Electrochemical migration and corrosion behavior of Sn-coated IC package substrate

Abstract

Chip on film (COF) is a packaging technology that is currently applied to the manufacturing process of liq-uid crystal display. The industrial trends toward the densification and miniaturization of electronic devices de-mands a steadily decreasing Cu lead spacing of IC package substrate. The electrical bias in the presence of moisture (or condensed water) can trigger an electrochemical phenomenon, called electrochemical migration (ECM) on printed circuit board (PCB) or IC package substrate. In ECM process, metal migrates as ions from anodic site to cathodic site where it is deposited in a form of dendrite. The insulating resistance (SIR) between the adjacent electrodes is significantly reduced by the growth of dendritic electrodeposit. Eventually a current leakage occurs, thereby leading to circuit failures of electronic components. Most research on the electrochemical migration of package substrate has been carried out under the DC bi-as condition. However, COF in the display package needs AC (square wave) power to provide the response time that is an important parameter in determining display image quality. Flat panel display shows a static image, and as the result, edge smearing in moving object is observed at low response time. Therefore, evaluation of the electrochemical migration on COF package for the display must be performed under AC bias condition. There are few publications on the issue of the electrochemical migration under AC bias (square wave voltage profile). Therefore, electrochemical migration behavior of COF package substrate under AC bias condition needs to be clarified in detail. The fine pitch IC substrate (COF) consists of Cu electrodes with a thickness of 8 $\mu$m on polyimide film with a thickness of 35 $\mu$m. The comb patterns were made using an anisotropic subtractive etching technology. The Cu comb patterns were coated with Sn of 0.4 $\mu$m thickness. The gap size between the electrodes on the fine pitch IC substrate was 15 $\mu$m with the electrode width of 10 $\mu$m in all cases. the ECM behavior of the fine pitch IC substrate was examined by a water drop test (WDT) in which 10 V square wave bias with 10 - 480 Hz frequency range were applied. The WDT and electrochemical tests were per-formed in a deionized water at room temperature.

Electrochemical migration behavior of a fine pitch IC substrate under square wave AC bias Effects of AC bias on the electrochemical migration response of a fine pitch IC substrate for a flat panel display were investigated by a water drop test and scanning electron microscope with energy dispersive spec-troscopy. Sn coated on Cu electrodes was found to be very effective in improving the resistance to ECM of fine pitch IC substrate under DC 3 V due to both the low corrosion rate of Sn and the low solubility product of Sn(OH)4. While the time to failure due to ECM (termed the ECM TTF in this work) for the fine pitch IC sub-strate is less than 1 s at a direct current (DC) bias of 10 V, it is approximately 330 s at an alternating current (AC) bias of 10 V at 10 Hz and approximately 3,940 s at 240 Hz. The ECM TTF increased in proportion to the fre-quency under AC 10 V bias.

Effects of frequency on the electrochemical migration and failure mechanism of a fine pitch IC substrate under square wave AC bias Effects of square wave frequency on the ECM TTF of the fine pitch IC substrate were examined with elu-cidating the failure mechanism of the Sn coated Cu in the fine pitch IC substrate under square AC bias, using the water drop test (WDT), SEM-EDS, XPS, and analytical analysis. The ECM TTF of the fine pitch IC sub-strate increased in proportion to the frequency. Dendrites of Sn or that of Cu was not formed between Sn coat-ed Cu electrodes. In contrast, a large amount of $Sn(OH)_4/SnO_2$ and $Cu_2O/Cu(OH)_2$ were precipitated between the electrodes under AC bias. The amount of the precipitates and the ratio of Cu to Sn in the precipitates also increased with testing time. The ECM failure mode under AC bias is an electric short failure caused by the flow-ing of leakage current resulting from the electrochemical reaction of exposed Cu, and also from a current flow through $Cu(OH)_x$ and $Sn(OH)_x$ precipitates with relatively low electrical resistance between electrodes. The re-gion in which the precipitates were formed depends on the applied frequency

at low frequency, the precipitates were formed in all region across electrodes, whereas, at high frequency, they formed in region near the surface of electrodes.

Effects of Cu-Sn intermetallic compound on the electrochemical migration of a fine pitch IC substrate un-der square wave AC bias Cu-Sn intermetallic compounds (IMC) were formed at an interface of the Sn coated Cu in the fine pitch IC substrate by annealing, and then the effects of the Cu-Sn IMC on the susceptibility to ECM TTF under square wave AC bias were explored using the water drop test, anodic polarization test, potential measurement, AES and SEM surface analysis. Cu-Sn IMC are formed by thermal annealing treatment during the process of COF manufacturing and dis-play module fabrication. The difference in the ECM TTF between Cu-Sn IMC formed electrode and Sn coated Cu electrode was not distinguished due to the high rates of nucleation and growth of dendrites under DC 10 V bias. Whereas, the ECM TTF of the Cu-Sn IMC formed Cu electrode (1030 s) was less than that of Sn coated Cu substrate (2370 s) under square wave 60 Hz AC 10 V bias due to the difference in corrosion current density for the two samples. The resistance of ECM depend on both the dissolution rate of electrodes and the for-mation of dendrites under DC bias, while that only depend on the dissolution rate of electrodes under square wave AC bias. Corrosion properties of Sn and Cu-Sn intermetallic compound in the electronics circuit system Electric corrosion behavior of surface finished materials (pure Sn, $Cu_6Sn_5$ and $Cu_3Sn$) on Cu plate and the difference in ECM between the surface finished materials were investigated on the base of electrochemical techniques, considering corrosion environment of electronics parts

small amount of electrolyte and aeration condition. In this environment, corrosion rate at 1 $V_{SCE}$ increase in the order of Sn, $Cu_6Sn_5$, $Cu_3Sn$. These are in good agreement with the results of their ECM TTF behavior under 2 V DC bias. In case of $Cu_3Sn$ on Cu plate, resistance of pseudo passive film increased with increasing anodic treatment time at 1 $V_{SCE}$. The pseudo passive film is composed of $SnO_2$, $Cu_2O$, $Cu(OH)_2$, as confirmed by XPS analysis. High concentration of $OH^-$ makes a stable barrier film on the surface due to the formation of metal hydroxide / metal oxide. Also, stability of barrier film is increased by the increase in ratio of Cu to Sn in the barrier film. Whereas, in pure Sn on Cu plate, local-ized corrosion occurred, confirming no barrier effect of the pseudo passive film.