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The effect of lead-free soldering on tantalum capacitors

AVX Corporation : 11 January, 2013  (Technical Article)
Legislation is being developed worldwide to reduce the lead content in many consumer electronic products. An historical paper from AVX has been identified which focuses on one component technology in particular - surface-mount tantalum capacitors with manganese dioxide and conductive polymer electrodes - and outlines a programme that will verify whether these devices are ready to meet the new legislation.
The effect of lead-free soldering on tantalum capacitors

Legislation developed world-wide to reduce the lead content in many consumer electronic products has been taken as an action to reduce environmental impact when such products are discarded.

Despite the fact that lead containing solders in electronic assemblies have accounted for only 0.49% of world lead consumption, the trend in legislation has been 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 is 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 the higher liquidus temperatures.of most lead-free solder systems.

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. 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 importantly, 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.

Some Japanese companies were the first to announce 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 semi-eutectic solder paste. These alloys require an increase of peak reflow temperature to 240 to 260C.

This historical paper from AVX focused 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. It summarised the current status of lead-free technology and AVX’s research to date and discusses the necessary development actions for component manufacturer. Sn (pure tin), SnAg and SnCu alloys were previously found to be the most serious candidates for replacement of the conventional tin-lead termination finish. However, pure tin was found to be the only one
available termination finish at the moment compatible with conventional tin-lead and new lead-free board mounting systems suitable for massive production of tantalum capacitors.

A wetting balance test, together with some experimental tests on tantalum capacitors proved that 240+/-5C would be the sufficient reflow peak temperature for lead-free SnAgCu /SnAgCuBi eutectic solder pastes. Industry peak temperature standard for the conventional tin-lead pastes is 220C. Reflow peak temperature should be increased to approximately 240C to reach the same wetting force and wetting time of the lead-free pastes. Higher reflow temperature further improve wetting characteristic; however higher component fall out may result.

Tantalum and ceramic capacitors indicate a better preparedness to higher peak temperature reflow then aluminum capacitor samples.

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