Solderability and Reliability of Sintered Nano-Ag Bond Pads of Printed Re-Distribution Layer (RDL)

Re-distribution layer (RDL) is one key enabling technology for advance packaging. RDL is usually fabricated in wafer level by photolithography process. An alternative approach for implementing RDL by additive manufacturing (AM) method is proposed in this study. This allows RDL to be fabricated on singulation chip. Nano-silver (nano-Ag) ink is printed on the silicon chip to form routing traces and bond pads. However, the Ag pad may be consumed by solder quickly if the process is not properly controlled. This paper studied the effect of nano-Ag ink sintering condition on the solderability of Ag pad. The solder joint mechanical integrity was evaluated by solder ball shear test. High temperature storage test was also carried out to evaluate the solder joint reliability. Experiment results showed that Ag pad fabricated by AM is SMT compatible. High temperature storage did not cause early failure to the samples. There was not significant change in the Ag3Sn IMC layer thickness and mechanical strength. The finding of the present study will serve as a very useful reference for future practice of forming solder joints on sintered nano-Ag pads.

Enhanced Ball Shear Testing Configuration For Substrate LGA Sensor Devices

Semiconductor assembly mass production environment has means of testing and verifying bond consistency and reliability during wire bonding. Common bond integrity assessment is ball shear testing (BST). This test enables analysis of the strength between the bond pad and a ball bond. This paper presents significant procedure on how ball shear testing parameters should be treated during wirebond integrity check. Device complexity in terms of performing ball shear testing specifically on sensor dice has different output responses. Frequent shearing on die resulted as bond pads are elevated by 30 µm (microns). To address manufacturing in-process controls challenges, shearing tool position, dage settings, and optical scopes are taken into consideration. Also, a study was performed on the execution correctness in combination with proper dage parameters was explored to meet good ball shear test process capability and break modes.

Investigation of oxygen followed by argon plasma treatment on LED chip bond pad for wire bond application

Purpose – The purpose of this paper is to investigate the efficiencies of argon (Ar), oxygen (O2) and O2 followed by Ar (O2→Ar) plasma treatments in terms of contaminant removal and wire bond interfacial adhesion improvement. The aim of this study is to resolve the “lifted ball bond” issue, which is one of the critical reliability checkpoints for light emitting diodes (LEDs) in automotive applications. Design/methodology/approach – Ar, O2 and O2→Ar plasma treatments were applied to LED chip bond pad prior to wire bonding process with different treatment durations. Various surface characterization methods and contact angle measurement were then used to characterize the surface properties of these chip bond pads. To validate the improvements of Ar, O2 and O2→Ar plasma treatments to the wire bond interfacial adhesion, the chip bond pads were wire bonded and examined with a ball shear test. Moreover, the contact resistance of the wire bond interfaces was also measured by using four-point probe electrical measurements to complement the interfacial adhesion validation. Findings – Surface characterization results show that O2→Ar plasma treatment was able to remove the contaminant while maintaining relatively low oxygen impurity content on the bond pad surface after the treatment and was more effective as compared with the O2 and Ar plasma treatments. However, O2→Ar plasma treatment also simultaneously reduced high-polarity bonds on the chip bond pad, leading to a lower surface free energy than that with the O2 plasma treatment. Ball shear test and contact resistance results showed that wire bond interfacial adhesion improvement after the O2→Ar plasma treatment is lower than that with the O2 plasma treatment, although it has the highest efficiency in surface contaminant removal. Originality/value – To resolve “lifted ball bond” issue, optimization of plasma gas composition ratios and parameters for respective Ar and O2 plasma treatments has been widely reported in many literatures; however, the O2→Ar plasma treatment is still rarely focused. Moreover, the observation that wire bond interfacial adhesion improvement after O2→Ar plasma treatment is lower than that with the O2 plasma treatment although it has the highest efficiency in surface contaminant removal also has not been reported on similar studies elsewhere.

Effect of Fe content on the interfacial reliability of SnAgCu/Fe-Ni solder joint

Fe-Ni films with compositions of Fe-75Ni, Fe-50Ni, and Fe-30Ni were used as under bump metallization (UBM) to evaluate the interfacial reliability of SnAgCu/Fe-Ni solder joints through ball shear test, high temperature storage, as well as temperature cycling. The shear strength for Fe-75Ni, Fe-50Ni, and Fe-30Ni solder joints after reflow were 42.57, 53.94, 53.98 MPa respectively, which are all satisfied with the requirement of industrialization (>34.3 MPa ). High temperature storage was conducted at 150°C and 200°C respectively. It was found that higher Fe content in Fe-Ni layer had the ability to inhibit the mutual diffusion at interface region at 150°C, and the growth speed of intermetallic compound (IMC) decreased with the increase of Fe concentration. When stored at 200°C, the thickness of IMC would reach a limitation for all these three films after 4 days, and cracks occurred at the interface between IMC and Fe-Ni layer. Temperature cycling tests revealed that SnAgCu/Fe-50Ni solder joint had the lowest failure rate (less than 10%), which has the best interfacial reliability among three compositions.

