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Lead-Free Processing


Lead-Free Processing | 20 July, 2000

Bob: Please help us understand the effect of changing to lead-free solder paste on our processes better. Compare the following for tin-lead and lead-free solders: * Size of stencil apertures � amount of solder necessary for a reliable solder connection. * Reflow profile and the effect of this on boards and components.

Many lead-free solder formulations are proprietary. What are thoughts on near to long term procurement issues surrounding proprietary lead-free solder formulations?


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Bob Willis


Re: Lead-Free Processing | 20 July, 2000

On all the trials I have done I have made no changes to the stencil thickness or apertures than the normal changes we make for our process. I have not changed any thing in terms on volume of solder for more reliable joints. I fact I have not read any thing to suggest this is necessary. You do seem to get more bulbus joints with wave soldering with tin/silver and tin/copper probally due to the poor drainage.

The basic profile is changed to achieve reflow by changing the last few zones. Boards dotend to sag a little more, more on wave than reflow. That is one of the reasons I like to see people reconsider the use of centre board supports rather than expensive pallets which I am not a fan.

Some General Observations on lead free

Although liquidus temperatures are higher, peak reflow temperatures are not significantly higher than with conventional Sn-Pb Eutectic alloy.

Sn-Pb Liquidus 183�C / Reflow ~215�C (~33�C above liquidus) Pb-free Liquidus 217�C / Reflow ~238�C (~20�C above liquidus)

Extending Liquidus Time better results Flux residues appear higher Flux may be more active Nitrogen could be used to reduce flux activity / residues Cleaning may be considered

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