Emerald | Soldering & Surface Mount Technology | Table of Contents

Growth behaviors of Sn whisker in RE-doped Sn-Zn-Ga solder

Fri, 21 Jun 2013 00:00:00 +0100

Abstract

Purpose - Investigation of the growth behavior and mechanism of Sn whiskers in RE-containing lead-free solderDesign/methodology/approach - Conducting aging test and microstructure analysisFindings - It is found that Sn whiskers showed a rapid growth from the surface of Pr-Sn intermetallics in the early exposure period, and then kept developing with the exposure time increased to about 60 days. The longest whisker reached to a length of about 100 µm. The whisker morphologies including whisker shape, section size and quantity were correspondingly changed during the growth.Research limitations/implications - Traditionally, Sn whisker growth has been occurred mainly on tin-based thin films such as on tin electroplating and Sn-Cu finish. Here we discovered that Sn whiskers grow directly on bulk Sn-Zn solders when adding rare earth. And the growth behavior and mechanism of Sn whisker were discussed. Practical implications - Tin whisker growth is a key reliability concern for electronic products. This research can give an enriched knowledge of Sn whisker growth. Originality/value - Sn whisker growth induced by RE Pr addition in Sn-Zn based solders is newly found. The growth behavior and mechanism were discussed.

Filling Analyses of Solder Paste in the Stencil Printing Process and Its Application to Process Design

literatinetwork@emeraldinsight.com (Jong-Bong Kim, Won-Sang Seo) — Fri, 21 Jun 2013 00:00:00 +0100

Abstract

Purpose - The purpose of this paper is to suggest an analysis methodology for stencil printing process and to obtain proper design parameters that guarantee the successful filling using suggested finite element analyses.Design/methodology/approach - Filling performance of solder paste in the stencil printing process is highly dependent on material properties such as viscosity and surface tension together with process parameters such as squeegee angle and squeegee speed. In order to investigate the effects of process parameters on the filling performance, the pressure built-up under the squeegee and the filling procedure of the solder paste into an aperture were analysed. Due to the limitations of the computational memory and time, the analysis domain was simplified. The pressure development under the squeegee was investigated for various values of squeegee angle and speed; then, the filling behaviour with the pressure boundary condition was analysed for only one aperture. Finally, the two analysis results were integrated to obtain the successful filling condition. In this analysis method, process parameters that guarantee filling performance were decided on.Findings - It was shown that higher squeezing pressure develops as the squeegee angle decreases and the squeegee speed increases. The filling performance, however, improves as the squeegee angle and the squeegee speed decrease. This is because the pressure duration time decreases as the squeegee speed increases.Originality/value - This study suggests a new design approach to obtain proper process design parameters for successful filling of solder paste in to an aperture. The direct analysis of filling with squeegee movement is impossible due to limitations of computer memory and computation time. To overcome these limitations, a two steps analysis approach is proposed and can be effectively applied in the design of stencil screen printing.

An alternative JEDEC test board design and analysis

literatinetwork@emeraldinsight.com (Fang Liu, Guang Meng, Junfeng Zhao) — Fri, 21 Jun 2013 00:00:00 +0100

Abstract

Purpose - The purpose of this paper is to propose an alternative test board design with only one loading condition and sufficiently large sample size, which is more suitable for the statistical package qualification. Except the board shape and size and package component layout, all other design strictly follows the JEDEC standard so that the board design can be easily implemented.Design/methodology/approach - A test board in round shape was introduced. First, drop tests were carried out. Then, the dye stain test and metallurgical analysis were performed in order to study the failure mechanism of lead-free solder joint under drop impact.Findings - The test results indicate that the combined effect of mechanical shock and PCB bending vibration is the root cause of solder joint failure under drop impact, and that the maximum peeling stress of the critical solder joint could be considered to be the dominant failure factor. On the other hand, the fracture of BGA lead-free solder joints occurs at intermetallic compound (IMC) interface near the package side, and failure mode is brittle fracture.Originality/value - These results are the same as those of JEDEC standard test board. Furthermore, the solder joint loading conditions in this design are simplified from six to one. The round test board can take the place of JEDEC standard test board to carry out drop test and conduct well solder joint life prediction and statistical analysis.

