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16 SMT007 MAGAZINE I JULY 2019 In this article, the author explains how the volume of low-melting-point alloy paste that delivers the optimum proportion of retained ball alloy for a particular reflow temperature can be determined by reference to the phase diagrams of the ball and paste alloys. The example presented is based on the equilibrium phase diagram of the binary Bi-Sn system, but the method could be applied to any combina- tion of ball and paste alloys for which at least a partial phase diagram is available or could be easily determined. Introduction The dependence of the electronics industry on solder to provide the reliable connections necessary to turn a collection of individual components into a functional circuit has cre- ated an ongoing dilemma that has challenged the industry since its inception. The main con- sideration in the design of electronic compo- nents is functionality; whether it be to provide a specific resistance, capacitance, or induc- tance in passive components; logical process- ing in integrated circuits; and responsiveness in sensors or just electrical connectivity. A need to survive the thermal profile required to form a joint with a molten metal has been an annoying complication. The industry has had a fairly easy start with the Sn-Pb eutectic that has a relatively low melt- ing point of 183°C and mechanical properties and microstructural stability that have been considered to be the benchmark for reliability in service. Because of the need to maximize heat transfer into the joint to get the substrates to wetting temperature, process temperatures (e.g., soldering tool tip temperature, wave sol- der bath temperature, reflow oven peak tem- perature) had to be substantially higher than that 183°C melting point. However, with proper process control, the temperature/time profile to which the most sensitive parts of the component were exposed could be kept within a safe limit. The move to Pb-free solder brought a renewed challenge because the alloy endorsed by IPC as "the Pb-free alloy of choice for the electronics industry"—Sn-3.0Ag-0.5Cu (SAC305)—does not start to melt until 217°C, which is 34°C higher than the melting point of the Sn-37Pb it was replacing. However, as long as the higher process temperatures could be accommodated by the use of resins and polymers that could survive the higher ther - mal profiles, the electronics industry was able to adapt to this new alloy. When the problem with higher process tem- peratures was not just thermal degradation of materials but gross deformation of compo- nent packages the challenge moved to a new level. Integrated circuit packages have evolved into complex stacks of a wide range of mate- rials with very different coefficients of thermal expansion so that as the package heats the dif- ferential expansion of bonded layers results in warpage of the package. The problem is exacerbated by the temperature gradients that develop within the package as a result of vari- ations in thermal conductivity and thermal mass. The extent of warpage can mean that at its peak the warpage in area array packages is sufficient to cause complete separation of joints at their extreme edges (Figure 1). Depending on the temperature at which that peak separation is reached for a particu- lar component, the separation can take differ- ent forms: 1. Unreflowed solder paste can be split with some adhering to the solder ball and some to the pad 2. Unreflowed solder paste adheres only to the solder ball and is lifted off the pad 3. Unreflowed solder paste adheres only to the pad with the solder ball detaching from the solder paste Depending on how the warpage changes as a function of temperature, the solder paste might reflow while separation is at its peak. When, later in the reflow profile, the component returns to its original shape, there is no cer- tainty that the separate and now molten solder will coalesce as the activity of the flux, which facilitates wetting and coalescence might have been exhausted. In the case of Type 1 sepa- ration, the result is a defect known as head-

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