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47.3.4.2 Solder. The solder alloy chosen dictates the peak temperature and time required for reflow and wetting. It may also influence the post-reflow cooling rate chosen, as some solders, especially Pb-free compositions, have preferred cooling ramps. 47.3.4.3 Flux and Other Solder Paste Components. As discussed previously, solder paste is typically composed of three main ingredients: solder alloy, flux, and other materials added to help with rheological properties critical for solder paste printing on the PWB. As mentioned, the solder alloy dictates the peak temperature regime, whereas the flux and other constituents dictate the pre-reflow heating ramp rate. Heating too rapidly may prematurely dry out solder paste components. Overly rapid heating may cause the solder paste to spatter. Too little heat may not allow proper activation of the flux. There is no universal profile recipe for reflow for a given solder alloy. The solder paste manufacturer will provide guidance for the time-temperature profile for each solder paste formulation. The reflow characteristics should be consistent from batch to batch for a given paste formulation.
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The Reflow Profile There are four distinct steps in the reflow profile that must be accommodated by the reflow oven for successful soldering. Each step must be tailored for the solder paste composition and must be accomplished in proper, controllable, and repeatable reflow equipment. A misstep at any of the four stages of reflow can result in product loss. Refer to Fig. 47.8 for this discussion on thermal profile.
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FIGURE 47.8 Schematic of a generic reflow profile for Sn-Ag-Cu solder paste. Time-temperature values are not precise and are shown for illustrative purposes for a discussion of reflow soldering.
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47.3.5.1 Initial Ramp. In this step of the reflow time-temperature profile, preheating of the boards, components, and solder paste is initialized. The solder paste begins to lose some of its volatile components and the flux becomes chemically active (activates). If the ramp gradient is too steep, volatiles will be given up too rapidly, boiling will result, and the solder paste will spatter. This can cause explosive solder ball formation and decrease local solder volume, and thus impair bond reliability. Unattached solder balls may form solder bridges between two closely spaced conductors and produce an electrical short circuit. Overly rapid heating is known to cause component cracking. This is particularly true of ceramic components.To be safe, restrict the ramp rate to between 2 to 4 C/sec., but investigate component manufacturers specifications as well as the solder paste vendor s recommendation for the maximum allowable heating ramp.
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47.3.5.2 Thermal Soak. During this step, the solder, board, and components are further heated. The flux from the solder paste flows onto all metal surfaces in which it is in contact, continues to react away surface oxides, and also acts as a barrier to prevent oxidation. The soak is designed to provide the requisite time and thermal energy for the flux s prolonged chemical reaction with oxides and tarnishes on metal surfaces and the solder. Were the flux to activate fully too early in the process, it could dry out or be spent too soon. If its potency is lost, fluxed metal surfaces will reoxidize in the critical moments before the onset of solder liquidus, and solder wetting (alloying) would be inhibited. 47.3.5.3 Spike 47.3.5.3.1 Onset of Solder Liquidus. In this step, the solder undergoes a phase change from solid to liquid.The liquid metal solder wets onto fluxed metallic surfaces and flows along the component lead-to-PWB pad interfaces drawn by surface tension, capillary forces, and convective flows within the liquid metal. This is the essence of soldering and effective solderjoint formation. Characterized by a rapid thermal rise, the spike s peak temperature is chosen to be well above the solder s melting point to ensure that all parts of the board, all components, and all solder paste deposits have surpassed solder liquidus onset. As examples, for eutectic Sn-Pb solder, the peak temperature is generally 25 to 40 C above the melting point. For Sn-Ag-Cu solders, the peak is a more conservative 15 to 30 C above the onset of liquidus. The lower temperature overage for SAC solder as compared to Sn-Pb is due to the higher melting range of the SAC alloy family and the potential damage to components and the circuit board due to the higher-temperature regime needed for reflow. Also, since the temperature is higher than for SnPb, oxidation of heated metal surfaces is more likely, as is solder paste decomposition or charring. 47.3.5.3.2 Surface Tension Effects. While the solder is molten, surface tension effects draw component leads into best registration with circuit board pads if the component is light enough to float upon the molten solder. Passive devices (resistors, capacitors, inductors, etc.), most components with formed leads, and plastic or laminate BGAs self-align during this step. Solder surface tension is typically not strong enough to effect self-alignment of heavy ceramic components such as ceramic ball grid arrays (CBGAs) and ceramic column grid array (CCGAs). 47.3.5.3.3 Intermetallic Compound Formation. As previously discussed, intermetallic compound (IMC) formation occurs when the molten solder wets to metal surfaces of component leads and PWB pads. IMC thickness is dependent on the temperature and time at or above liquidus. The longer the solder is heated, the thicker the intermetallic layer, and thick IMCs result in brittle bond lines and degraded solder-joint reliability. For this reason, overheating the circuit board during the spike should be avoided. Time-above-liquidus (TAL) should be in accordance with the solder-paste manufacturer s recommendation, which is typically 30 to 60 sec., although there are no hard and fast rules for TAL. A longer TAL for some components may be evidenced on large thermal mass boards that are slow to heat and heat unevenly. In this case, some devices may overheat while others barely reach soldering temperature. Similarly, some component or board metallurgies may be slow wetting and a longer TAL may be necessary. Hot air solder leveling (HASL) or reflowed tin on copper bond pads possess a thin intermetallic layer between the applied surface finish and the pads. During surface-mount technology (SMT) reflow, the addition of molten solder on top of this preexisting intermetallic layer will further its growth. Therefore, the TAL should be limited to minimize IMC but sufficient for good solder wetting. 47.3.5.3.4 Materials Degradation. During spike, board laminates, component bodies, etc., can degrade, so trials should be performed to ensure that all materials are compatible with chosen reflow parameters. If connectors are being applied during SMT reflow, make sure that normal contact force has not been jeopardized by distortion or softening of the connector body during the spike. Inspection of connector contact gaps before and after reflow and comparison to the connector manufacturer s specifications will provide clues as to whether the connector is truly compatible with the reflow process. This is most important during the
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