This push for more environmentally friendly products has addressed two problems inherent with PCB technology. The first problem relates to the presence of Lead (Pb) and the second relates to the presence of HALOGENS. The restrictions on the use of Lead are now currently in place within the European Union, while restrictions on the use of Halogens are currently voluntary.
Lead has been an integral element in the electronics industry for over half a century. Found in solder (comprising 60% tin and 40% lead), the compound exhibited characteristics which even today are difficult to surpass. With the banning of lead under the European RoHS directives a number of alternatives ar e now commonly available and offered by TN.
Halogens comprise of five none metallic elements defined within Group VII of the Periodic Table. These five elements are FLOURINE, CHLORINE, BROMINE, IODINE and ASTATINE. "Halogen" meaning "salt former" with the 5 elements all being referred to as "Salts". Fluorine (Fl), Chlorine (Cl) and Bromine (Br) are found in compounds which are used in the manufacture of PCB laminate to give the laminate its fire retardant characteristics, in most cases to UL-94V-0 flammability compliance. TN is currently utilising a number of different laminates classified as "Halogen Free".
LEAD FREE OPTIONS
TN offer the following Lead Free solutions:
LHAL (Lead Free Hot Air Level): This finish utilises a compound primarily comprising of Tin. PCBs with this finish have a long shelf life (prior to soldering) and come at a similar price to the traditional Tin/Lead solder finish. The finish is applied using the same technique and equipment as Tin/Lead solder, making it attractive for many PCB manufactures as they are able to convert their existing HAL equipment. This finish has a higher melting temperature than traditional solder and hence subjects the PCB to higher thermal stresses during manufacture. In addition, as the finish is leveled using a jet of hot air, the flatness of the finish is poor compared to other options.
Gold Plate: This finish consists of a thin layer of gold (Au) electroplated over a thicker layer of Nickel (Ni). The Nickel is electroplated directly over the bare copper. This finish has the advantage of a long shelf life and due to the low oxidization rate of the finish, can be left unprocessed to form electrical contact connections such as switches or circuit edge connectors. The soldering process requires the solder compound to bond to the gold, unlike the LHAL finish where the solder compound can melt into the LHAL finish and mix on a molecular level. As such a PCB with a gold plated finish requires accurate temperature profiling to ensure reliable solder joints across the entire PCB. While the finish is durable, the soldering process is susceptable to contamination from handling and the environment.
In addition the finish contains a degree of Phosphor contamination within the gold due to processes involved during manufacture. When the PCB is subject to a heat process, the Phosphor tends to rise the surface and detracts from the solderability of the finish. Temperature profiling needs to take this into consideration when re-flow and wave soldering processes are utilised. In particular the problem arises if a PCB needs to be exposed to more than one heat process during manufacture, for example soldering the SMD components through a re-flow process may cause solderability problems in subsequent soldering processes. The finished assembly consist of solder joints comprising a copper PCB pad, a layer of electroplated nickel, then a layer of gold and then a layer of solder material. The layers of different metals do suffer from an electrolysis affect, which in turn can cause the connection to corrode and fail. Though this is rare it is a documented phenomenon which needs to be considered particularly in high volume production. |
