We were just asked the SN62 question yesterday by a subcomponent supplier!
So, more than just you questioning the need.  Here's what we said:

        The 2% silver was added to reduce silver leaching from thick film
conductor pads.  Since hybrid manufacturing was our start and remains half
our business, it makes sense to have only one type of solder paste in the
building.  Avoids confusion, even though there is a cost factor.  There are
some other benefits as well.

Overall advantages:
1)  Reduce silver leaching from thickfilm conductor pads.
2)  Higher shear strength.
3)  Higher creep resistance.
4)  Better cosmetic appearance (shiny joint, especially if gold on solder
pad)
5)  Process considers SN62 the same as SN63.

Disadvantages:
1)  Higher cost.

Glenn

Reference:  "Soldering in Electronics," R.J. Klein Wassink


-----Original Message-----
From: Alan Kreplick [mailto:[log in to unmask]]
Sent: Wednesday, October 03, 2001 12:27 PM
To: [log in to unmask]
Subject: [TN] Sn62 vs. Sn63


Hello Technet:

We're currently using a water-soluble Sn62 solder paste, in which the
formulation is five plus years old.  We're beginning a solder paste
evaluation using the current solder paste, a newer formulation from that
manufacturer plus a half dozen other solder pastes from other leading paste
manufacturers.

Many of the paste manufacturers have asked why we're using Sn62, and we're
now asking ourselves the same question, "Why not make the switch to Sn63
during the paste evaluation?".

So, two questions:

   What are the major Pro's & Con's of Sn62 vs. Sn63 (mechanical,
   electrical, processing, etc.)?

   Anybody willing to share their solder paste evaluation/qualification
   (techniques, results, lessons learned, etc.)


As, always, thanks in advance for your responses.


Al Kreplick
Sr. Mfg. Eng.
Teradyne, Inc.
500 Riverpark Drive
North Reading, MA
Tel: 978-370-1726
Fax: 734-661-5352