Factors impacting long term reliability

Although there is a wide range of Lead-Free alloys commercially available, it is likely that those based on tin-silver-copper (SAC alloy) will be most commonly used in Europe . These alloys have been widely tested and there is now sufficient reliability data to confirm that, in most applications, SAC alloy solder-joint reliability can be at least equal to or better than tin-lead solder joint reliability. However, this statement is a broad generalisation and there are many process details which need to be understood if such results are to be achieved.

Design

Design can have a significant effect on solder joint reliability. Important issues include; the type of board and solderable finish as well as the board layout and the choice and type of components used. These will affect the compliance of the board as well as influence the stresses that will be experienced in devices such as large area ball grid arrays etc.

In order to increase reliability at the design stage there are number of factors that should be considered. These can include:

• The specification of a suitable laminate with an appropriate solderable finish for the application.
  
  
• The type and location of the components on the board to reduce the chances of warping or sagging.
  
  
• Increasing the solder pad areas to compensate for the inferior wetting properties of Lead-Free solders.
  
  
• Redesign the size and spacing of vias and through-holes.
  
  
• Ensuring components have temperature ratings compatible with higher soldering temperatures.

Assembly

There are also factors and variables to consider during the assembly process if reliability is to be achieved. These can include the specific Lead-Free alloy composition and flux, the solder joint formation conditions i.e. reflow and cooling profiles, the quality of the printed circuit board and component termination solderable finishes (e.g. level of tarnishing or degree of oxidation). The incorporation of metals from the board and component finishes into the subsequently formed solder joint has a significant impact on its subsequent metallurgy and hence reliability.

The characteristics of the solder paste, the design of the stencil used and the printing parameters can all influence the geometry and integrity of the solder joints that are formed. They can also have an influence on the level of void formation or occurrence of other defects at the solder/substrate interface.

The soldering process and the parameters used also have an influence, as the use of higher temperatures for longer periods not only impacts on associated materials but also on the metallurgy of the joint itself. Higher temperature soldering for longer periods will cause changes in the intermetallics that form part of the joint.

In order to achieve good yields and reliability it is essential that all aspects of the assembly process are optimised and that no undesirable material and process interactions occur. In particular, careful attention should be paid to Lead-Free alloy manufacturers' guidance notes regarding the solder paste printing process. Also, the use of optimised temperature/time profiles during soldering will help to ensure that only the required amount of heat is used. When soldering by hand, the poorer wetting of Lead-Free solders may encourage operators to revisit the joint in order to improve the coverage. This may cause pad lifting and operators should be made aware that the relatively poor pad coverage may well be perfectly acceptable and that there is no need to expose the joint to additional heating.

Contamination

It is known that the contamination of SAC alloys with either lead or bismuth can seriously impact solder joint reliability. Consequently, it is important that great care is taken to avoid the use of components or solderable finishes that may contain these materials.

Obviously, in the change-over period, when there are still many tin-lead based components in circulation, this is a very real possibility and one that should be avoided both at the supply stage as well as through the use of carefully controlled component segregation and storage. The inadvertent introduction of bismuth is perhaps less likely than lead but it is widely used in solders and finishes in the Far East and so there is still a possibility of bismuth contamination occurring, especially if components or modules are supplied from this region.

In order to avoid contamination and subsequent reliability problems it is important that combinations of solder alloy and solderable finishes are properly understood. During the transition to Lead-Free there is a possibility of lead-containing and Lead-Free finishes being mixed and used on the same board. Procedures should be put in place to prevent this happening. This could involve dialogue with suppliers as well as careful control and segregation of stock. Care should be taken to ensure that solder baths do not become contaminated with lead once they have been converted to Lead-Free alloy as this may well lead to subsequent reliability problems and expensive corrective actions.

Strain, mechanical shock and fatigue

Tin-silver-copper (SAC) alloy solder joints can out-perform tin-lead solder based joints where low to moderate strain rates occurred during thermal cycle testing. This type of strain scenario is considered to be a more realistic representation of conditions likely to be encountered in real life applications. However, if higher strain rates are used in testing, it has been found that tin-lead joints can provide better reliability than their Lead-Free counterparts.

It has also been found that SAC alloy solder joints can perform less well than tin-lead in mechanical shock tests. This can be an important consideration for devices such as boards used in products such as mobile telephones which must survive a one metre drop test.

When comparing Lead-Free and tin-lead solder joint reliability it is important to understand how solder joints typically fail. One of the main causes of solder joint failure is known as low cycle fatigue (LCF). This term used to describe what happens when temperature cycling or power cycling induced stresses in the solder joints that can subsequently lead to failure. Each type of solder alloy will have different physical properties and thus a varying ability to survive this type of cycling.

In theory, many Lead-Free solder joints should exhibit better reliability than tin-lead. Their higher melting points tend to indicate that any stresses generated during service are less likely to approach the alloy yield strength. This appears to be confirmed by the better low cycle failure properties of SAC alloys in bulk form. Lead-Free alloys also possess higher strength, particularly creep strength. However, the properties seen in the bulk metal may not always be a reliable predictor of performance on the individual joints.

For many applications requiring mainstream operating conditions, Lead-Free reliability should be at least equivalent to that of conventional tin-lead alloys. The key factors that can have a large impact on subsequent reliability are the soldering process parameters and the correct choice of materials.