Chaoran Yang

Impact of Ni Concentration on the Intermetallic Compound Formation and Brittle Fracture Strength of Sn-Cu-Ni (SCN) Lead-free Solder Joints

Abstract Cu 6 Sn 5 and Cu 3 Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu 6 Sn 5 and Cu 3 Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni) 6 Sn 5 /Cu 6 Sn 5 and Cu 3 Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni) 6 Sn 5 and Cu 3 Sn under thermal aging. The thicknesses of (Cu, Ni) 6 Sn 5 /Cu 6 Sn 5 and Cu 3 Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.

Effects of corner and edgebond epoxy adhesives on board level solder joint reliability of BGA mezzanine connectors

In this paper, the button shear tests of seven kinds of epoxy used for corner/edge bonding of BGA mezzanine connectors are performed firstly for determining adhesive strength between the epoxies and each relevant surface material. The effect of corner/edge bonding epoxy on the board level solder joint reliability of BGA connectors is also investigated including mechanical drop and 4-point bending tests. In addition, the pre-conditioning effect of thermal aging (1000 hours at 125°C) and accelerated temperature cycling (ATC, −40~125°C, 500 cycles) on the reliability of connectors with corner/edge bonding epoxy is discussed. The failure modes and fracture strengths of the button shear and boardlevel tests are cross-referenced for comparison and assessed for correlation. The results from the present study not only contribute to the characterization and selection of corner/edge bonding epoxies for BGA connectors, but also improve the understanding of the corresponding relationship between simple button shear and the more complex and costly boardlevel reliability tests.

Experimental investigation of the failure mechanism of Cu–Sn intermetallic compounds in SAC solder joints

Abstract A detailed experimental study on the fracture mechanism of Cu–Sn intermetallic compounds (IMCs) in the Pb-free solder was presented in this paper. The growth behaviors of the Cu 6 Sn 5 and Cu 3 Sn IMCs were inspected and the respective evolution pattern of their microstructures was investigated. Then, a detailed fractographic analysis on brittle fractured solder joints was conducted after the high speed ball pull test. The fracture locations in the Cu–Sn IMC layers during different periods of aging process were identified. The fracture modes of Cu 6 Sn 5 and Cu 3 Sn were determined as well. Afterwards, the fracture energies of different Cu–Sn IMC materials were directly compared using the Charpy impact test with a specially designed specimen. It was found that the grain boundary of Cu 3 Sn is the weakest link in the Cu–Sn IMC system. Finally, based on these three parts of study, a mechanism to explain the thermal degradation of Cu–Sn IMCs was proposed.

Investigation of lead-free BGA solder joint reliability under 4-point bending using PWB strain-rate analysis

Board level solder joint reliability was evaluated using a 4-point monotonic bending test on different BGA package assemblies, including a 51×51 mm 2400 CBGA and a 40×40 mm 1517 PBGA. The edgebond epoxy was applied between the package samples and solder-attached printed wiring boards (PWBs). During the bending test, the PWB strain, bending force and daisy-chain resistance were monitored in real time. The daisy-chain resistance and PWB strain-rate during the bending test were cross-referenced for detailed comparison. The test results indicated that the PWB strain-rate showed an obvious change at the moment of package failure. The PWB strain-rate can be a useful monitoring method for detecting the onset of the solder joint failure during the monotonic bending test.

Effect of interfacial strength between Cu 6 Sn 5 and Cu 3 Sn intermetallics on the brittle fracture failure of lead-free solder joints with OSP pad finish

The brittle failure mechanism of solder joints has gained more and more attention due to the introduction of lead-free solders and also the extensive use of different kinds of portable electronic products. The excessive growth of the IMC is blamed to increase the propensity of the brittle failure. Typically the thickness of the IMC layer is used to indicate the risk of failure of solder joints. Cu<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn are the most common IMCs found in Sn-Ag-Cu (SAC) lead-free solder joints with OSP pad finish. Previous studies revealed that the brittle failure mostly occurred in the IMC region. However, the respective role of Cu<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn in the solder joint brittle failure still remains unclear. In the present study, different amounts of Nickel (Ni) were doped in the Sn-Cu based solder. Its effects on the growth rate of (Cu, Ni)<inf>6</inf>Sn<inf>5</inf>/Cu<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn were characterized at first. Subsequently, the results of characterization were utilized to produce different growth rates of (Cu, Ni)<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn under thermal aging. The thicknesses of (Cu, Ni)<inf>6</inf>Sn<inf>5</inf>/Cu<inf>6</inf>Sn<inf>5</inf> and Cu<inf>3</inf>Sn after different aging hours were measured using SEM (with EDX and BSE). High speed ball pull/shear tests were performed. The correlation between IMC strength and IMC layer thicknesses was established. An investigation on the effect of Ni on the micro-hardness of solder is also included in this study.

