Fine Pitch Solder Joint Research Articles

Temperature Dependence of Elastic–Plastic Properties of Fine-Pitch SAC 0307 Solder Joint Using Nanoindentation Approach

This paper highlights the advantages of using the nanoindentation approach over the conventional method for the mechanical characterization of components used in electronic packaging. The limitation of the conventional method has become more critical with the miniaturization of electronic packages, giving inadequate information regarding the mechanical properties of metallurgical interconnections. The load–displacement approach via nanoindentation was used in this study to determine the micromechanical properties of a fine-pitch solder joint subjected to aging for 1000 hours. This approach is more advantageous than tensile testing, as it focuses on the elastic behavior unlike that in conventional mechanical testing. The nanoindentation analysis results showed that the elastic–plastic behavior before failure can be assessed in a wide range of temperatures and thus help study the temperature dependence on the mechanical properties of fine-pitch solder joints. The characterization was done beyond the elastic range beforehand of conventional method. The modulus and hardness of the fine-pitch SAC 0307 solder joint decreased while its plastic and elastic behaviors became pronounced at higher aging temperatures. This implies that solder joints become weaker and less resilient with increasing temperature, at least for a duration of 1000 hours.

Stencil printing behavior of lead-free Sn-3Ag-0.5Cu solder paste for wafer level bumping for Sub-100 μm size solder bumps

Stencil printing for flip chip packaging using fine particle solder pastes is a low cost assembly solution with high throughput for fine pitch solder joint interconnects. The manufacturing challenges associated with both solder paste printing increases as electronic device size decreases due to trend of miniaturization in electronic components. Among multiple parameters, the two most important stencil printing parameters are squeegee pressure and printing speed. In this paper, the printing behavior of Pb free Sn-3Ag-0.5Cu solder paste with a particle size distribution of 2–12 μm for wafer level bumping using a stencil printing method (stencil opening dimension −30 μm) was evaluated by varying the printing speed and squeegee pressure to fabricate solder bumps with a sub 100 μm size. The optimal squeegee pressure and print speed for the defect free printing behavior and fairly uniform size distribution of reflowed paste were found to be 7 kgf and 20 mm/s, respectively. The average size of the reflowed printed paste decreased with the increasing squeegee pressure.

Creep and Mechanical Properties of Cu6Sn5 and (Cu,Ni)6Sn5 at Elevated Temperatures

Cu6Sn5 is the most common and important intermetallic compound (IMC) formed between Sn-based solders and Cu substrates during soldering. The Cu6Sn5 IMC exhibits significantly different thermomechanical properties from the solder alloys and the substrate. The progress of high-density three-dimensional (3D) electrical packaging technologies has led to increased operating temperatures, and interfacial Cu6Sn5 accounts for a larger volume fraction of the fine-pitch solder joints in these packages. Knowledge of creep and the mechanical behavior of Cu6Sn5 at elevated temperatures is therefore essential to understanding the deformation of a lead-free solder joint in service. In this work, the effects of temperature and Ni solubility on creep and mechanical properties of Cu6Sn5 were investigated using energy-dispersive x-ray spectroscopy and nanoindentation. The reduced modulus and hardness of Cu6Sn5 were found to decrease as temperature increased from 25A degrees C to 150A degrees C. The addition of Ni increased the reduced modulus and hardness of Cu6Sn5 and had different effects on the creep of Cu6Sn5 at room and elevated temperatures.

合金としてのウィスカ抑制Pbフリーはんだ

Electronic products with Sn-Pb solder are being now excluded due to the harmful effect of Pb on the environment. Generally, Sn-Ag-Cu solder is used in place of Sn-Pb solder. In the solder joint, acicular crystals called whiskers often grow. Whiskers often cause a short-circuit problem in a fine-pitch solder joint. In this study, several lead-free solders added with a small amount of various elements were prepared to investigate the effectiveness validity of added elements in restraining whisker growth. The elements effective in inhibiting whisker growth were identified.

