Lead-Free Soldering Effect On Tantalum Capacitors

Lead-Free Soldering Effect On Tantalum Capacitors
Written By: T. Zednicek | P. Vasina | Z. Sita | B. Vrana
Legislation is being developed worldwide to reduce the lead content in many consumer electronic products. This is being taken as an action to reduce environmental impact when such products are discarded.

Despite the fact that lead containing solders in electronic assemblies account for only 0.49% of world lead consumption, the trend in legislation will likely be to require not only reduced lead content, but also its complete elimination in such products.

There are three principal sources of lead in electronic circuit assemblies; the solderable traces on the circuit card, the solderable finish on the components themselves and the solder alloy used to connect the two (either solder paste for reflow, or liquid solder for wave).

A typical component has negligible lead content in its termination finish in comparison to the amount of solder alloy used in the pcb (print circuit board) process. Nevertheless, changing to a lead-free solder alloy for the soldering process will require the component to have a compatible termination finish to achieve the correct soldering/wetting characteristics with the reduced lead or lead-free soldering system being used. Depending upon the component type, this in itself can be either a straightforward or a complex change. But, regardless of the technology requirements to provide a part with the correct termination characteristics, the major concern will be the compatibility of the component with the higher temperature profiles associated with many reduced lead or lead-free soldering systems. In many cases, this will require modification of current technology relating to internal design or new material development in order to ‘survive’ the more aggressive reflow or wave soldering conditions as a result of most lead-free solder systems’ higher liquidus temperatures.

Many papers have been written that discuss alternate lead-free solder systems, and the emerging consensus is that, in terms of solder joint characteristics, Sn (Cu, Ag, Bi, etc.) and other solders are at least comparable to traditional lead containing alloys. Of these, Sn / Cu has seen most usage to date. Is this option becoming the de facto standard? Some of the main reasons for not pursuing the other alternatives are cost, limited compatibility with the current lead containing systems and metallic property issues (intermetallic alloy formation). More important, from a component perspective, are the higher peak temperatures required for soldering. In an ideal world, all pcb manufacturers would change their lead process to the same lead-free system and all components would be supplied with compatible terminations and the ability to survive the higher thermal stress reflow. But who will make the first move…? This question has been answered recently – some Japanese companies have announced their “green” product plan of reduced lead by the replacement of tin-lead with a lead-free solder 96Sn-2.5Ag-1Bi-0.5Cu as the soldering medium. Many large companies throughout the world have already converted to lead-free assembly based on SnAgCu semieutectic solder paste. These alloys will require an increase of peak reflow temperature to 240 – 260°C. Component suppliers will be required to meet this specification by March 2001 for the first, mainly Japanese, companies to introduce complete lead-free products on the market.

This paper focuses on these issues in relation to one component technology – surface mount tantalum capacitors with MnO2 and conductive polymer electrodes – and outlines a program that will verify whether these devices are ready to meet this specification.