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February 2010

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Subject:
From:
Nigel Burtt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Nigel Burtt <[log in to unmask]>
Date:
Tue, 23 Feb 2010 03:35:41 -0600
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Whilst I was with Dolby we actually moved to ImmAg from ENIG as a finish of 
preference for SMT/mixed technology when swapping over to RoHS compliant 
assembly = lead-free soldering (using SAC305 for SMT paste, SAC387 for 
wave/selective, SAC305 hand solder.) I was encouraged to try this having 
seen the Hillman/Pauls lead-free pony show at a SMART Group conference in 
the UK back in Nov 2004.

With good PCB suppliers (able to meet and maintain IPC4553 thicknesses 
reliably) we had far fewer random or batch dependent solderability issues with 
ImmAg than ENIG. Also we saw an improvement in solder joint quality at wave 
solder (fewer bridges, better topside hole pull-thro'.) You just have to be a bit 
more careful with storage and keep sources of sulphur away. 

There was an increase in blowholes at wave solder, however, as the 
underlying nickel in ENIG seals the hole walls against micropores in the copper 
PTH, which the thin immersion Ag can't achieve. And concurrently the more 
robust PCB laminates used for lead-free are harder to drill and more prone to 
hole tear-out which can make hole wall problems more prevalent. Again, we 
worked with our supplier to understand and control these issues.

We did have one supplier that routinely gave us ImmAg problems, but they 
were also responsible for most of the occasional ENIG solderability problems 
too which caused us to look at ImmAg in the first place. We stopped using 
them...

I'm not a metallurgist but creating a solder joint with Ag over Cu on the PCB 
seems more logical using a Sn/Ag/Cu based solder alloy than 
introducing "foreign" elements Au and Ni into the mix. 


Nigel Burtt
[log in to unmask]
www.enjaybee.com

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