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

Interfacial reaction in Cu/Sn/Cu system during the transient liquid phase soldering process

Kinetics of Ag3Sn growth in Ag-Sn-Ag system during transient liquid phase soldering process

A lead-free solder alloy suitable for use in flow soldering of electronic components to printed wiring boards comprises 0.1-3 wt % of Cu, 0.001-0.1 wt % of P, optionally 0.001-0.1 wt % of Ge, and a balance of Sn. The solder alloy may further contain at least one element of Ag and Sb in a total amount of at most 4 wt %, and/or at least one element of Ni, Co, Fe, Mn, Cr, and Mo in a total amount of at most 0.5 wt % in order to strengthen the alloy, and/or at least one element of Bi, In, and Zn in a total amount of at most 5 wt % in order to lower the melting point of the alloy.

Lead-free solder alloy

In the big data era, Si chips are integrated more and more to satisfy the fast growing demand from customers. In addition, in the post-COVID-19 virus era, the trend of distance teaching and home office has increased greatly the need of advanced consumer electronic products. To provide more processing tolerance and to build a wider temperature window in manufacturing, it becomes necessary to develop low melting temperature solders because a hierarchy of solders is needed in the packaging technology. Much research has been on Pb-free and Sn-based solders with a melting point from 180 to 230 °C. In this review, we will concentrate on low melting point solder alloys with a melting point lower than 180 °C and even below 100 °C. We review eutectic SnBi, eutectic SnIn, and the alloys with trace addition of a third element to them. Eutectic Sn-Bi solder is too brittle, and Sn-In solder is too soft. However, third element addition can only improve the solder properties partially; thus, we will further review the properties of Sn-Bi-In ternary alloys and the effect on the addition of a fourth element to them. This approach, when we include the Cu substrate, becomes a 5-element system, which leads us to consider high entropy alloys for soldering. With these Sn-, Bi-, and In-based alloys, we hope to develop new ways to design industrial applicable low melting point solders.

Low melting point solders based on Sn, Bi, and In elements

Comparison of interfacial reactions of Ni and Ni–P in extended contact with liquid Sn–Bi-based solders

The control of the interfacial reaction rate is of great importance for liquid solder interconnects for high temperature electronic assemblies. Conventional electroless Ni-P barrier metallizations have been found to be inadequate for providing long term protection of the underlying metallization from the attack of molten solders. In this paper, binary Ni-P was modified with the co-deposition of a refractory alloying element, tungsten (W), from its soluble metal salt added to the plating bath. Critical parameters for the deposition of ternary Ni-W-P were identified. The long term reaction between Ni-W-P and molten Sn-Bi solder at 200 degC was studied. The results indicated that Ni-W-P barrier coatings with higher W contents have much longer service lifetime as a barrier than normal Ni-P coatings. A mechanism for the reaction between Ni-W-P and molten Sn-Bi solder, and a failure mechanism for the Ni-W-P layer, are also proposed

/pdf/electroless-ni-w-p-alloys-as-barrier-coatings-for-liquid-192jws7moy.pdf

Electroless Ni-W-P Alloys as Barrier Coatings for Liquid Solder Interconnects

This paper reports on the interfacial reactions and lifetime of electroless Ni-P coatings in contact with molten Sn-Bi based solders. A layer of approximately 4 mum thick electroless Ni-P in contact with the molten Sn-58Bi solder began to fail at 48 h at temperatures between 200degC and 240degC . Elemental additions to modify the solder, included 1-2wt.% of Al, Cr, Si, Zn, Ag, Au, Ru, Ti, Pt, Nb, and Cu. Of these, only Cu modified the interfacial intermetallic compound growth from Ni3Sn4 to (Cu,Ni)6Sn5 , resulting in significantly decreased consumption rates of the Ni-P substrate in contact with the molten solder and increasing the lifetime of the Ni-P layer to between 430 and 716 h. Micro cracks were observed in all but the thinnest Ni-P layers, allowing the solder to penetrate.

/pdf/interfacial-reaction-between-molten-sn-bi-based-solders-and-3nlsu0eggy.pdf

Interfacial Reaction Between Molten Sn-Bi Based Solders and Electroless Ni-P Coatings for Liquid Solder Interconnects

Molten Solder Interconnects I: Evaluation of Sn-Bi-X/Cu systems and failure mechanisms of LGA assemblies

The stability of Ni/sub 3/P and its effect on the reaction with molten tin were evaluated in the present study by comparing the reaction behaviour of Ni-P coatings in the as-plated and heat-treated conditions It was found that after the solder reaction a dark layer of Ni/sub 3/P was formed at the interface on Ni-P coatings in both conditions, but its thickness was thinner on heat-treated coatings than on as-plated coatings From the compositional analysis on the cross-sections, it was found that not all the P ejected from the interfacial reaction between Sn and Ni-P was consumed by the formation of Ni/sub 3/P, as P was detected in a second layer of interfacial phase which also contained Ni and Sn The formation of Ni/sub 3/P was found to be characteristic of a diffusion process, whilst the thickness of Ni-P coating consumed during the reaction was found to be linearly proportional to the reaction time It is concluded that the Ni/sub 3/P phase formed during the solder reaction is not stable and therefore cannot act as an effective barrier to reactions with liquid solders

Stability of Ni/sub 3/P and its effect on the interfacial reaction between electroless Ni-P and molten tin

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