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July 2007

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Subject:
From:
Leo Higgins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Leo Higgins <[log in to unmask]>
Date:
Wed, 18 Jul 2007 02:45:55 -0700
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Hi folks,
     In the different world of IC assy and pkgg, use of imm Sn on BGA pads is not uncommon.  After eutectic Sn-Pb flip chip reflow in Nitrogen with low Oxygen (about 20ppm ), followed by several thermal processes at between 125 - 165C for substrate baking and underfill curing in air, PbFree SAC305 ball attach is performed with excellent results.  Imm Sn is about 1 um thick.  

Thanks.



Best regards,
Leo

          
Leo M. Higgins III, Ph.D.
Sr. Vice President, Engg and Technology
ASAT, Inc. 
13809 Research Blvd., Suite 635
Austin, Texas     78750

office phone   512-249-4758
mobile           512-423-2002
[log in to unmask]
www.asat.com



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stadem, Richard D.
Sent: Tuesday, July 17, 2007 3:45 PM
To: [log in to unmask]
Subject: Re: [TN] Immersion Tin


Larry, Reuven
Any thickness of tin is going to oxidize somewhat after going through
two reflow passes. Even lead-free components that are baked out
overnight at 150 C show a marked darkening on the tin-plated
terminations, even with 90 uinches of tin or more. Circuit boards with
immersion tin finish are also susceptible to oxidation.

The nitrogen exposure is only in the reflow portion of the tunnel. But
it helps a little, as that is the hottest.

However, any time you heat tin above 100 deg. C or so, it oxidizes
somewhat quickly. That is one of the reasons (just one) that immersion
tin is not always a preferred finish. Is there a particular reason why
you wanted to use it?

You may wish to try immersion silver instead, but without looking at
your assembly configuration and its intended use it is hard to recommend
a finish.
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
Sent: Tuesday, July 17, 2007 3:14 PM
To: [log in to unmask]
Subject: Re: [TN] Immersion Tin

The thickness of the Tin finish is below the minimum.

Short term solution:
Lower peak profile as possible.
Use Nitrogen

Long term
Replace supplier of PCBs
Corrective actions of existing supplier 

Best Regards
Reuven ROKAH 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Larry K
Sent: Tuesday, July 17, 2007 10:35 PM
To: [log in to unmask]
Subject: [TN] Immersion Tin

We are having problems with solderability of the PCB on oursecond side
reflow. Everything looks good after reflowon the first side but when we
run the second side we have non-wetting of the pads of this side. 
   
  Just to ensure we were doing everything correctly,we switched sides
(we did the second side first and then did the first side as a second
reflow) and the problem was still after the second reflow. We are
thinking this validated our process because the side causing us problems
switched with our experiment. As I said in the suject line, this is
Immersion Tin. Any suggestions for a cure?
  thanks,
  Larry

 
---------------------------------
 Get your own web address.
 Have a HUGE year through Yahoo! Small Business.

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