Comparative study of PWB pad cratering subject to reflow soldering and thermal impact

Pad cratering is one of the dominant failure modes that occur in various board level reliability tests, especially under the dynamic loading. In recent years, the increasing propensity of pad cratering due to the implementation of the lead-free technology calls for the necessity of re-assessment of PWB qualification related to the pad cratering resistance. The objective of the present study is to establish a test method for evaluating the pad cratering resistance of PWB and studying the PWB degradation mechanism subject to different high temperature conditions. Three kinds of PWB with different resin systems were investigated, including one phenolic-cured system board and two dicy-cured system boards. Tg and Td of the selected PWBs were characterized using TMA and TGA. The detailed cold ball pull test procedure is presented and different pull jaws with/without standoff are evaluated in this paper. The effects of multiple reflows, thermal aging and temperature cycling on the pad cratering resistance of PWBs are discussed as well.

Development of innovative cold pin pull test method for solder pad crater evaluation

Hot pin pull test is one of the testing methods frequently used in the pad crater evaluation. Although it carries several merits comparing with the other testing methods, it still has certain drawbacks such as time consuming and the potential thermal impact on the PCB copper pad during the pin attachment process. In this paper, an innovative cold pin pull testing method is introduced. Using a set of fixtures designed and manufactured in advance, an array of pins can be attached to the pre-deposited solder balls at one time. Therefore, instead of attaching the pin one by one, the test panel with pin arrays can be prepared after only two times of reflow. Compared with conventional cold ball pull tests, this newly developed method can not only equally exhibit the pad crater characteristics of the PCB, but also reduce the testing variations, which could be proven by experimental data generated in the present study.

Assessment of solder pad cratering strength using cold pin pull test method with pre-fabricated pin arrays

Hot pin pull test is frequently used for the pad crater evaluation. Although it carries several merits, it still has certain drawbacks such as time consuming and causing potential thermal impact on the PCB during the pin attachment process. In this paper, an innovative cold pin pull test method with pre-fabricated pin arrays is introduced. A set of fixture is designed and manufactured in advance. An array of pins can be attached to the test board at one time. With this fixture, instead of attaching the pin one by one, the test panel with pin arrays can be prepared after only two times of reflow without severe thermal impact. Compared with the conventional cold ball pull tests, this newly developed method cannot only equally exhibit the pad crater characteristics of the PCB, but also reduces the scattering in testing results. These conclusions are well proven by the experimental data generated in the present study.

Comparison of thermal fatigue reliability of SnPb and SAC solders under various stress range conditions

Most accelerated temperature cycling (ATC) tests reported the observations that SnAgCu (SAC) solders have better thermal fatigue reliability than SnPb solders due to their lower creep strain rate. But material studies revealed that this is true only when stress level is below a certain level. When the stress level in the solder increases, eventually the creep strain rate of SAC solder may outgrow that of the SnPb solder. Therefore, lead-free soldering under thermal cyclic loading may lead to more reliability concern than people expected. Further investigations on ATC tests with various stress ranges are still in demand. In the present study, by making use of distance-from-the-neutral-point (DNP) effect, a simple but effective custom-designed dummy package with various solder joint spacing is designed and manufactured to evaluate the thermal fatigue reliability of SnPb and SAC solders under different stress levels. It was found that SnPb solder could outperform SAC solder under a higher stress level condition. For the lower stress level condition, although the ATC test is still in progress, based on the present preliminary result, it is reasonable to predict that SAC solder would be better than SnPb solder in this case. The finite element analysis using the same package configuration was also conducted. The computational creep strains in the solders agreed with the ATC test results. Further discussion on the thermal fatigue life prediction is also presented in this paper.

Brittle fracture of intermetallic compounds in SAC solder joints under high speed ball pull/pin pull and Charpy impact tests

A comprehensive experimental study on the brittle fracture of intermetallic compounds (IMCs) of a Pb-free solder was performed and presented in this paper. The solder joints were subject to thermal aging for accelerating the growth of IMCs. The brittle fracture of IMCs was investigated using high speed ball pull/pin pull and Charpy impact tests. In addition to qualitative analysis of failure mechanism, quantitative characterization of ball pull force and impact toughness was also performed. It was found that the decrease in the ball pull force and the impact toughness due to thermal aging could not be simply attributed to the growth of IMC thickness. In order to understand the whole scenario of failure mechanism, the variation in IMC composition and the shift of fracture location must be considered as well. From direct test data, it seems that Cu3Sn should be the weakest link in the solder joint. But in cases subject to limited thermal aging, the brittle fracture propagated along the interface between Cu3Sn and Cu6Sn5, instead of inside Cu3Sn. With detailed fractographic analyses, the number of stacks of Cu3Sn grains was considered the key factor for determining the fracture location.