Sami T. Nurmi

The effect of solder paste composition on the reliability of SnAgCu joints

Abstract As the electronics industry is moving towards lead-free manufacturing processes, more effort has been put into the reliability study of lead-free solder materials. Various tin–silver–copper-based solders have become widely accepted alternatives for tin–lead solders. In this study, we have tested three different SnAgCu solder compositions. The first consisted of a hypoeutectic 96.5Sn/3.0Ag/0.5Cu solder, the second of a eutectic 95.5Sn/3.8Ag/0.7Cu solder, and the third of a hypereutectic 95.5Sn/4.0Ag/0.5Cu solder. A eutectic SnPb solder was used as a reference. The test boards were temperature-cycled (−40 to +125 °C) until all samples failed. The results of the temperature cycling test were analyzed, and cross-section samples were made of the failed joints. Scanning electron and optical microscopy were employed to analyze the fracture behavior and microstructures of the solder joints. The reliability of lead-free solders and the effect of microstructures on joint reliability are discussed.

The influence of multiple reflow cycles on solder joint voids for lead‐free PBGAs

Lead‐free soldering is becoming a common practice in the electronics industry because of the growing general opposition to lead‐containing solders. The reliability of lead‐free solders has been studied a lot recently, but knowledge of it is still incomplete and many issues related to them are under heavy debate. This paper presents results from a study of the formation of voids with regard to the number of reflow cycles in three different kinds of solder joints: first the ones prepared with lead‐free solder paste and lead‐free plastic ball grid array (PBGA) components, next the ones prepared with lead‐free solder paste and tin‐lead‐silver PBGA components, and last the ones prepared with tin‐lead solder paste and tin‐lead‐silver PBGA components.

The effect of PCB surface finish on lead‐free solder joints

Purpose – To study the behaviour of voids in PBGA solder joints and their influence on the lifetime of lead‐free solder joints.Design/methodology/approach – The behaviour of voids was studied using micro via and land pad PWBs, PBGA components, and by measuring voids in the solder joints. The lifetimes of solder joints were tested using accelerated temperature tests.Findings – Number of factors affecting the solder joint lifetimes were found. The voids were discovered to have a significantly large influence on the solder joints.Practical implications – The findings can be used to achieve better soldering results, methods, and designs.Originality/value – In this paper, the effect and the behaviour of voids were studied profoundly. The findings can be valuable to researchers and process personnel.

Pull testing of lead-free QFP solder joints

Eutectic 63Sn/37Pb solder has been the dominant soldering material in the joints of electronic components for decades. The harmful impact of lead on environment and human health has, however, led to an international trend to ban Pb from all electronic products. Efforts have been made to find a viable lead-free solder for replacing tin-lead. The SnPb replacing lead-free solder should have good enough mechanical properties both in the as-soldered condition and in the service. Many studies have shown that SnAgCu solder is one of the most promising soldering materials. This paper concentrates on determining the best composition of SnAgCu solder paste to guarantee reliable joints between substrate and components. QFPs (quad flat packages) with Ni/Pd/Au plating were reflow soldered on NiAu plated circuit boards. The used lead-free solder compositions were 96.9Sn/2.7Ag/0.4Cu, 95.5Sn/3.8Ag/0.7Cu and 95.5Sn/4.0Ag/0.5Cu. Eutectic SnPb 63Sn/37Pb solder paste was used as a reference. The pull strength, microstructure and fracture surfaces of the joints after pull tests were examined in the as-reflowed condition and after 2500 thermal cycles.

Effect of PCB surface finish on creep properties of lead‐free solder joints

Purpose – To provide further knowledge of the effect of solder composition and PCB surface finish on the creep properties of lead‐free SnAgCu solder joints.Design/methodology/approach – Single‐overlap shear specimens were prepared for the creep testing. The test matrix included three different SnAgCu pastes with hypoeutectic, eutectic, and hypereutectic compositions. An Sn63Pb37 solder paste was used as a reference. The PCB finishes used were NiAu, organic solderability preservative (OSP) and immersion tin. The creep tests were performed at 85 and 105°C using a dead‐weight system.Findings – According to the results, the SnAgCu solder with eutectic or near‐eutectic composition is the safest choice when the creep behaviour of solder joints is considered. Of the three different PCB surface finishes, immersion tin is the most favourable choice for use with SnAgCu joints when creep is the predominant deformation mechanism in the joints. On the NiAu finish the creep properties of SnAgCu solder joints were signi...

Impact of component placement in solder joint reliability

Abstract Component placement in an electronic product is usually derived using manufacturability or electromagnetic effects as the defining factors. The effect of placement on component reliability is rarely studied. High integration level of modern products provides advantages in high speed electronics but can severely degrade the reliability of components, unless certain design rules are met. In this paper the relation between placement and the solder joint reliability of a BGA components is studied with computational methods in 3-D and verified with experimental tests. Finite element method is utilized to calculate the accumulation of plastic work in solder joints. Based on the failure criteria obtained in the process, simple design rules are extracted and presented.

The Effect of Multiple Reflow Times on Lead-Free Solder Joint Microstructure

As environmental issues are raising more interest and are becoming crucial factors in all parts of the world, more and more environmental-friendly electronics products are emerging. Usually this means the introduction of products with lead-free solders. However, the reliability of lead-free solders is still a serious concern despite the vast research done in this field. This paper will describe the interconnect reliability of three kinds of solder joints respectively prepared with lead-free solder paste and lead-free PBGA components, lead-free solder paste and tin-lead-silver PBGA components, and tin-lead solder paste and tin-lead-silver PBGA components. Lead-free and tin-lead solders were composed of eutectic tin-silver-copper and tin-lead, respectively. In addition, the study also presents the effect of multiple reflow times. The study focuses on the microstructures of different assemblies. The particular interest is on the assemblies soldered with lead-free solder paste and tin-lead-silver PBGA components, since the SnPbAg solder on the bumps of the PBGA components were exposed to the reflow profile meant for the lead-free SnAgCu solder. Thus, these SnPbAg solder bumps were in the molten state almost twice as long as the rest of the solders. This had a notable effect on the reliability of these solder joints as we will be showing later in this paper. The test boards were temperature-cycled for 2500 cycles between −40 and +125°C (a 30-minute cycle). PBGA solder joint failures were monitored with a real time monitoring system. Optical and scanning electron microscopy was used to inspect the broken solder joints and their microstructure. The results of tests indicate that the number of reflow times can significantly affect the lifetime of PBGA solder joints. The most notable changes can be seen in the solder joints made with tin-lead-silver PBGA components and tin-silver-copper solder paste soldered with a lead-free reflow profile. The general trend was that the reliability of the solder joints increased in proportion to the number of reflow times. Mainly two factors are believed to have the major effect on the reliability of PBGA solder joints, voids, and microstructural changes in solder.Copyright