Optimizing Reflow

Estimated reading time: 9 minutes

Successful reflow soldering is key to productivity and profitability.

When infrared (IR) ovens were the norm and solder pastes relatively unsophisticated, initial reflow profiles were developed. These profiles were called "ramp to dwell — ramp to peak" (Figure 1). Since then, IR technology has bowed to the superior capabilities of convection technology.

Figure 1. The 'Ramp to Dwell — Ramp to Peak' reflow profile initially developed for IR reflow ovens.

Recent work by Lee1 indicates that these IR reflow profiles are not optimum for convection ovens and modern solder pastes. Through analysis of defect mechanisms, the work reveals that a gentle ramp to about 175°C and a gradual rise above liquidus, followed by a ramp to a peak temperature of 215°C will result in the highest yields (Figure 2). Considering almost all ovens used in SMT assembly are convection ovens, this distinction in reflow profiles is significant.

Figure 2. The optimized Lee reflow profile.

For a reflow oven to achieve this profile, the user must select the appropriate oven setups. Because a modern reflow oven is designed to support countless different setups, modern process setup tools help identify the best combination of zone temperatures and conveyor speed. This process optimization should take place quickly to minimize production downtime.

The ramp to dwell — ramp to peak (RDRP) profile was developed for IR reflow ovens. This technology tends to heat unevenly and, in some respects, more slowly than convection ovens. Consequently the dwell, usually at 140° to 160°C, was developed to ensure even heating with IR technology in the solvent evaporation and flux activation stages of the reflow process. The ramp to peak then was used to minimize the time above liquidus (TAL) and the possibility of singeing components. When IR technology was deployed, 0402 passives, ultra-fine-pitched PQFPs, BGAs, CSPs and other high-tech components were far in the future. Unfortunately, each of these now dominant technologies has a problem with the RDRP profile. Many failure mechanisms, common today with these components, can be traced back to the use of the RDRP profile.

Tombstoning. This failure mode is almost unavoidable with RDRP. As the RDRP profile goes from the soak temperature of 140° to 160°C, it shoots directly to the peak reflow temperature. This fast temperature rise from below to above liquidus often will cause the solder paste at one end of a passive to melt before the paste at the other end. The surface tension of the melted solder will cause the passive to tombstone. The Lee profile minimizes tombstoning by establishing a brief dwell as the profile goes through liquidus. This dwell allows for more even temperatures as solder paste at the component leads goes through liquidus. This profile melts the solder paste at both ends of the passive simultaneously, minimizing tombstoning.

Wicking. This occurs when the component leads become significantly hotter than the printed circuit board (PCB) pads during reflow. Since solder flows to where the temperature is highest, opens can result.

The RDRP profile typically ramps from 1° to 2°C/second from its dwell. This high ramp rate and the lack of a dwell at liquidus can result in the leads being much hotter than the pads. Wicking often will follow. The Lee profile's gentler heating rate of 0.5° to 1.0°C and a brief dwell at liquidus minimizes such wicking.

Solder Balling. The RDRP's profile has a rapid ramp rate that can cause the solvents to escape so rapidly that paste spattering occurs. Additionally, the long time at a relatively high dwell temperature can result in oxidation. The combination of these two factors can create solder balling. The Lee profile's gentler ramp rate minimizes spattering and the lack of a long dwell reduces oxidation. Therefore, the Lee profile has a tendency to minimize solder balling. However, many experiments have also shown that stencil design has a strong effect on solder ball reduction.

Hot Slumping — Bridging. Hot slumping occurs when the solder paste is at too high a temperature for too long. Hot slumping can lead to solder bridging. The long dwell of the RDRP profile can result in hot slump. The gentle ramp of the Lee profile minimizes this failure mode as the solder paste is exposed to high temperatures for a shorter time.

Poor Wetting. The RDRP profile can expose the solder paste and leads and pads to excessive temperature and time. This can cause oxidation, resulting in poor wetting. The Lee profile minimizes the time at high temperatures, reducing the chance of excessive oxidation and making good wetting more likely.

Voiding. The combination of oxidation and the flux remnant being too viscous can result in voids. A viscous flux remnant cannot move through the molten solder and escape through the surface. The longer times at high temperatures the RDRP profile provides can cause oxidation. However, this profile also can drive off too much solvent and leave a viscous flux remnant. The shorter time at higher temperatures provided by the Lee profile minimizes oxidation and leaves the flux remnant fluid, minimizing voids.

Peak temperatures that are too high can result in component charring. The combination of high peak temperature or excessive TAL also can create intermetallics that are too thick, resulting in reliability concerns. The Lee profile recommends an absolute peak temperature of 228°C, but encourages the user to strive for 215°C as a target peak temperature. The Lee profile minimum TAL can be as short as 30 seconds. Some RDRP profiles suggest peak temperatures of 235°C and minimum TAL of 45 seconds. These types of profiles also can cause charring of components or form intermetallics that are too thick.

