Solder joint reliability

The lead-free solders are mostly based on Sn-containing binary and ternary alloys. Among them, the Sn-Ag system is one of the earliest commercially available lead-free solders and has been recommended for general-purpose use as a substitute for Sn-Pb eutectic solder. Addition of nano-particles of second phase helps in improving thermo-mechanical properties such as melting temperature, mechanical strength, mechanical fatigue resistance, creep resistance and solder-joint reliability. 

Lau, J. H. (1997), Solder Joint Reliability of BGA, CSP, Flip Chip, and Fine Pitch SMT Assemblies, McGraw-Hill, New York. 

The FEM analysis has also shown that the stress of the first-level interconnection in the encapsulated flip chip bump is comparably same to that of the alumina package which has high hrst-level interconnection reliability. We evaluated the solder joint reliability of EGA and CSP by TCT. We confirmed that we could obtain approximately three to ten times longer fatigue life by this high TCE ceramic material package than that of an alumina material package 151. 

FIGURE 1.1.11 Solder joint reliability by TCT (—40 to 123°C) for wire bonded chip assembly type CSP made of the high TCE ceramic materials. 

We have developed new high TCE ceramic material of 13ppm/°C. This material and 11.5 ppm/° C material have almost same properties with the exception of TCE. By using the 13 ppm/°C material, the TCE mismatch between the substrate and the potting compound decreases, and the improvement of solder joint reliability is obtained as shown in Figure 1.1.11. 

Lau, J.H., ed. 1991. Solder Joint Reliability. Van Nostrand Reinhold, New York. 

Ball grid arrays (BGAs) are a new style of leadless SMT component with an areal array of solder joints. The reliability of these components has come nnder intensive stndy. Plastic BGAs are less susceptible to solder joint fatigue failures than ceramic BGAs becanse the laminate and plastic body match the CTE of the PCB much better. At this time, it seems that there will be size and power limitations to ensure solder joint reliability. 

A. Emerick, J. Ellerson, J. McCreary, R. Noreika, C. Woychik, and P. Viswanadham, Enhancement of TSOP Solder Joint Reliability Using Encapsulation, Proceedings of the 43d Electronic Components and Technology Conference, lEEE-CHMT, 1993, pp. 187-192. 

Although environmental impact has a large effect on interconnect reliability, there are some basic design variables that can have a positive or negative impact on solder joint reliability. These critical design variables and their impact on reliability are discussed in the following sections. 

FIGURE 58.11 Effect of board thickness on solder joint reliability of stacked lead-free extremely thin CSP (PS-etCSP). (Reprinted with permission from Amkor Technology, Inc/ [2004] lEEEECT )... 

FIGURE 58.16 Solder-mask defined (SMD) and non-solder mask defined (NSMD) pad construction. For optimum reliability, the pad size on the PWB should be kept within 80 to 100 percent of the wetted pad area on the package side. This allows the stresses to be vectored equally between both ends of the solder joint, and it the PWB pad is slightly smaller, it allows a slightly higher stand off height, which also tends to enhance solder joint reliability. 

To verify the impact of heatsink loading on package solder joint reliability, temperature cycling can be performed on packages with heatsinks and compared with control samples. However, there are two caveats ... 

Ball Pitch and Ball Size. Ball pitch is the center-to-center distance between two adjacent solder balls (see Rg. 58.25). In theory one could change the ball pitch and keep the same solder joint size. In practical applications though, decreases in ball pitch also require a reduction in solder ball diameter and pad size for physically accommodating the decrease in distance between adjacent balls. This decrease in ball size, with decreasing pitch, results in the formation of smaller balls with smaller stand-off between the package and the PWB. Smaller solder balls and decreased stand-off have been shown to decrease solder joint reliability (see Rg. 58.26). 

Ball Array (Full or Depopulated). Ball array is another variable that has an indirect impact on solder joint reliability.There is a coupled effect between the location of the edge of the packaged die, the solder ball array, and the state of stress in the solder joints. The silicon has a much lower CTE than plastic substrates. Thus the die can create a state of overconstraint, producing excessive stress in the solder joints under and near its edge.This overconstraint tends to be worst in joints near the edge of the die. 

Package Substrate Material. The choice of package substrate material can have a large impact on solder joint rehability. Some substrates require employing solder ball alternatives (such as the ceramic solder column carriers shown in Fig. 58.4) or even the elimination of a soldered solution due to interconnect reliability considerations. A wealth of information regarding ceramic and plastic substrates can be found in the literature. This section focuses on the impact of those materials on solder joint reliability. 

Rocky Shih, Sam Dai, Francois Billaut, Sue Teng, Mason Hu, Ken Hubbard, The Effect of Printed Circuit Board Thickness and Dual-sided Configuration on the Solder Joint Reliability of Area Array... 

Lei L. Mercado, Vijay Sarihan,Yifan Guo, and Andrew Mawer, Impact of Solder Pad Size on Solder Joint Reliability in Flip Chip PBGA Packages, IEEE Transactions On Advanced Packaging, Vo. 23, No. 3, August 2000, pp. 415-420. 

Shelgon Yee, Lodgers Chen, Justin Zeng, Roger Jay, Ternary Intermetallic Compound - A Real Threat To BGA Solder Joint Reliability, Journal ofSMT, 2004, Vol. 17, Issue 2, pp. 29-36. 

Zequn Mei, Mudasir Ahmad, Mason Hu,Gnyaneshwar Ramakrishna, Kirkendall Voids at Cu/Solder Interface and Their Effects on Solder Joint Reliability, Electronic Components and Technology Conference, 2005, pp. 415-420. 

COMPONENT-TO-PWB RELIABILITY ESTIMATING SOLDER JOINT RELIABILITY AND THE IMPACT OF LEAD-FREE SOLDERS... 

A myriad of experimental tools have been developed to assess solder joint reliability rapidly. These include ... 

The peak reflow temperatnre of lead-free solders is higher than that of tin-lead solders. Assembling lead-free components with Pb/Sn paste wonld involve limiting the peak reflow temperatnre to that of Pb/Sn components ( 220°C). Consequently, the lead-free solders may not fnlly melt during reflow, resulting in unpredictable long-term solder joint reliability. 

Pan, N., Henshall, G.A., Billaut, R, Dai, S., Strum, M. J., Lewis, R., Benedetto, E., and Rayner, J., An Acceleration Model for Sn-Ag-Cu Solder Joint Reliability under Various Thermal Cycle Conditions, SMTA International, 2005, pp. 876-883. 

Hua, Fay, Aspandiar, Raiyo, Rothman, Tim, Anderson, Cameron, Clemons, Greg, and Klier, Mimi, Solder Joint Reliability of Sn-Ag-Cu Bga Components Attached with Eutectic Pb-Sn Solder Paste, Journal ofSMT, Vol. 16, No. 1,2003, pp. 34-42. 

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