Effect of Structural Design Parameters on Wafer Level CSP Ball Shear Strength and Their Influence on Accelerated Thermal Cycling Reliability

Ball shear testing is typically conducted in Wafer level chip scale package (WLCSP) fabrication to estimate the strength of the solder ball attachment. Generally, the solder ball shear strength is dependent on the solder ball size, pad size, solder/pad interface treatment, reflow temperature and time. Solder ball strength is also a function of ram speed and height at which the ball is sheared with respect to the wafer. Recent investigations suggest that ball shear test is being used as an indicator for board level reliability of assemblies. In current market lead time for launching a new product is very short. Unfortunately, it takes several weeks to qualify a new product by board level qualification process. If there is a methodology through which one can predict the board level performance by extrapolating the wafer level test, it will save great amount of resources in testing and millions of dollars worth of testing time. In the first part of this study, we conducted a wafer level ball shear test. A DOE was created for varying wafer level structural parameters like solder ball size and type. Ball shear tests and Accelerated thermal cycling have similar failure signatures of compression on inner side and tension on outer side. Thus, for specific cases there is a possibility of correlating the two failure methodologies based on their failure signatures. Strain rate for ball shear test was determined based on shear speed and solder pad diameter. Strain rate for accelerated thermal cycling was determined based on difference in CTE between board and package. In this paper, results from ball shear test and accelerated thermal cycling are compared to find correlations for specific cases. The correlations derived from this study are statistical and empirical.

Effect of Cu content on the mechanical reliability of Ni/Sn–3.5Ag system

Copper was supplied to Sn–3.5Ag by electroplating Cu/Ni double under-bump metallization (UBM), and the amount of Cu was controlled by varying the Cu UBM thickness. Supposed Cu contents in the solder were; 0.2, 0.5, and 1.0 wt%, respectively, and the solder joint microstructure was investigated after 1, 5, and 10 reflows. In the case of specimens with 0.2 and 1.0 wt% Cu, only one type of intermetallic compound (IMC) formed, either (Cu,Ni)6Sn5 or (Ni,Cu)3Sn4, while two types formed in specimen with 0.5 wt% Cu. No correlation could be found between the solder joint microstructure and the ball shear test. However, drop test results showed two opposite trends. The drop resistance of 0.2 and 1.0 wt% Cu specimens was quite good initially but degraded dramatically with multiple reflows, in contrast to that of the 0.5 wt% Cu specimen, which was very poor after one reflow but improved substantially later on. The former was ascribed to thickening of IMC during reflow, while the latter was related to (Ni,Cu)3Sn4 thickening beneath (Cu,Ni)6Sn5 and subsequent spalling of (Cu,Ni)6Sn5 from (Ni,Cu)3Sn4.

Effects of Tooling Tip Wear and Fixture Rigidity on Solder Ball Shear and Ball Pull Tests

The present study is aimed at evaluating the effect of ball shear tool wear and fixture rigidity on ball shear and ball pull tests respectively. In particular, the emphasis is placed on understanding the progressive failure mechanism during the ball shear test. The location of crack initiating is investigated on two kinds of shear tool with different wear features. In this paper, the experimental investigation is presented. Specimens with PBGA solder balls are fabricated and a series of ball shear and pull tests are conducted. In the shear test, the shear tool is stopped at a certain stage during test, and then the specimens are inspected by SEM. The failure modes and location of cracks are characterized. From the ball attachment strength and crack location of the ball shear test, no significant difference is found between the shear tools with different wear features. For investigating the effect of fixture rigidity on the ball pull test, two kinds of PBGA package with different sizes was fixed on the fixtures with and without gluing on a rigid plate. The failure modes and ball pull strength with different fixture rigidity were compared. The test results indicate that more brittle failures are found on the specimens without gluing on the rigid plate during the ball pull test, both on two kinds of package with different sizes. In addition, the data scattering of ball pull strength is large on the case without gluing on rigid plate.