EMSNow - NEW WHITE PAPER - Voiding Behavior in Mixed Solder Alloy Systems

EMSNow - NEW WHITE PAPER - Voiding Behavior in Mixed Solder Alloy Systems:

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NEW WHITE PAPER - Voiding Behavior in Mixed Solder Alloy Systems

By Yan Liu, Derrick Herron, Joanna Keck, and Ning-Cheng Lee.

Jun 21, 2013

The voiding behavior of mixed solder alloy systems using a nitrogen reflow atmosphere during BGA assembly was investigated. 

The solder joints were comprised of a combination of Sn62/Pb36/Ag2 (Sn62), Sn63/Pb37 (Sn63), Sn98.5/Ag1/Cu0.5 (SAC105), and Sn96.5/Ag3/Cu0.5 (SAC305) as the solder ball or solder paste, except for the SnPb solder ball, which used lead-free solder pastes. Assembly of each BGA employed a high temperature (HT) profile for lead-free and mixed systems, and a low temperature (LT) profile for mixed and SnPb systems. 

Mixed systems had higher voiding than lead-free systems when reflowed at HT. SnPb systems had a wider voiding range when compared with mixed systems reflowed at LT. Voiding was found to decrease with reduced alloy surface tension; earlier melting of the bottom solder paste; a narrower pasty range; smaller joint overheating; when the top ball remained solid during reflow; and by using flux alone instead of solder paste.

Mixed solder paste patent from Nihon Handa

US20120018048A1: A cream solder obtained by kneading an Sn—Ag—Cu alloy together with a flux, wherein the Sn—Ag—Cu alloy includes a mixture of a first powdery alloy and a second powdery alloy, the first powdery alloy is represented by an Sn—Ag phase diagram having a solid-liquid coexistence region and has a given silver amount which is larger than that in the eutectic composition (3.5 wt. % silver), and the second powdery alloy has a silver amount which is that in the eutectic composition (3.5 wt. % silver) or which is close to that in the eutectic composition and is smaller than that in the first powdery alloy. This cream solder has excellent strength and thermal stability, and satisfactory bonding properties. It is based on an inexpensive Sn—Ag—Cu solder alloy. It is suitable for use as a high-temperature-side lead-free solder material conformable to temperature gradation bonding. Also provided is a method of soldering.

Nihon Handa Co., Ltd., Tokyo, Japan
Fuji Electric Holdings Co., Ltd., Kawasaki-shi, Japan

Published / Filed: 2012-01-26 / 2007-07-03

Application Number: US2007000307370

Mixed alloy paste patent Senju

espacenet: A solder paste using a Sn-Ag base, Sn-Cu base, or similar alloy powder has a high melting point, so it causes thermal damage to electronic devices. Sn-Ag-In base lead-free solder alloys having a low melting temperature have been studied, but they are difficult to use because they cause much occurrence of chips standing up during reflow. The present invention forms a solder paste by separating a Sn-Ag-In base lead-free solder into first and second solder alloy powders for which the difference in their peak temperatures measured by differential thermal analysis is at least 10 DEG C and blends the mixed powders with a flux.

Senju patent: TWI308510 (B) ― 2009-04-11

Material behavior of mixed formulation solder joints

Scopuss: "In this work, we have explored the physical properties and mechanical behavior of mixed formulation solder materials. Seven different mixture ratios of 63Sn-37Pb and SAC305 solder materials have been formed, which include five carefully controlled mixtures of the two solder alloys (by weight percentage) and the two extreme cases (pure Sn-Pb and pure SAC).

For the various percentage mixtures, the melting point, pasty range, stress-strain curves, mechanical properties (modulus, strength), and creep curves have been characterized. The variations of the mechanical properties and creep rates with aging at room temperature (25'C) and elevated temperature (100'C) have also been measured.

Finally, the microstructures realized with the various mixtures have been found and correlated to the mechanical measurements and microstructures found in actual mixed formulation BGA solder joints.