The behavior of solder pates in stencil printing with electropolishing process

Fri, 21 Jun 2013 00:00:00 +0100

Abstract

Purpose - The purpose of the present work is to study the effect of the electropolishing time of stencil manufacturing parameters and solder-mask definition methods of PCB pad design parameters on the performance of solder paste stencil printing process for the assembly of 01005 chip components. Design/methodology/approach - During the study, two types of stencils were manufactured for the evaluations: electroformed stencils and electropolished laser-cut stencils. The electroformed stencils were manufactured using the standard electroforming process and their use in the paste printing process was compared against the use of an electropolished laser-cut stencil. The electropolishing performance of the laser-cut stencil was evaluated twice at the following intervals: 100 s and 200 s. The performance of the laser-cut stencil was also evaluated without electropolishing. An optimized process was established after the polished stencil apertures of the laser-cut stencil were inspected. The performance evaluations were made by visually inspecting the quality of the post-surface finishing for the aperture wall and the quality of that post-surface finishing was further checked using a scanning electron microscope. A test board was used in a series of designed experiments to evaluate the solder paste printing process.Findings - The results demonstrated that the length of the electropolishing time had a significant effect on the small stencil’s aperture quality and the solder paste’s stencil printing performance. In this study, the most effective electropolishing time was 100 s for a stencil thickness of 0.08 mm. The deposited solder paste thickness was significantly better for the enhanced laser-cut stencil with electropolishing compared to the conventional electroformed stencils. In this printing-focused work, print paste thickness measurements were also found to vary across different solder-mask definition methods of printed circuit board pad designs with no change in the size of the stencil aperture. The highest paste value transfer consistently occurred with solder-mask-defined pads, when an electropolished laser-cut stencil was used.Originality/value - Due to important improvements in the quality of the electropolished laser-cut stencil, and based on the results of this experiment, the electropolished laser-cut stencil is strongly recommended for the solder paste printing of fine-pitch and miniature components, especially in comparison to the typical laser-cut stencil. The advantages of implementing a 01005 chip component mass production assembly process include excellent solder paste release, increased solder volume, good manufacture-ability, fast turnaround time, and greater cost saving opportunities.

Comparison study of SAC405 and SAC405+0.1%Al lead free solders

Fri, 21 Jun 2013 00:00:00 +0100

Abstract

Purpose - The effect of addition of a small amount of Al (0.1%) on the properties of lead-free solder type Sn-4Ag-0.5Cu (SAC 405) was studied.Design/methodology/approach - The soldering properties of wettability and spreadability on a Cu substrate were also studied, and the effect of Al on the growth of intermetallic compounds (IMCs) was observed. The shear strength of soldered joints was assessed. For comparison, soldering and strength tests were carried out on SAC 405 and SAC 405 + Al solders. Soldering was performed with an activated flux type ZnCl2-NH4Cl, with non-activated flux (rosin), and without flux in the air.Findings - Experimental results show that Al addition slightly reduces the wettability and spreadability of SAC 405 solder. Also, the shear strength is moderately reduced, dropping by 8 MPa on average. Differential scanning calorimetry (DSC) analysis showed that the melting point of SAC 405+0.1%Al solder was increased to 221°C.Originality/value - The positive effect of a small Al addition is due to the fact that it hinders the growth of IMCs formed on the contact surface with Cu substrate. The width of the transition zone of IMC was reduced by approximately 2 to 3 µm, depending on the soldering temperature.

Non article content issue 25.3

literatinetwork@emeraldinsight.com (John Ling) — Mon, 22 Apr 2013 00:00:00 +0100

Abstract

Not available.