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BGA repair for mixed assemblies: arm yourself with knowledge of the material construction.

Just as the industry is becoming familiar with BGA assembly technology, another challenge in the form of Pb-free solders has presented itself. Companies are phasing in Pb-free technologies at different times and using different metallurgies, and industry will have to learn to work with mixed solder technologies. BGA devices are especially sensitive to mixed solders in the rework process.

Studies of BGAs assembled with mixed solder technologies show successes and some areas of concern. The biggest problem arises when attaching a BGA device with Pb-free interconnect balls using SnPb solder paste at reflow temperatures below 217[degrees]C. (1) Solder interconnects solidify in zones, and uneven stresses tend to develop in the SnPb rich areas. Early cracking has also been reported in PWB interconnects in SnPb rich areas.

Successful interconnects can be made with Pb-free BGA interconnects using SnPb paste, but only if complete mixing of the BGA solder interconnect and the SnPb paste occurs. Industry studies indicate that full mixing is required to prevent yield and reliability reduction. (2,3) Reflow profiles should be above the Pb-free solder alloy melting point to ensure full mixing of the metallurgy. As industry continues to work with Pb-free materials and more data are collected, the best approach is to not mix solder technologies.

Before starting the rework process, understand the BGA component solder ball composition and the PWB finish. Material certifications may be requested from the component and PWB manufacturers. Ensure that they identify the construction materials used in their products (solder alloy, coatings, etc.) If the BGA interconnect ball is the same material as the PWB solder, then proceed using a solder paste with the same alloy composition. If certifications are not available prior to reworking the device, several techniques can be used to identify the solder alloy.

PWB solder joints may be easily analyzed with x-ray fluorescence spectroscopy (XRF). Other methods such as optical emission spectroscopy (OES), energy-dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) can also help identify solder compositions. Time and budget contraints might be concerns with analytical tools like these.

A simple analysis technique is to place solder shavings on a controlled hot plate set to 195[degrees]C (verified with a calibrated thermometer). If the solder reflows, there is a good chance that the solder is SnPb eutectic and can be reworked with standard SnPb eutectic solder using a new device with SnPb solder interconnects. This method may be used for Pb-free solders by identifying the melting range (Table 1) and setting the hot plate to the melting point. However, this is more difficult for Pb-free solders as the melting temperatures are similar and have overlapping ranges.

After identifying the solder composition, rework the BGA using a solder paste alloy identical to the BGA interconnects ball and PWB coating.

If time and access to equipment are limiting factors, it is best to carefully rework the device. Ensure that the old solder is removed as thoroughly as possible from the PWB before proceeding with the reattach process. Match the solder paste with the BGA device interconnects solder alloy and proceed with rework. SnPb solders contaminated with Pb-free solders of <1%, reflowed at >225[degrees]C, should not create any reliability concerns.

References

1. David Hillman, et al, "The Impact of Reflowing a Pb-free Solder Alloy Using a Tin/Lead Solder Alloy Reflow Profile on Solder Joint Integrity," International Conference on Lead-Free Soldering, May 2005.

2. Fay Hua, et al, "Solder Joint Reliability Assessment of Sn-Ag-Cu BGA Components Attached with Eutectic Pb-Sn Solder," SMTA International Proceedings, September 2003.

3. Polina Snugovsky, et al, "Theory and Practice of Lead-free BGA Assembly Using Sn-Pb Solder," International Conference on Lead-Free Soldering, May 2005.

4. Ning-Cheng Lee, "The Relationship of Components, Alloys and Fluxes," CIRCUITS ASSEMBLY, October and November 2005.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.
Table 1. Pb-Free Solder Comparison (4)

Solder Type Composition Melting Range ([degrees]C)

SnAg eutectic SnAg3 221[degrees]C
 SnAg2 221[degrees] to 226[degrees]C
SnCu eutectic SnCu0.7 227[degrees]C
SnAgBi SnAg3.5Bi3 206[degrees] to 213[degrees]C
 SnBi7.5Ag2 207[degrees] to 212[degrees]C
SnAgCu eutectic SnAg3.8Cu0.7 217[degrees]C
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Title Annotation:Soldering Tips
Publication:Circuits Assembly
Date:May 1, 2006
Words:723
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