The results for the mechanical and physical properties show a very complicated dependence on the mixture ratio."

SnZnBi + SAC

Factiva: "The formation of intermetallic compounds and the shear strength of Sn-Zn-Bi solder alloys with various (0, 1, 3, 5 and 7 wt%) weight percentages of Sn-Ag-Cu were investigated on Au/Ni metallized Cu pads depending on the number of reflow cycles. In Sn-Zn-Bi solder joints, scallop-shaped AuZn3 intermetallic compound (IMC) particles were found at the interfaces and in the solder ball regions, fine Bi- and needle-shaped Zn-rich phase were observed in the Sn matrix,' investigators in Hong Kong, People's Republic of China report.

'After Sn-Ag-Cu additions, an additional Ag-Zn intermetallic compound layer was adhered to the top surface of the AuZn3 layer at the interface and fine spherical-shaped AgZn3 intermetallic compound particles were detected in the solder ball regions together with Bi- and Zn-rich phase volumes. After the addition of Sn-Ag-Cu, the shear strength of Sn-Zn-Bi solder joints increased due to the formation of the fine AgZn3 intermetallic compound particles,' wrote A.K. Gain and colleagues, City University of Hong Kong."

Mixed soldering SAC, SnZnAl

Factiva: "With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn-Ag-Cu (SAC) Pb free solders, as well as Sn-Zn-Al solder, melted together with eutectic Pb-Sn solder paste and also various SAC solder pastes, on a copper substrate,' investigators in Toronto, Canada report.

'It was observed that using eutectic Pb-Sn solder paste with an SAC solder ball introduced some Pb-Sn eutectic microstructure and changed the ternary eutectic present from Ag3Sn-Cu6Sn5-Sn to Ag3Sn-Pb-Sn. Use of an SAC solder paste with Sn-Zn-Al solder introduced an apparent Ag-Cu- Zn ternary compound, replacing Zn lamellae of the Sn-Zn eutectic. With eutectic Pb-Sn solder paste, the Pb-Sn-Zn ternary eutectic was formed,' wrote L. Snugovsky and colleagues, University of Toronto.

The researchers concluded: 'It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.'"

Combining Tin/Lead and Lead-free

SMT: "With the electronics industry movement to lead-free components, hybrid manufacturing may be a necessary solution if a non-RoHS assembly uses tin/lead solder and lead-free BGAs are being introduced to the assembly and process. Diligence and process controls are required to verify the acceptable case temperatures for all components on the BOM, reflow profile parameters, and appropriate alloy mixing. Additional testing and failure analysis (FA) is also recommended to verify the reliability of the design and overall assembly's technology."

Investigation of mixed SAC and SnPb solder balls under high speed ball shear and pull tests

Scopus : "This paper presents the results of microstructural analysis of mixed solder joints which are formed by attaching different alloys of solder balls and pastes on the OSP pad finish.

The effect of thermal aging on the strength and fracture energy of the mixed solder balls during high speed ball shear/pull tests was also investigated in the study. The test specimens were aged at 125' for durations of 200, 500 and 1000hours. The correlations between the solder ball fracture force/energy, failure modes and inter metallic compound (lMC) thickness of the mixed alloy solder joints are also established."

Microstructure, defects, and reliability of mixed Pb free / SnPb assemblies

Scopus: "The results of an intensive reliability study on Pb free BGA / SnPb solder assemblies as well as some lessons learnt dealing with mixed assembly production at Celestica are described in this paper. In the reliability study, four types of Pb free ball grid array components were assembled on test vehicles using the SnPb eutectic solder and typical SnPb reflow profiles with 205-220'C peak temperatures. Accelerated thermal cycling was conducted at 0 to 100'C The influence of the microstructure on Weibull plot parameters and failure mode will be shown. Interconnect defects such as non-uniform phase distribution, low melting structure accumulation, and void formation are discussed. Recommendations on mixed assembly and rework parameters are given"