A review : On the development of low melting temperature Pb-free solders

The objective of this review is to study the interfacial intermatallic compounds (IMCs) between Sn–Ag–Cu based solders and common substrates, which play a crucial role in solder joints typically present in Pb-free electronics manufacturing. The microstructural evolution of IMCs at the solder/substrate interfaces is analyzed, while the models and theories describing the formation/growth mechanism of interfacial IMCs are also introduced. We focus on the influence of a variety of factors that have been reported recently, including substrates, minor alloying, mechanical stress, electromigration and thermomigration etc., as full understanding of the mechanisms that determine the formation and growth of interfacial IMCs is important to reach for developing high reliability solder joints. In the end of this review, the characteristics of the IMCs are compared and illustrated, which have marked effect on the mechanical properties and fracture behavior as well as reliability of solder joints.

A review on the interfacial intermetallic compounds between Sn–Ag–Cu based solders and substrates

Size effect model on kinetics of interfacial reaction between Sn-xAg-yCu solders and Cu substrate

/pdf/size-effect-model-on-kinetics-of-interfacial-reaction-cesqr2f4zw.pdf

Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate

The effects of solder volume and Cu concentration on the consumption rate of Cu pad during reflow soldering

The focus of this study is on the role of Cu content in the dissolution kinetics of Cu in high-Sn solders during the solid/liquid reaction accompanied by interfacial intermetallic compound formation. Small additions of Cu (0.7%, 1.5%) in high-Sn solders dramatically decrease the dissolution rate of Cu at low temperatures. Sn-3.5Ag, as expected, has a dissolution rate similar to that of pure Sn. The difference in dissolution rate of Cu in various molten solders is explained in terms of the solubility limit of Cu in molten solders based on the Cu-Sn phase diagram. The correlation between the metallurgical aspects of interfacial η(Cu6Sn5) phase formation and dissolution kinetics of Cu in molten solders leads to an understanding of the mechanism that controls the dissolution rate of Cu in molten solders.

Role of Cu in dissolution kinetics of Cu metallization in molten Sn-based solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni3Sn4, SnNiP, and Ni3P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)6Sn5 and Ni3P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni3Sn4 IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)6Sn5 IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)6Sn5 phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.

Morphology and Growth Kinetics of Intermetallic Compounds in Solid-State Interfacial Reaction of Electroless Ni-P with Sn-Based Lead-Free Solders

Innovative technologies for the realization of packages and System-in-Packages (SIP) with embedded components will be described Embedding of semiconductor chips into substrates has several advantages At first it allows a very high degree of miniaturization, since multiple layers of embedded components can be sequentially stacked A further advantage is the beneficial electrical performance by short and geometrically well controlled interconnects Furthermore the embedding gives a homogeneous mechanical environment of the chips, resulting in good reliability The development of different technologies for component embedding will be described One technology, called "Chip in Polymer", uses thin chips which are die bonded and embedded by lamination of Resin Coated Copper (RCC) layers The electrical contacts are made by laser drilling and Cu metallization An other approach, which is mainly of interest for flexible substrates, uses flip chips with very thin interconnect The chips are soldered or glued to flex substrates, followed by embedding in an adhesive layer Both technologies result in chips which are fully integrated in a flat substrate, either rigid or flex In both cases further layers an be applied or other components can be conventionally assembled on top

Strategies for Embedding of Active Components

Stretchable electronics can be realized using different manufacturing methods and hybrids thereof. An example of the latter is the combination of stretchable circuit boards with screen-printing, which will be discussed in this work. The hybrid stretchable electronics structures are based on photolithographically structured and rigid copper islands and screen-printed silver ink interconnections. This enables the assembly of components with a high number of contacts onto the copper islands and deformable silver ink lines between islands. The transition area between islands and lines is critical due to local stress concentration. The effect and potential mitigations were studied by measuring the electrical resistance of test interconnections under mechanical loading. The first set of samples was elongated up to 30% in tensile tests. The second set of samples was elongated 10%, 20%, and 30% in cyclic tests up to 10 000 cycles. After the tests, extensive failure analysis, e.g. scanning electron microscope, and finite element analysis were conducted. In tensile tests at maximum load, the interconnections either snap apart or their resistance increases by 640% in the transition area. Adding protective structures around the transition area, the resistance increase can be reduced to 12%. Stress concentration in the transition area can be controlled with the layout of the structures, as shown in the cyclic tests. Depending on a layout, the structures protect interconnections in the transition area (resistance <4 Ω at 10% and 20% throughout 10 000 cycles, and up to 5000 cycles at 30% elongation), or with particular designs, cause fatal damage of the circuitry and fail early. The identified failure mechanism is typically fatigue damage caused by the repeated bending of the protective structure. The observed resistance increase at the interface was closely related to the crack propagation phase in the protective structures.

https://iopscience.iop.org/article/10.1088/2058-8585/acda46/pdf

Mechanical properties of structured copper and printed silver hybrid stretchable electronic systems

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Papers

Role of Cu in dissolution kinetics of Cu metallization in molten Sn-based solders

Morphology and Growth Kinetics of Intermetallic Compounds in Solid-State Interfacial Reaction of Electroless Ni-P with Sn-Based Lead-Free Solders

Strategies for Embedding of Active Components

Mechanical properties of structured copper and printed silver hybrid stretchable electronic systems