Solder intermetallic compounds

The material complexity of solder joints arises from the presence of the base metal(s), the intermetallic compound layers, and the bulk solder [5]. Each one of those materials has different mechanical properties (e.g., yield strength, ductility, ultimate strength, etc.). In addition, Pb-Sn materials have strength properties that are very strain rate sensitive. The yield strength and ultimate tensile strength values increase dramatically with faster strain rates [29]. Also, there are two interfaces associated with each intermetallic compound layer—the base metal/intermetallic compound interface and the solder/intermetallic compound interface— which can exhibit significantly different adhesion properties. 

Nonideal solid-liquid TX diagram at 1 atm for Cu and Al (only about the left half of the diagram is shown). The two-phase regions are indicated. There is a very limited solubility of Cu in Al this is phase a. There is similarly a limited solubility of Cu in the stoichiometric phase or intermetallic compound CuAI2 (called the 6 phase). The liquid solution of Al in Cu freezes at the lowest possible temperature ( 540°C) for 32 mass % Cu this is the eutectic point (which is technologically useful in solders). 

The basic function of the circuit board is to act as a mechanical and electrical connection between components of an electrical device. Application of solder to a PCB surface results in the formation of an intermetallic compound. The metals in the solder and the metals on the PCB participate in the intermetallic formation. The intermetallic acts as the physical glue holding the solder to the circuitry. The type of intermetallic depends on the type of solder and the type of PCB finish. 

FIGURE 48.2 Soldering steps required for first-pass soldering of a bottomside component on a PWB and success reflow steps encountered during first-pass soldering and repair. Note that the intermetallic compound formation thickness is not truly linear with each step. Thickness depends on materials of the soldering system, time above solder alloy liquidus, peak temperature, etc. The illustration is meant to show that with each reflow cycle, there is increased intermetallic layer thickness. 

Shelgon Yee, Lodgers Chen, Justin Zeng, Roger Jay, Ternary Intermetallic Compound - A Real Threat To BGA Solder Joint Reliability, Journal ofSMT, 2004, Vol. 17, Issue 2, pp. 29-36. 

Fig. 7. The galvanic current densities of the solder with respect to intermetallic compounds CueSns and CuaSn, and Cu substrate, in a 3.5 wt.% solution [45].

Fig. 12. Morphology of intermetallic compounds formed at the interfaces of the as-reflowed solder joints (a) Sn3.5AgO.5Cu, ( h) (a) magnifications (c) d Sn3.5Ag0.5Cu-0.75TiO2 (d) (c) magnifications [58].

The interactions between solders and substrates are described in two classes liquid sol-der/substrate reaction during soldering and solid solder/substrate reaction during subsequent aging. During soldering, two processes occur simultaneously (a) the substrate metal dissolves into the molten metal and (b) the active constituents in the solder combine with the substrate metal to form intermetallic compounds (IMCs) at the substrate/solder interface. 

In order to predict the mechaiucal properties of the solder joint, it is necessary to understand the various structures and manner in which their properties effect those of the whole joint. The solder joint is comprised of the solder alloy, the substrate materials that are being joined together, and the interfaces between the substrate materials and the solder that is typically comprised of one or more intermetallic compound... 

Many North American companies are concerned that lead-free components and assemblies will not meet the reliability or functionality requirements necessary for high-end equipment supplied to banks, air traffic control systems, web-based businesses, and other mission-critical applications. Accordingly, manufacturers of aerospace and military electronics have no plans to introduce lead-free solders. There are too many reliability concerns to utilize lead-free solder materials in high-reliability equipment related to the mechanical characteristics of the materials themselves and the effects of high temperatures to process them. The effect of new and modified intermetallic compound phases within solder joints and at the interfaces is yet an additional concern, and there are many more. 

The interface reaction product formed between Pb-Sn solders and most base metals is commonly referred to as an intermetallic compound layer. Intermetallic compounds exhibit highly directional bonding similar to ceramic materials and, as such, typically have a well-defined stoichiometry, high melting temperatures, high strength, and very low ductility. Unfortunately, the binary alloy phase diagrams provide only an approximate indication of intermetallic com-... 

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