Xingjia Huang

Assessment on the effects of electroless nickel plating on the reliability of solder ball attachment to the bond pads of PBGA substrate

This paper presents an experimental study to assess the reliability of solder ball attachment to the bond pads of PBGA substrate with various plating schemes. The basic structure of under bump metallization is Cu/Ni/Au. Three different kinds of electroless plating solutions are used to deposit the Ni layer, namely, Ni-B, Ni-P (5%), and Ni-P (10%). Also, the conventional electrolytic Ni/Au plating is performed for benchmarking. After solder ball attachment, mechanical tests are conducted to characterize the ball shear strength for comparison. Furthermore, some specimens are subjected to multiple reflows to investigate the thermal aging effect. The testing results reveal that, compared to the electroless Ni-P plating, the electroless Ni-B plating may lead to better solder ball shear strength. However, the conventional electrolytic plating still gives the highest ball shear strength. For most cases under investigation, multiple reflows seem to slightly reduce the solder ball shear strength. In addition to the ball shear tests, scanning electron microscopy is performed to inspect the cross-section and the fracture surface of the tested specimens. From the failure analysis, certain characteristics are identified for various Ni plating schemes.

Experimental investigation on the progressive failure mechanism of solder balls during ball shear test

The present study is aimed at establishing the mechanics foundation of solder ball shear tests for evaluating the solder ball attachment strength of BGA packages. In particular, the emphasis is placed on understanding the progressive failure mechanism during the ball shear test. In this paper, an experimental investigation is presented. Specimens with BGA solder balls are fabricated and a series of ball shear tests is conducted. The shear ram is stopped at various stages during the ball shear test. The specimens are cross-sectioned for SEM inspection. The observed failure modes are characterized and correlated to the corresponding shear loading curves. The current experimental results can lead to a profound understanding in the failure mechanism of solder balls under mechanical shear loading. Furthermore, the outcome of the present study may provide a valuable database for the validation of computational modeling.

An in-process form error measurement system for precision machining

In-process form error measurement for precision machining is studied. Due to two key problems, opaque barrier and vibration, the study of in-process form error optical measurement for precision machining has been a hard topic and so far very few existing research works can be found. In this project, an in-process form error measurement device is proposed to deal with the two key problems. Based on our existing studies, a prototype system has been developed. It is the first one of the kind that overcomes the two key problems. The prototype is based on a single laser sensor design of 50 nm resolution together with two techniques, a damping technique and a moving average technique, proposed for use with the device. The proposed damping technique is able to improve vibration attenuation by up to 21 times compared to the case of natural attenuation. The proposed moving average technique is able to reduce errors by seven to ten times without distortion to the form profile results. The two proposed techniques are simple but they are especially useful for the proposed device. For a workpiece sample, the measurement result under coolant condition is only 2.5% larger compared with the one under no coolant condition. For a certified Wyko test sample, the overall system measurement error can be as low as 0.3 µm. The measurement repeatability error can be as low as 2.2%. The experimental results give confidence in using the proposed in-process form error measurement device. For better results, further improvement in design and tests are necessary.

A Study of Key Optical Profiler Parameters for Form Error Measurement

Optical profiler is a typical modern device for precision form error measurement. In our use of the equipment, we found that the surface form profile after stitching is ambiguous if the lens magnification is changed. The error in terms of PV value can be up to 3200% when the lens magnification changes from 2.5 times to 30 times. This has been confirmed by the equipment supplier. It is worthwhile to offer a comprehensive study as many users may use the instrument of the kind straightforward without detailed investigation of the performance. We found that, among the 6 key parameters studied, 3 independent parameters are more important. For the 3 independent parameters, we recommend to set the magnification A=2.5 times, the resolution r=0.5, and the overlap p=20%. With the recommended settings, the measurement error can be less than 0.5%. Backscan and length for scanning in the vertical direction, and cut off frequency for delivering form profile are less critical compared with the three independent parameters.

Interlaminar fracture behaviour of re-formed bamboo/aluminium sheet laminates

Sandwich laminates containing re-formed bamboo core and aluminium face sheets were produced using two different types of adhesive: an epoxy and a modified polyethylene. The interlaminar fracture behaviour of the laminates was characterized based on peel and lap-shear tests. It was shown that the laminates bonded with polyethylene had much higher peel and shear strengths than those bonded with epoxy. For the polyethylene-bonded laminates, the major failure mechanisms were a combination of cohesive and interfacial failure, whereas for the epoxy-bonded laminates, the fracture occurred almost exclusively along the aluminium/epoxy interface. There was a significant dependence of the failure mechanism and interlaminar strength on the loading direction relative to the bamboo fibre axis and on whether the aluminium sheets were bonded to the inner or outer bamboo surface.

Experimental characterizations for the shear strength of Pb-free solder balls on a PZT substrate with Pd/Ag pads

Two lead-free solder alloys, 95.5Sn-3.8Ag-0.7Cu and 91.8Sn-3.4Ag-4.8Bi, and the conventional 63Sn-37Pb eutectic solder were reflowed on a PZT ceramic substrate with Pd/Ag solder pad metallization. The specimens were subjected to thermal aging for 1000 hours at temperatures of 85/spl deg/C, 125/spl deg/C and 150/spl deg/C, respectively. Ball shear tests were performed to evaluate the effects of thermal aging on the various solders. In general, experimental data indicated that the shear strength of the solder balls gradually decreased with respect to increase in thermal aging temperature. Also, the longer the thermal aging time, the lower the ball shear strength. The shear strength of 95.5Sn-3.8Ag-0.7Cu solder balls was found to be the least sensitive to thermal aging. After thermal aging for 250 hours, with a few exceptions, the predominant failure mode in the ball shear tests appeared to be fracture at the interface between the Pd/Ag metallization and the PZT substrate. After thermal aging for 1000 hours, all specimens failed at the aforementioned interface. Using high magnification SEM inspection, a mixed-mode failure was observed. Further EDX analysis of the fracture surface confirmed the identified failure mode.