To achieve the higher yields, it is not sufficient to simply select any oven recipe that provides an in-spec profile. A smaller subset of oven recipes, those that yield a profile that conforms to Lee profile specifications, must be identified. For example:

• From room temperature, ramp at 0.5° to 1.0°C/second to 175° ±3°
• Ramp at <0.25° through 183°C but for less than 45 seconds
• Ramp to peak (target 215°C, range 208° to 228°C) with TAL 30 to 90 seconds
• Ramp down 2° to 4°C/second.

New software-driven technologies simplify this process.

Recipe Search Engine. A modern reflow oven typically has between five and 12 independently controlled temperature zones. Conveyor speed represents another variable. Each of these variables can operate in a wide range and the technician is faced with billions of alternative oven setups. Only a small percentage of the possible oven setups will yield an in-spec process, and only a fraction of those setups will conform to the Lee specs.

Figure 3. The Lee profile process window specifications.

In a recipe search engine, Lee profile specifications are entered to define the required process window (Figure 3). The technician runs a profile to measure the time vs. temperature on the relevant PCB or part in the current oven setup. The software now has information on the relationship between the oven settings and the resulting profile (Figure 4). An automatic computer simulation routine then is initiated. The software makes a small incremental change in one of the variables (an individual zone temperature or conveyor speed), simulates the new profile that would result from such a new oven setup, and determines how well this profile fits the Lee process specifications. This "fit" is mathematically calculated using the process window index (PWI) concept. The PWI assigns a single number, which represents each profile's fit to the process specifications. Any number less than 100 percent is in spec, and more than 100 percent is out of spec. A PWI of 0 percent represents the very center of the process window.

Figure 4. A product optimized for the Lee process window.

This procedure is repeated billions of times in a matter of seconds, and the Oven Recipe Search Engine selects the optimum recipe. Typically within 60 seconds, the optimum oven recipe is displayed, ready for downloading to the oven control system. The process engineer decides the criteria for the optimum oven recipe. They typically fall into one of three categories, or a combination:

1. The oven recipe that positions the profile toward the center of the Lee specifications.
2. The oven recipe with the fastest conveyor speed that still yields a profile within the Lee specifications.
3. An oven recipe that eliminates or minimizes the oven changeover time. (The search engine will attempt to find an acceptable profile by searching exclusively on different conveyor speeds, rather than temperatures. It may take from five to 30 minutes for an oven to stabilize on new temperature settings; changes in the conveyor speed may be reached quickly.)

Conclusion

It is critical to refresh efforts at process optimization continually. Implementing the latest proven tools frequently is the surest way to remain the vendor of choice.

Reference

¹ .N.C., "Reflow Soldering Processes and Troubleshoot-ing," Newnes, Boston 2002, p. 239.

Bjorn Dahle, president, may be contacted at KIC, 15950 Bernando Center Drive, Suite E, San Diego, CA 92127; (858) 673-6050; Fax: (858) 673-0085; E-mail: bdahle@kicmail.com; Web site: http://www.kicthermal.com. Ron Lasky, Ph.D., senior technologist, Indium Corp., may be contacted at 26 Howe St., Medway, MA 02053, (508) 930-2242, Fax: (508) 533-5678, E-mail: rlasky@indium.com.

Defining the PWI

The process window index (PWI) is a measure of how well a profile fits within user-defined process limits. This is done by ranking process profiles on the basis of how well a given profile "fits" the critical process statistics. A profile that will process product without exceeding any of the critical process statistics is said to be inside the process window. The center of the process window is defined as zero, and the extreme edge of the process window as 99 percent. A PWI of 100 percent or more indicates that the profile will not process product in spec. A PWI of 99 percent indicates that the profile will process product within spec, but it is running at the very edge of the process window. A PWI of less than 99 percent indicates that the profile is in spec and tells users what percentage of the process window they are using. For example, a PWI of 70 percent indicates a profile that is using 70 percent of the process spec. The PWI tells users exactly how much of their process window a given profile uses. The lower the PWI, the better the profile. A PWI of 99 percent is risky because it indicates that the process could easily drift out of control. Most users seek a PWI of <80 percent, and profiles with a PWI between 50 and 60 percent are commonly achieved (if the oven is sufficiently flexible and efficient).

Figure 5. The process window index (single statistic — peak temperature of one thermocouple).

Figure 5 shows the PWI for the peak temperature of a single thermocouple. The PWI for a complete set of profile statistics is calculated as the worst case (highest number) in the set of statistics. For example, if a profile is run with six thermocouples, and four profile statistics are logged for each thermocouple, then there will be a set of 24 statistics for that profile. The PWI will be the worst case (highest number expressed as a percentage) in that set of profile statistics (Figure 6).

Figure 6. The Process Window Index (multiple statistics for a single thermocouple and final PWI calculation).

Calculating the PWITo calculate the PWI: I = 1 to N (number of thermocouples); j = 1 to M (number of statistics per thermocouple); measured_value[i,j] is the [i,j]th statistic's value; average_limits[i,j] is the average of the [i,j]th statistic's high and low limits; and range[i,j] is the [i,j]th statistic's high limit minus the low limit (Figure 7).

Figure 7. Process Window Index formula.

Thus, the PWI calculation includes all thermocouple statistics for all thermocouples. The profile PWI is the worst-case profile statistic (maximum, or highest percentage of the process window used), and all other values are less.