Grain refinement of solder materials by minor element addition

C. K. Chung, J. J. Yu, T. L. Yang, C. R. Kao

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

5 Scopus citations

Abstract

Owing to environmental concern, SnAg and SnAgCu alloys have been widely applied to electronic packaging industry since the RoHS announcement. However, there are still some reliability issues waiting for systematic research to discover promising ways to alleviate them. For instance, electromigration- resistance issue of advance packaging becomes more critical due to increased current density of smaller solder joint. It has been characterized that different dominating diffusion flux would cause distinct failure model of electromigration (EM). When the interstitial diffusion flux of Cu/Ni in Sn crystals dominates, intermetallic compounds (IMCs) and metallization layer dissolutions will be the main model of EM failure. On the other hand, solder joints tend to degrade by pancake void formation-and-propagation model when the Sn self-diffusion flux dominates under current stressing. Recently, tin's anisotropic properties have been widely reported. Especially, some researches proved that the interstitial diffusion of Cu/Ni along the c-axis of tin grains is much faster than along the a-axis and b-axis. This phenomenon will lead to serious EM degradation via IMCs and metallization layer dissolutions if the electron flow parallels to the c-axis of Sn grain. Unfortunately, there are only few Sn grains or even one grain existing in a single joint of Sn-rich solder alloys. Consequently, the probability of occurring c-axis effect causing EM-failure mentioned above would be increased much greater. The objective of this work is to refine the microstructure of solder joints. With finer tin grains and more crystal-orientations, EM-resistance might be enhanced theoretically. Several researches about reducing undercooling or improving mechanical reliabilities of Sn rich solders with minor additions of Ti, Mn, Zn, respectively, have been published. Nevertheless, the effect of grain refinement via minor addition has not been clearly understood yet. Hence, the microstructures of the Sn2.4Ag solder alloys on Cu pads without/with minor additions of Ti, Mn, and Zn, respectively, were investigated by Optical Microscopy (OM) cross-polarized images meticulously including as reflowed and thermal aging under 150°C conditions in our work. Interfacial reactions between these solder alloys and Cu pads were also observed by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) to characterize the IMC growths of solder joints with minor additions systematically. According to our experimental results, Ti addition is able to refine the microstructure effectively. We assume that Ti addition might provide extra sites which enhance the nucleation upon solidification. After the critical nuclei form, the pre-existing Ti-Sn IMC may pin the grain boundaries. As a result, grain growth after nucleation could be suppressed. However, Zn and Mn additions seem not to influence the microstructures of Tin. After 1000 hours prolong aging, the growths of the Cu6Sn5 are similar with/without Ti and Mn additions but the Cu3Sn could be suppressed slightly by the minor additions of Ti and Mn. On the other hand, Zn addition can suppress the interfacial IMCs including Cu6Sn5 and Cu3Sn drastically.

Original languageEnglish
Title of host publication2013 8th International Microsystems, Packaging, Assembly and Circuits Technology Conference - Green and Cloud
Subtitle of host publicationCreating Value and Toward Eco-Life, IMPACT 2013 - Proceedings
Pages124-127
Number of pages4
DOIs
StatePublished - 2013
Event2013 8th International Microsystems, Packaging, Assembly and Circuits Technology Conference - Green and Cloud: Creating Value and Toward Eco-Life, IMPACT 2013 - Taipei, Taiwan
Duration: 22 Oct 201325 Oct 2013

Publication series

NameProceedings of Technical Papers - International Microsystems, Packaging, Assembly, and Circuits Technology Conference, IMPACT
ISSN (Print)2150-5934
ISSN (Electronic)2150-5942

Conference

Conference2013 8th International Microsystems, Packaging, Assembly and Circuits Technology Conference - Green and Cloud: Creating Value and Toward Eco-Life, IMPACT 2013
CountryTaiwan
CityTaipei
Period22/10/1325/10/13

Keywords

  • electromigration
  • grain refinement
  • minor addition

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