Christopher,
I forgot to mention one other exception, BGAs.
Mixing of unleaded BGA solder ball alloys with leaded solder paste alloys can be a reliability issue, and there are two distinct schools of thought on this. The first is that the alloys should not be mixed, as this can lead to lead-rich regions in the finished solder joint which are potentially weaker, and the second is that this alloy mixing is not an issue if the finished solder joint is completely homogeneous such that there are no lead-rich regions.
I personally prefer to solder BGAs without using solder paste, at least for BGAs with 63/37 solder balls. From many years of experience, I have learned that the paste only increases the solder volume about 2%, which has no real reliability advantage. Using solder paste for BGAs can introduce head-in-pillow defects (improper reflow profile), bridging (from imperfect printing), entrapped solder fines and voiding (poor paste handling practices), and other issues. However, YOU NEED TO QUALIFY YOUR PROCESS.
I also prefer to use only BGAs with 63/37 solder balls or columns and matching 63/37 solder paste for exempt product, rather than mix them and have to solder the PWB and the rest of the components at the higher temperature required only for the BGA alloy. This sometimes requires re-balling of the BGAs, but there is no impact on reliability of a re-balled BGA if done properly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Christopher Young
Sent: Wednesday, January 26, 2011 5:18 AM
To: [log in to unmask]
Subject: [TN] Lead Free / Lead policy
Hi, anyone know what the policy is regarding the use and protection
from failure of mixed lead and non lead components and soldering
process. Does anyone have a company philosophy or policy as I believe
this is a requirement for Fire and Avionics environment.
Thanks
chris
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