

Choosing solders, fluxes and equipment

The Lead-Free alloys that are recommended for use in most applications have been detailed elsewhere in this toolkit but of these the recommended alloy for hand soldering is tin-silver-copper. This is preferred over both tin-silver and tin-copper alloys because its wetting properties are better and its melting point is lower.

Multi-cored Lead-Free solder wire is available in a range of diameters. The flux used in these solders will have been carefully selected to be compatible with the specific solder alloy and it should not be necessary to use any additional flux. If there is a choice of flux, it is best to avoid those with high activity for hand soldering operations, as these will tend to corrode the soldering iron tips more rapidly. Soldering at the optimum temperature is also important as fluxes are prone to oxidation at higher temperatures.

Many of the Lead-Free alloys are available as multicored wire with a range of different fluxes. The choice of flux has been found to have a significant impact on solderability. More active fluxes may be required in order to accommodate the influence of higher process temperatures the reduced wetting forces associated with Lead-Free alloys. Lead-Free flux cored solder wires may also have an increased volume of flux. Tin-lead alloys typically have around 1% flux but, for Lead-Free alloys, this may need to increase to 2% or more. The use of more aggressive fluxes in greater volumes may also mean that some cleaning may be necessary after soldering. This will add to the overall manufacturing costs.

Efficient heat transfer can be achieved by the correct choice of soldering iron tip. Flat, spade-like tips offer better heat transfer than round tips and should be approximately the same width as the part being soldered. The use of the correct size of solder wire is also important. Larger diameter wires offer better heat transfer than thinner wires. Some newer soldering systems now feature advanced electronics to monitor the thermal demand at the tip which enables the iron to respond immediately by driving the optimal amount of heat into the joint at safer, lower temperatures.

Soldering at higher temperatures

Although the absolute temperature of the soldering iron tip is important, it is only one of several key factors. Increasing the temperature of the iron when moving to Lead-Free soldering may only provide part of the solution for producing good quality solder joints. In many cases there may be no need to increase soldering temperatures. For optimum performance, a number of other factors will also need to be considered. These can include the iron tip shape and condition, the power output of the soldering iron and the length of time the joint is heated.

Generally, successful joint formation can be achieved by elevating the joint temperature to around 40C above the melting point of the solder for between two and five seconds. For tin-silver-copper alloys this means taking the joint temperature to just under 260°C. This can normally be achieved either by increasing the soldering iron tip temperature or increasing the contact time. For Lead-Free solders, it has been found that the tip temperatures typically need to be set to between 345°C and 370°C. Temperatures higher than 400°C are unlikely to be required for most hand soldering applications. Using a higher soldering temperature may improve the solder wetting characteristics but this could result in board and component damage. The preferred approach is to use longer contact times if possible.

Too much heat tends to produce a thicker intermetallic layer between the solder and the component/board leading to joint brittleness. Insufficient energy input produces too little intermetallic formation and can result in dry joint formation. Use of excessive temperatures will also cause increased oxidation of the soldering iron tip and reduced wetting.

Inerting

For critical applications, it is possible to use hand soldering irons that provide an inert atmosphere of nitrogen around the tip. The use of nitrogen helps to reduce oxidation, thus enabling less active fluxes to be used, as well as reduced quantities of flux. These irons are costly so careful consideration will have to be given to the costs versus the benefits.

Inspection

The appearance of a solder joint has always been a good indicator of the joint quality.

This is not the case with many of the preferred Lead-Free solders.

Solders such as the tin-silver-copper alloy have significantly different appearances and properties to conventional tin-lead which are immediately apparent in their different soldering characteristics and joint appearances. These solders do not flow as readily as tin-lead alloys and they also do not wet copper as easily. The shapes of the joints are different to conventional solder joints and the alloys have a much duller and more grainy/granular appearance. This often leads to the formation of what may look like poor quality joints but are in actual fact perfectly acceptable.

Solder pads may not wet completely and this can lead to operators making a second visit to the joint in an attempt to improve the quality. This should be avoided whenever possible because the joints formed are probably of perfectly good quality and the application of additional heating can cause damage. It is vital that operators are trained to understand the differences between tin-lead and Lead-Free solder joints so that they can appreciate what exactly constitutes a good quality solder joint with Lead-Free alloys.

