The Relationship of components, alloys and fluxes, part 1: call it a love triangle--or a Bermuda Triangle. Either way, the best alloy may be determined by the end-application.

Ed.: This column can be found in its entirety at www.pcdandm.com.

LEAD HAS BEEN an indispensable element in solders due to its high ductility, reasonable mechanical strength, low eutectic melting temperature with tin, low surface tension and low cost. Although the dust has not settled, a picture of this new relationship is emerging. Changes in alloys not only directly impact components and fluxes, but also the relationship between components and fluxes, which in turn impacts the alloys.

Alloys vs. Components

High temperature stability, Perhaps the biggest impact on components is the requirement of higher thermal stability. With mainstream solder for SMT changing from eutectic SnPb to higher melting temperature systems such as eutectic SnAgCu (217[degrees]C), eutectic SnAg (221[degrees]C) or eutectic SnCu (227[degrees]C), the soldering temperature inevitably has to be raised, with a consequent increase for component thermal stability. As reflected in IPC/JEDEC J-STD-020C, with an increase in melting temperature to 217[degrees]C from 183[degrees]C, the thermal stability requirement rises about 20[degrees]C. On the other hand, moisture sensitivity would drop one to three levels with 260[degrees]C reflow peak temperature, and new molding compounds would be desired to improve the performance--at additional packaging cost. (1)

1. Internal Solder Alloys

High reciting solders. Due to the process hierarchy consideration, the internal solder joints of a package often need to be high in melting temperature so that the subsequent board-level assembly will not cause remelt of those internal joints. Remelt of internal solder joints may cause die drifting, solder extrusion and damage on wire bonding. With SAC alloys having a melting temperature around 217[degrees]C being adopted as a mainstream solder for SMT assembly, and with components possibly reaching 260[degrees]C upon reflow, internal solder joints should have a solidus temperature above 260[degrees]C, preferably above 280[degrees]C.

Medium melting solders. For internal solder joint applications where remelting is not a concern, solder mechanical or thermal fatigue behavior becomes the focus. Frear et al. reported in 2001 that eutectic SnCu performed best in flip-chip applications. (2) Clech in 2004 analyzed some published data (FIGURE 1) expressing the correlations of characteristic life to cyclic shear strain range for bare chip assemblies for SnCu, SnPb and SAC assemblies by trendlines. (3)

[FIGURE 1 OMITTED]

Alpha particle emission, An alpha particle is a nuclear particle that contains two protons and two neutrons. The alpha particle strips electrons from atoms such as 28Si as it passes through the electron cloud of an adjacent atom, thus producing charge along their path, leaving a trail of electrons and holes. When enough electrons were knocked out of an IC and accumulated in a capacitor, it switched the capacitor from 0 to 1 or from 1 to 0 and resulted in a so-called "soft error."

2. BGA Solder Balls

Fragility. Similar to SMT assembly applications, SAC has also been adopted for BGA solder balls. The alloys employed include SAC305, SAC387, SAC396 and SAC405. However, unlike other SMT components, the fragility of BGA SAC solder joints appears to be considerably poorer than that of BGA SnPb joints for both NiAu and OSP surface finishes, particularly in the former case. This difference manifests especially in the drop test. BGA SAC joint fragility has been attributed to inconsistent plating quality of the copper pad, (4) Kirkendall void formation, (5) low solder ductility and intermetallics. (6)

Intermetallic plates, Ag in lead-free solders contributes to fragility of joints and to the formation of large intermetallic plates. The plates may grow across the entire solder joint, and may protrude out of the joint itself. The probability of AgSn plate formation is aggravated by slow cooling and thermal aging, and decreases with decreasing content of Ag.

3. Small Forms

With continuous miniaturization of electronics, discretes such as chip capacitors or resistors are also shifting toward 0402, 0201 or 01005. An immediate impact is the rapid increase in tombstoning rates. Tombstoning of SAC is affected by solder composition, and is dictated by the wetting at the onset of the paste melting stage. A maximal tombstoning rate is observed at the ternary eutectic composition Sn95.5Ag3.5Cu1. The tombstoning rate decreases with increasing deviation in Ag content from this composition.

Ed.: Part 2, a look at the relationship between alloys and fluxes and components and fluxes, will be published in December.

REFERENCES

(1.) Lead-Free Component Team, "Component Implications of Lead-Free Reflow Assembly," IPC/iNEMI Symposium on Lead-Free Electronics, September 2002.

(2.) D.R. Frear, J.W. Jang, J.K. Lin and C. Zhang, "Pb-Free Solders for Flip Chip Interconnects" JOM, June 2001, pp. 28-38.

(3.) J-R Clech, "Lead-Free and Mixed Assembly Solder Joint Reliability Trends," IPC Apex, February 2004.

(4.) C. Chiu, K. Zeng, R. Stierman, D. Edwards and K. And, "Effect of Thermal Aging on Board Level Drop Reliability for Pb-free BGA Packages" ECTC, June 2004, p. 1256.

(5.) D. Henderson, "On the question of SAC solder alloy--Cu pad solder joint fragility," Webcast on SAC Solder Joint Fragility, September 2004.

(6.) M. Date, T. Shoji, M. Fujiyoshi and K. Sato, "Pb-free Solder Ball with Higher Impact Reliability," Intel Pb-free Technology Forum, July 2005.

DR. NING-CHENG LEE is vice president of technology at Indium Corp. of America (indium.com); [e-mail].