Reflow soldering temperature profiles

The reflow time-temperature profile is of greatest importance in the oven soldering process. It is the relationship of temperature with respect to time required to bring a PCB assembly to solder liquidus and back to solidus before exiting the oven. 

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 whl spatter. This can cause explosive solder bah formation and decrease local solder volume, and thus impair bond rehabihty. Unattached solder balls may form solder bridges between two closely spaced conductors and produce an electrical short circuit. 

Overall rework time was about eight minutes for lead-free and six minutes for SnPb profiles. In most cases, board temperature 150 mils from the reworked component was above the liquidus-reflow temperatures, which was also the case during SnPb rework. Time above liquidus (TAL), which was often close to 90 seconds, combined with higher peak temperatures for lead-free solder rework, let to increased solder joint voiding. More solder paste development work is needed to support the elevated lead-free solder-temperature profiles. 

FIGURE 40,23 Generalized time-temperature profiles for Pb-free reflow soldering (a) the soak-reflow profile (b) the continuous ramp or hat profile. Peak temperatures will vary, depending on the particular circuit board product. 

Once the defective component has been removed from the circnit board, the second step is to remove excess solder from the Cu pads. This step is also referred to as dressing the pads. Remaining solder bumps and spikes interfere with the placement of the new component. More importantly, excess residual solder, the quantity of which is not tightly controlled, causes variabihty to the replacement component s solder joints, which can lead to possible opens, shorts, or a loss of long-term reUabiUty. Eecanse the cleaning or dressing step requires a solder reflow heat cycle (the secx>nd of three), the time-temperature profile of that cycle must be... 

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. 

Successful soldering is dependent on several factors including suitable, well-maintained, and controlled reflow equipment good-quahty solderable parts a thermally balanced board designed for the reflow process a well-tested and rehable solder paste a proven time-temperature profile and good thermometry techniques. 

Understanding the thermal impact of the oven on the board and the board on the oven is critical for a controlled and reproducible reflow process. Thermometry is the only practical method for validating these influences by means of oven profiling. Oven-and-board interactions are explored in this presoldering step and the oven is adjusted for proper time-temperature profile of the PWB with the parts to be soldered. Some of the most important process points determined through board and oven profiling are ... 

In the case of a thermocouple sandwiched between a component lead and PWB pad (see Fig. 47.11) and an accidental junction formation nearby, the thermocouple will register the oven air temperature rather than the conducted temperature of the component lead, the solder, and the PW. Since the oven air temperature is generally much hotter than the PWB traveling through it, the resultant soldering time-temperature profile (based on the false thermocouple reading) may be cooler than desired and cold (under-reflowed) solder joints may result. 

IPC-7530 Guidelines for Temperature Profiling for Mass Soldering (Wave and Reflow)... 

Reflow Process. The key parameter for the reflow profile is the peak temperature. Adequate reflow temperature is needed for the solder to melt, flow and wet, interact with the copper on the pad and the component termination, and form sound intermetallic bond when cooled and solidified. Typically, 30 °C (55 °F) superheat (above the melting temperature) is desired. Eor lead-free soldering, because of concerns about the thermal stability of the components, efforts are needed to minimize the soldering temperature. For SAC alloy with the eutectic temperature at 217 °C (422 °F), the minimum reflow... 

The removal of flux residues from soldered assemblies is more difficult than the removal of solder paste from stencils and misprints. In cooperation with a reputable local university, various lead-free solder pastes were printed onto standard test substrates and then soldered in a reflow oven at the specific temperature profiles of each respective solder... 

Reflow temperature profile for Sn-based Pb-free solders (Kim and Jung 2007)... 

Activity increases with temperature until disproportionation of abietic acid becomes significant. Therefore, it is generally preferred that the temperature profile consist of a steep ramp in the range of solder melting, especially for alloys that melt near or above 300°C. Typically, the ramp of a reflow temperature profile increases slowly to allow solvents to evolve, has a soak plateau between the solvent boihng point and solder melting temperature, and has a rapid ramp to the solder melting temperature. 

A flux system must be designed to be compatible with a particular solder alloy system. For example, typically the activation and volatilization temperatures for the eutectic Sn-Pb system are lower than lead-free alloys. In lead-free reflow, the soak or stabilization temperature may be in the range of 160 to 170°C compared with 140 to 155°C for eutectic Sn-Pb solder reflow. In addition, the lead-free peak temperature may approach 240 to 250 °C compared with 220 to 230°C for eutectic Sn-Pb. At the higher lead-free process temperatures, fluxes vaporize more rapidly and completely compared to standard temperature profiles utilized for eutectic Sn-Pb solder. Often there is insufficient flux left on a board due to volatilization. The volatilization... 

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