Hi Raye,

From the metallurgical point of view there is not much of the difference whether it's SAC BGAs with Sn-Pb or vice versa.

However!!! When one puts Sn-Pb BGA through a Pb-free profile,  long term reliability of the component itself might be jeopardized.
Vladimir

SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca

-----Original Message-----
From: "Rivera, Raye" <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Thu, 1 Mar 2012 12:11:12 
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>,
        "Rivera, Raye"
	<[log in to unmask]>
Subject: [TN] BGA Reverse Backwards Compatibility

Hello TechNet Gurus.

I have seen lots of discussion and studies on the reliability of using BGAs with SAC solder balls on an assembly with SnPb paste. Many people are interested in this topic because of backwards compatibility issues when SnPb BGAs aren't available. At the risk of oversimplifying a complex topic, I understand this can be successful using a backward compatibility thermal profile such as the one shown on Figure 8-20 of IPC-7095B to ensure complete melting of both types of solder. Of course we must take care that the profile does not exceed the temperature limitations of the SnPb paste.

Is anyone aware of any study done on the reverse situation? That is, SnPb BGAs used with SAC solder paste and a SAC thermal profile? I would think it would be at least as reliable as the above situation. We definitely get complete melting and the temperature profile is correct for the SAC solder paste chemistry so we are not as concerned about holding a narrow process window.

This is not just academic curiosity on my part. We have some prototype boards built with SnPb BGAs SAC paste, and SAC profile. They are not for sale to customers, but some of our engineers are of the opinion that any mixed process joint is suspect. I'd like to find some data on the topic.

I hope everyone is enjoying Apex.

Best regards,
Raye Rivera

QA Manager * Canoga Perkins
20600 Prairie Street * Chatsworth * CA 91311-6008
818-678-3872  * [log in to unmask]


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