Solder intermetallic compound formation

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. 

I. Effects of Intermetallic Compound Formation on the Mechanical Properties of Pb-Sn Solder Joints... 

Brothers, E. Intermetallic compound formation in soft solders. The Engineer (Western Electric Pub.), 49-63. 

Erickson, K. Hopkins, P. Vianco, P. Preferential dissolution and intermetallic compound formation with multi-component base metals and solder alloys, presented at the TMS Annual Meeting, Seattle, WA, 2002. 

The formation of IMCs in Pb-free solder systems is in many respects reminiscent of those observed in Pb-Sn, with several intriguing added complications. In Pb-free solders such as Sn-Ag-Cu, the Cu and Ag additives form IMCs with Sn. These elements also diffuse rapidly in Sn, thus a high flux of these constituents is available for otherwise favorable reactions, such as the formation of ternary IMCs at metallization interfaces. The evolution of the microstructure of Pb-free solder joints during reflow, cooling, and subsequent annealing has been shown to be a complex subject. As far as intermetallic compound formation and growth at metallization interfaces are concerned, Sn-Ag-Cu solder alloys were found to behave similar to eutectic Sn-Pb solder on Cu substrates. In contrast, distinct differences were observed on Ni surfaces between the behavior of Sn-Ag-Cu and Sn-Pb solder. 

FIG. 33 Comparison of the growth of intennetallic alloys for different reflow times with Sn-Ag-Cu solder. The Sn-Ag Cu solder was reflowed on chips with NiV under bump metallization. Then the bumped chips were reflowed to substrates with Cu/OSP pads above the liquidus of (a) 545 , and (b) 745 °, and a peak temperature of 238 C. An increase in time above the liquidus resulted in an increased intermetallic compound formation. 

TABLE 7 Causes of Improper and Uniacceptable Intermetallic Compound Formation During Soldering... 

Intermetallic compound formation usually accompanies solder reflow operations during the assembly of PCBs. The base metal and materials utilized to enhance solderability dictate the intermetallic compound species that form. The most common PCB pad finishes are hsted in Table 6. 

Zhang et al. reported the reconfiguration of AuSn4 in the bulk solder to a more thermodynamically stable Cu-Sn-Au phase at interfacial intermetallic compound formations after aging at 150°C. The conversion of CugSns to a Cu-Sn-Au phase was followed by the formation of a Cu-Sn-Au-Ni phase with continued aging and the addition of Ni from the underbump metallurgy (UBM). A faster rate of reaction was reported in Sn-Pb solders compared to Sn-Ag-Cu solders with either 0.7 or 1.2 wt.% Au content [16]. 

In addition to the metallurgical aspects discussed above, process parameters can have important effects on reliability as well. Good-quality paste printing, peak reflow temperature, dwell time above the liquidus temperature, cooling rate, and solder atmosphere reactions all affect the microstructure of solder joints and ultimately their reliability. Higher processing temperatures increase the dissolution rates of metal finishes and/or conductor metals (i.e., minor elements) into the solder and can increase the rate of intermetallic compound formation in the bulk solder. This increases the joint stiffness (i.e., reduces the compliance). 

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. 

In the presence of molten solder, the interface reactions include the simultaneous process of base metal dissolution or base metal erosion as well as intermetallic compound layer formation. The extent of dissolution depends upon the composition of the base metal as demonstrated in Fig. 9 for the case of molten 40Pb-60Sn solder and the base metals Au, Ag, Pd, Pt, Ni, and Cu [14]. The dissolution rate of Cu, specifically as a function of temperature for several molten Pb-Sn compositions is shown in Fig. 10 [15]. A lower Sn content reduces the extent of base metal dissolution. 

The CusSn intermetallic compound (sub)layer composition is favored under conditions in which Sn availability is limited. A reduced Sn supply occurs when there is only a very thin Pb-Sn solder layer (e.g., an electroplated Pb-Sn coating) present over the Cu base metal, or when the solder composition has a low Sn content. For example, use of high-Pb solders on Cu, such as the 95Pb-5Sn composition, favors the formation of the CusSn composition [19]. 

Lead-tin solder joints are commonly made to nickel (Ni), the Ni being either a bulk material or a thin solderable finish. The reaction results in the formation of primarily Ni3Sn4 intermetallic compound. The Pb component of the solder has no explicit role in the interface reaction. The reactivity between the Sn component of the Pb-Sn solder and Ni is considerably slower compared to Cu for the liquid-state dissolution reaction (see Fig. 9) as well as the... 

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