Lead

The most significant impact of the RoHS Directive is the proscription of lead as this will result in the removal of lead from most solders used in electronics assembly. Conventional solders are based on an alloy of tin and lead such as the widely used tin-lead eutectic composition that melts at 183°C.

As lead will no longer be allowed in solder used for most electronics assembly operations, alternative solders will have to be introduced. These are already widely available and there is now established experience of their use. However, there are many different types of solder, each with its own specific advantages and disadvantages.

Lead is also used in various other ways and it may therefore be encountered in other components, products and parts of the electronics assembly process. A good example is in the printed circuit board (PCB) manufacturing process where, lead is often a component of the tin-lead etch resists used in the patterning process for PCB outer layers.

In this process a tin-lead pattern coating is plated onto the surface of the board to selectively protect the underlying copper during etching. After etching, the tin-lead coating is removed with a proprietary stripper.

The subsequent waste solutions are very acidic and after treatment they are often consigned to landfill. Although the use of lead in this process will not, strictly speaking, be banned, the removal of lead from related manufacturing and assembly processes means that PCB manufacturers will change to Lead-Free etch resists. The American PCB industry has largely changed over to using pure tin, and the European industry is now changing.

Another important area where lead is encountered in the electronics manufacturing process is in the provision of a solderable final finish on PCBs. The pads and holes of a PCB are where components are soldered during assembly, and it is important that a final finish be applied to the underlying copper of the circuit board that will be solderable when the board is assembled. (Bare copper oxidises and loses its solderability). The most popular type of finish is known as a hot air solder levelled (HASL) finish, and this method applies a thin layer of tin-lead solder to the pads and holes of the board. With the implementation of the RoHS Directive it will no longer be possible to apply a conventional tin-lead HASL finish to boards.

Manufacturers will either have to convert to Lead-Free HASL using an alloy such as the tin-copper eutectic or convert to another type of finish such as those based on silver, nickel-gold or organic solderability preservatives (OSPs). There are many Lead-Free final finishes commercially available and they have established track records. The key for assemblers will be to know what solderable finish is used, especially if a HASL process has been used, since it is important that suitable combinations of solderable finish, solder and component finish are used to avoid solderability and subsequent reliability issues.

Another place where lead is routinely found is in the solderable finishes of component leads. Again, tin-lead alloys are routinely used to provide solderability to leads or terminations, and these will also have to change to Lead-Free. In the past there have been concerns about the use of tin as an alternative finish because it can be prone to a phenomenon called whiskering, which can lead to reliability concerns. Manufacturers have recently developed new tin chemistries that are claimed to deposit whisker-free tin, and these are now starting to be used. The biggest challenge for assemblers is the identification of Lead-Free components, since there is, as yet, no agreed standard convention for identifying whether or not component finishes are Lead-Free. Given that components with and without Lead-Free finishes may appear visually identical, it is clear that careful attention needs to be paid to component finishes, selection and storage and so on.

Lead also finds use in electronics in a number of other areas where it will be exempted from the RoHS Directive and thus will be able to continue in use. One example is high melting point solders where there are no viable alternatives. Such solders are use in the so-called 'die attach process' where a silicon chip is soldered to a lead frame or some other substrate. A high melting point is essential so that the solder does not remelt during subsequent soldering operations. Lead is also a vital component of various ceramic materials that find wide use in electronic components. In these examples the lead does not exist in its metallic form but is chemically bonded to other elements to form a more inert and chemically stable material. The small loudspeaker devices found in novelty cards and electronic toys are examples of where the lead would be bound up in a more inert ceramic material.