Just as the industry was becoming familiar with BGA assembly technology, another challenge in the form of lead-free solders has presented itself. As companies are phasing-in lead-free technologies at different times and using different metallurgies, the electronics 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 report both some successes, but also some areas of concerns. The biggest problem arises when attaching a BGA device with lead-free interconnect balls using SnPb solder paste at reflow temperatures below 217ºC. [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 lead-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 lead-free solder alloy melting point to assure full mixing of the metallurgy. As industry continues to work with lead-free materials and more data is collected, the best approach is not to mix solder technologies when possible.

Before starting the rework process, it is necessary to 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 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 aid in identifying solder compositions. Time and budgetary contraints might be concerns with analytical tools like these.

A simple technique is to place solder shavings on a controlled hot plate set to 195ºC, verified with a calibrated thermometer, to see if it reflows. 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 other solders by identifying the melting range (See Table 6-1) and setting the hot plate to the melting point. However, this is more difficult for lead-free solders as the melting temperatures are close together and have overlapping ranges.

After identifying the solder compositions, rework the BGA using a solder paste alloy identical to the BGA interconnects ball and PWB coating.
In a case where 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 your solder paste with the BGA device interconnects solder alloy and proceed with the rework. SnPb solders contaminated with lead-free solders of <1%, reflowed at >225ºC, should not create any reliability concerns.

REFERENCES
[1.] D. Hillman, etal, Rockwell Collins Internal Report, "The Impact of Reflowing A Pbfree Solder Alloy Using A Tin/Lead Solder Alloy Reflow Profile On Solder Joint Integrity"
[2.] Fay Hual, etal, Intel Corporation, "Solder Joint Reliability Assessment of Sn-Ag-Cu BGA Components Attached with Eutectic Pb-Sn Solder"
[3.] P. Snugovsky, etal, Celestica, "Theory and Practice of Lead-Free BGA Assembly Using Sn-Pb Solder"
[4.] Dr. Ningcheng Lee, iNEMI, "The Relationship of Components, Alloys and Fluxes"