Long enough to provide some contact with the solder tip or wave so as to provide good thermal transfer into the work piece. Otherwise, the heat energy must pass first into the PWB then via convection (through the flux or solder itself) to the work piece. 
We used a DFM guide of .020" or about .5 mm as a minimum.

Not so long that there is danger of the lead bending over to contact adjacent leads, or so long as to present a safety hazard, or so long as to pierce and render useless ESD protective packaging. IPC suggests 1.5 mm for leads that can safely be trimmed. 

Guy

--- [log in to unmask] wrote:

From:         Ken Barton <[log in to unmask]>
To:           [log in to unmask]
Subject: [TN] LEAD LENGTH
Date:         Wed, 2 Jun 2010 12:15:30 -0700

Hi Folks

 

What is the preferred lead length protrusion on the secondary side for
either wave or hand solder?

 

Thanks,

 

Ken

 

Ken Barton

Luma Technologies

(425) 643-4000 x304

[log in to unmask]

Visit us at www.lumatech.com

 



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