Lead alternatives

Apart from the use of lead in solders for electronics assembly it is also used in various other stages of the electronics manufacturing process.

More details of the alternative Lead-Free solders that are likely to be used to replace tin-lead can be found in the Lead-Free section of this toolkit.

Circuit board etch resists

The outer surface conductor tracks of printed circuit boards have traditionally been formed using a process that employed an electroplated tin-lead alloy. This coating is typically removed after the patterning process, and thus does not appear in the final product containing the circuit board. Theoretically, therefore, the use of lead in these alloys would not, strictly speaking, be proscribed.

However, for companies converting their manufacturing processes to Lead-Free, it would be a good idea to avoid the use of tin-lead etch resists. There is a readily available alternative in the form of pure tin and this is becoming widely used as a replacement for traditional tin-lead based etch resists. The PCB industry in the USA has almost completely converted to the use of pure tin and the European industry has also made good progress in changing over.

Circuit board solderable finish

One of the final stages of the PCB manufacturing process requires the deposition of a solderable finish onto the copper surfaces and this has traditionally contained lead in the form of the so-called 'Hot Air Solder Levelled' (HASL) finish.

Many Lead-Free planar finishes have been available for a number of years and these were adopted, initially, not because they were Lead-Free but because they gave the degree of planarity that was required to accommodate fine pitch surface mount device technology. There is already an established history of use and finishes such as nickel-gold, immersion tin, immersion silver and those employing organic solderability preservatives (OSPs). These are just a few of the many types that are widely used.

The preferred choice will depend on the application; OSPs, for example are the lowest cost option but they may not be able to survive multiple solder cycles and wetting with Lead-Free solders may be inferior to that achieved with conventional tin-lead solders. A nickel-gold finish will give very good performance over a wide range of applications but is relatively expensive. For those companies wishing to continue to offer a HASL finish, it may be possible to convert existing equipment to a Lead-Free process using tin-copper or a similar solder alloy.

Solder

With a Lead-Free circuit board ready to solder it will be necessary to use Lead-Free solder pastes and in Europe the preferred alloys are those based on the tin-silver-copper system with melting points/ranges in the region of 215 to 220°C or the tin-copper eutectic alloy with a melting point of 227°C.

Tin-silver-copper is preferred for reflow soldering and tin-copper for wave soldering.

There are many other solder alloys available but care should be taken as there are incompatibility and reliability issues when alloys containing different alloying elements are mixed. (See the Lead-Free section of this toolkit for more details).

Component solderable finishes

Lead has also been traditionally used in the solderable finishes applied to component leads and terminations. This will no longer be possible when the RoHS Directive comes into force and alternatives such as pure tin will have to be used.

Although matte tin finishes are likely to be one of the most popular Lead-Free finishes for components, there are still some concerns about the possibility of reliability issues related to tin whiskering. Component finish manufacturers have worked hard to understand the causes of tin whiskering. They have modified their process chemistries to minimise the risk of whsikering occurring, although it is not absolutely certain that it can be completely eradicated. (See the Lead-Free section of this toolkit for more details)

There are also a number of other Lead-Free finishes that may be used and these include alloys of tin with silver, copper or bismuth as well as nickel - palladium alloys. The best choice of finish will depend on a number of factors including the type of component and the application, for example, tin - bismuth finishes have been predicted to become popular with semiconductor devices, whilst alloys using palladium may be prohibitively expensive for many non-specialist applications.

Summary - lead

Although lead is widely used in the electronics manufacturing process and, although there are a certain number of issues to be addressed, in most cases there are plenty of viable alternatives and the places where lead is likely to be encountered are largely well known.