Procedures for Metallographic Examination and Mechanical Strength Testing of Fine Pitch Surface Mount IC Package Solder Joints

The reduction in IC package lead pitches in surface mount solder assembly and the current high emphasis on quality and reliability of printed circuit assemblies have created a requirement for microanalysis of fine pitch solder joints in manufacturing situations. Of particular interest are metallographic analysis, detection of solder joint defects and mechanical strength testing of solder joints. Much has been published in the literature on the results of such evaluations in specific applications but little has been available on procedures for use in the microanalysis itself, particularly for fine pitch solder joints. Detailed procedures for fine pitch solder joint microanalysis, which the authors have verified down to 0.5 mm (0.02 in.) lead pitches, are presented. In particular, the authors present procedures for metallographic examination of tin‐lead and tin‐lead‐silver solder joints. In addition, test parameters are given for a repeatable technique of fine pitch solder joint mechanical strength testing that allows mechanical strength measurements to be obtained from almost every lead on a fine pitch surface mount IC package.

Case Studies in Quality and Reliability Analysis of Fine Pitch Solder Joints

Rapid feedback on the effect of changes in materials or assembly process parameters on solder joint quality is essential in process optimisation for fine pitch surface mount solder assembly. This is particularly the case where the demands of high volume production do not allow lengthy experimentation to be carried out on the production line. The greater precision and care required in analysis of fine pitch solder joints places a further constraint on the speed at which the analysis can be carried out. The techniques of microstructural analysis, solder joint mechanical strength testing and short duration environmental stress testing must be used to meet a rapid turnaround quality and reliability analysis requirement and to maximise the amount of information obtained from each stage of the analysis. Three case studies are detailed which demonstrate the use of these techniques in a high volume production context to provide rapid feedback on joint quality. The case studies have been selected for presentation not only to demonstrate the analysis techniques but also because they address issues which are of current interest due to the increased usage of fine pitch packages in production and the constraints on the use of solvents for cleaning of circuit boards. The studies are:

Effect of Gold on the Reliability of Fine Pitch Surface Mount Solder Joints

The effect of gold (Au) on the reliability of 0.65 mm pitch surface mount solder joints between plastic quad flat packs and Cu‐Ni‐Au FR‐4 printed circuit boards was investigated. Cu‐Ni‐Au is a desirable printed circuit board finish for multi‐chip modules or printed circuit boards that would otherwise require a selective Au finish, for example for edge connectors or wire bondable parts. However, Au is known to embrittle solder when it is present in sufficiently high concentrations, creating a concern that solder joint fatigue life in service will also be adversely affected. This paper reports the results of mechanical shock, mechanical vibration and thermal cycling testing of fine pitch solder joints containing varying amounts of Au. Tests were performed on as‐soldered joints and on joints that had been heat‐treated to evolve the microstructure towards equilibrium. The tests were designed to accelerate in‐service conditions in a typical industrial environment. Under these conditions, the Au concentrations tested did not promote solder joint failures. Microstructural characterisation of the distribution and morphology of the Au‐, Ni‐ and Cu‐Sn intermetallics in the joint before and after accelerated testing was also performed. On the basis of these observations it is recommended that the Au concentration in solder joints between plastic quad flat packs and Cu‐Ni‐Au FR‐4 printed circuit boards not exceed 3.0 wt.%.

Solder joint reliability of fine pitch surface mount technology assemblies

The reliability of fine-pitch solder joint interconnection is studied. It consists of two different fine-pitch surface mount components (SMC), namely, the Electronic Industry Associates of Japan (EIAJ) 160-Pin Quad Flat Pack (QFP), and the Joint Electronic Device Engineering Council (JEDEC) 132-Pin Plastic Quad Flat Pack (PQFP). They each have a lead-spacing (from lead-center to lead-center) of approximately 0.025 in. For comparison purposes the conventional JEDEC 68-Pin Plastic Leaded Chip Carrier (PLCC) and the 84-Pin PLCC have also been used in the same manufacturing process. Solder joint reliability of these four SMCs has been quantified for the mechanical shock, mechanical vibration, and thermal environmental stress factors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>