Journal of SMT Article

A Comprehensive Analysis of the Thermal Fatigue Reliability of SnPb and Pb Free Plastic Ball Grid Arrays (PBGA) Using Backward and Forward Compatible Assembly Processes

Authors: Richard Coyle, Peter Read, Steven Kummerl, Debra F
Company: Alcatel-Lucent, Texas Instruments, and Flextronics
Date Published: 10/30/2008   Volume: 21-4

Abstract: High reliability electronic equipment producers are continuing to manufacture tin-lead (SnPb) electronic products using the European Union Pb-in-solder exemption to enable RoHS (restriction on certain hazardous substances) compliance. These companies face challenges from a component supply chain that is rapidly converting to Pb free offerings and has minimal fiscal motivation to continue to produce SnPb product. When a critical SnPb ball grid array (BGA) component becomes unavailable, one manufacturing option is to use the Pb-free version of the component with the SnPb assembly process. A significant number of industry studies have focused on various aspects of this so-called mixed alloy processing. However, implementation of mixed alloy or backward compatible processes has been limited because the long term attachment reliability of these solder joints has not yet been characterized thoroughly.

This paper presents the results of attachment reliability testing of backward compatible (SnPb paste/Pb free BGA) mixed alloy test boards assembled using parameters that produced both full and partial Pb mixing. The assembled test boards were thermally cycled from 0 °C to 100 °C in accordance with the IPC-9701 industry test guidelines using 10 and 30 minute dwell times. To enable a more comprehensive reliability comparison, additional test cells were incorporated using SnAgCu (SAC) Pb free assembly, SnPb/SnPb assembly, and SAC paste/SnPb BGA assembly (forward compatible).

Post-cycling failure analysis was conducted on representative test samples from each cell including optical metallography and scanning electron microscopy with backscattered electron imaging (BEI) and energy dispersive X-ray spectroscopy (SEM-EDS) to identify the failure mode and evaluate the microstructure. The accelerated temperature cycling (ATC) test data and failure analysis results are discussed in terms of the relationship to the initial and evolving microstructures and mechanical behavior that result from temperature cycling. The thermal fatigue reliability of the mixed alloy assemblies is discussed relative to that of the pure SAC and SnPb assemblies and the results indicate that mixed assemblies can provide acceptable reliability, even when mixing is not complete.

Key words: Pb-free, SnPb, microstructure, mixed alloy soldering, backward compatible process, forward compatible process.

Members download articles for free:

Not a member yet?

What else do you get when you join SMTA? Read about all of the benefits that go along with membership.

Notice: Sharing of articles is restricted to just your immediate work group. Downloaded papers should not be stored on an external network or shared on the internet.


SMTA Headquarters
6600 City West Parkway, Suite 300
Eden Prairie, MN 55344 USA

Phone +1 952.920.7682
Fax +1 952.926.1819