High-temperature lead-free solders with solderability

J. Smetana, Plated Through Hole Reliability With High Temperature Lead Free Soldering, The Board Authority (Technical Supplement to CircuiTree magazine), Vol 4, No. 1, April 2002, p 50-64... 

High-Temperature Lead-Free Solders with Dispersoids... 

HIGH-TEMPERATURE LEAD-FREE SOLDERS WITH DISPERSOIDS... 

PEEK is a semi-crystalline polymer, insoluble in all common solvents and can be used at temperatures of up to 300 °C. This is a particular benefit when it is used in applications which involve the use of lead-free solder. The polymer has excellent chemical and mechanical stability. Victrex has achieved success in the Asian market with PEEK, which has been selected by Shod, a Japanese capacitor and miniature rechargeable battery manufacturer and distributor, to replace PPS resin in all the resin mould cases of its PetitCap aluminium electrolytic capacitors. PEEK-HT (high temperature polymer), which maintains its physical and mechanical properties at temperatures 30 °C higher than standard grades, has been introduced by Victrex. 

TAB packages are usually naturally lead-free at the component level. Assembly to the apphcation board does require lead-free solder along with a requirement for the package to be able to withstand the high temperatures associated with lead-free processing. 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

Two issues have arisen regarding RoHS requirements for the flexible circuit materials flame-retardant molecules with bromine in adhesive resins, and heat resistance for high-temperature processing with lead-free soldering. Although the issue of bromine is not actually a part of the RoHS requirement, it has been linked to the general environmental issues of printed circuit materials and processes. 

If adhesiveless laminates are used as the major materials, high temperatures can be used with lead-free soldering. In the case of SMT assembling, the flexible circuit should be fixed on a carrier frame. This allows it to be processed by the same mounting and soldering equipment as used for rigid boards. 

For wave soldering, since the solder pot temperature is typically already quite high for the tin-lead solder, the increase needed for the lead-free solder is generally less as compared with reflow, and the dwell time with the solder pot is also generally very short (a few seconds). 

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