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Using RoHS Components in a leaded paste world

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#71302

Using RoHS Components in a leaded paste world | 31 October, 2013

I work in a leaded paste world due to aerospace environmental requirements. There are a few cases where parts are only available from suppliers as RoHS. Can anyone recommend how to process these into a leaded assembly? Is it just reflowing at a higher temp, needing to pre tin component leads, or something else? The types of components in this scenario are QFPs and QFNs and are either on tape and reel already or waffle trays from suppliers.

Thanks!

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Pete B

#71303

Using RoHS Components in a leaded paste world | 31 October, 2013

You should have no difficulty with QFN or QFP parts when using non-RoHS solder. The QFP's are most likely to have tin plated leads and the metallisation on the QFN lands the same, or ENIG.

Check the component part datasheets to verify the plating and if they are as I have noted above then use your standard Tin/Lead process.

The only time you really need to make process changes is if you are using BGA's that may have RoHS compliant solder balls. In this case a 'hybrid' profile that ensures full mixing of the solder alloys should suffice. If you do this be sure to note the maximum process temperature tolerance of the other components on your assembly.

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Pete B

#71304

Using RoHS Components in a leaded paste world | 31 October, 2013

You should have no difficulty with QFN or QFP parts when using non-RoHS solder. The QFP's are most likely to have tin plated leads and the metallisation on the QFN lands the same, or ENIG.

Check the component part datasheets to verify the plating and if they are as I have noted above then use your standard Tin/Lead process.

If you have any RoHS compliant BGA's as well then it is a different scenario and there could be reliability implications. Sometimes a 'hybrid' profile is all that is needed to ensure you get full mixing of the solder alloys, but depends on your product reliability requirements in this case.

Hope this is helpful.

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#71308

Using RoHS Components in a leaded paste world | 31 October, 2013

Thanks Pete!

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#71311

Using RoHS Components in a leaded paste world | 31 October, 2013

Keep in mind that you may need to assure that there are no pure tin finishes remaining on your parts.

A general idea is that parts with pure tin (RoHs) finish will need to be plated with tin-lead solder to avoid the possibility of tin whiskers forming as time goes by. For most small passive parts this happens automatically, but larger parts may need to be retouched to assure that tin-lead plating of the leads is complete. If you use a .006" stencil foil, then generally parts that are .030" or less in solder termination height are considered "self mitigating". Parts with terminations taller than that will require touch up to eliminate pure tin finish. Speaking in generalities...of course. Your experience may vary.

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