The IPC has changed the visual inspection criteria of J-STD-001 and IPC-A-610 to take into account the different appearance of Lead-Free solder joints compared to tin-lead joints.

Problems

Hand soldering with tin-rich Lead-Free solders can accelerate soldering iron tip wear and more frequent replacement will probably be necessary. Tip life is reduced because the tin-rich solders attack the iron plating and also because the effect of the higher temperatures and more aggressive fluxes. It is essential to use high quality soldering iron tips; the use of low cost inferior tips may prove to be a false economy.

Health and safety

From an operator health and safety perspective, more care may be required when soldering with Lead-Free materials. The use of stronger fluxes means that there is a greater requirement to protect employees from the effects of fumes given off during soldering. This will require the use of suitable fume extraction equipment. Highly effective units are now available which combine a pre-filter, an efficient HEPA filter (down to 0.3 micron particle size), and chemically treated gas filters. Exposure to flux fumes must be avoided as some workers could become sensitised to them. However, there is the benefit that workers will not be exposed to lead.

Rework and repair

Hand soldering is also used in the rework and repair of circuit board assemblies and it is essential that the same alloy is used as in the original assembly process. The lowest possible working temperature should be used along with the largest possible soldering iron tip that can be accommodated for the application. The optimized transfer of heat through a properly tinned tip will also help to provide a rapid and efficient solder connection. In some cases, it may be advisable to pre-heat the circuit board before attempting to undertake rework or repair in order to improve the soldering efficiency.

De-soldering of Lead-Free components can be achieved as long as the correct tools and techniques are used. The use of manual solder suckers may be acceptable for de-soldering on single sided circuit boards but for multilayer boards, professional heated vacuum de-soldering equipment should be used. This is particularly true with thicker multilayer boards where the plated through holes will provide an increased degree of heat-sinking that will make de-soldering more difficult. As with conventional tin-lead based solders, de-soldering braids can also be used for some de-soldering operations. If vacuum de-soldering tools are used, sufficiently high temperatures must be maintained right up to the entrance to the filter; otherwise the solder may solidify and block the tool before being completely extracted.

Hand soldering summary

Lead-Free hand soldering need not cause problems if attention is paid to process control. Parameters such as soldering iron power output, flux type, temperature, tip shape and condition all need to be optimized to produce good quality Lead-Free solder joints. Operators need to understand that switching to Lead-Free alloys places a much greater emphasis on process control, if excessive temperatures and subsequent thermal damage to assemblies are to be avoided.

Stronger and more aggressive fluxes may be needed to cope with the higher temperatures, the greater oxidation rates and weaker wetting forces. The knock-on effect is that fume extraction systems are essential to protect employees, and post-soldering cleaning may also be required. The thermal profile needs to account for the higher melting points of Lead-Free alloys, while providing sufficient thermal energy to the joint and ensuring that fluxes are not burnt off before they can do their job. The overall heat transfer can be maximized by choosing a good quality tip that matches the size and shape of the object being soldered.

In order to achieve the required process control, many manufacturers are turning to more sophisticated soldering systems, where the tip is automatically maintained at a constant temperature. This means the tighter process window becomes less of a problem, and the risk of thermal damage to the component or PCB is virtually eliminated.

• Lead-Free soldering is perfectly possible but the solders used behave differently to conventional tin-lead solder.
• The post popular Lead-Free solders for hand soldering are the tin-silver-copper (SAC) alloys and these have melting points around 215 to 220°C, which is significantly higher than for tin-lead alloys (mp 183°C).
• The SAC alloys do not wet as well as tin-lead solder and they form solder joints that are less shiny and more granular in appearance. There may also be exposed copper on the solder pads.
• It is important that operators understand the differences in appearance of Lead-Free solder joints compared to those from tin-lead alloys.
• Lead-Free hand soldering should not cause any major difficulties for most applications, as long as the soldering process is carried out in the correct way.