Reflow Profile Optimization

Estimated reading time: 7 minutes

A key tool for process optimization and defect minimization, a "generic" reflow profile is recommended for use with solder paste. However, this must be modified to achieve optimum solderability results with the fewest defects for a given assembly.

By David Scheiner

In surface mount assembly, a key step in producing finished assemblies is reflowing deposited solder paste. Solder paste commonly is reflowed in a convection IR oven, which heats the circuit board in stages. The time and temperature graph while the board is in the oven is referred to as the reflow profile. The heat profile that the oven uses to reflow the solder paste is fully adjustable. To maximize product throughput and minimize defects, the reflow profile should be customized specific to the board.

Solder paste manufacturers supply a recommended reflow profile for use with their specific solder paste. These "generic" profiles offer a convenient starting place for setting up a reflow profile. An example of a generic profile can be seen in Figure 1. Reflow profiles typically are divided into three zones.

Generic Profile

Zone 1: The Preheat Zone. During the preheat zone, board temperature quickly increases from room temperature to 150°C. While the board is heating, the solvent is evaporating from the deposited solder paste. In Zone 1, both the board and components begin warming up. The board temperature is ramped up at a rate in the range of 1.5° to 2.5°C per second, resulting in an overall time in the preheating zone of about 50 to 90 seconds.

Zone 2: The Soak Zone. During the soak zone, the board temperature increases from 150° to 180°C. At these temperatures, the flux becomes more active and prepares the metallized surfaces for the soldering process. The soak zone also ensures that the entire board, including components of various masses, is at nearly equal temperature prior to the solder melting. If components of different masses are not all at the same temperature prior to reflow, defects such as tombstoning or poor wetting may occur. In the soak zone, the activated flux also prevents the solderable surfaces from re-oxidizing.

During the soak zone, the board temperature is ramped up at a rate in the range of 0.3° to 0.8°C per second, resulting in a soak time of approximately 40 to 100 seconds. The amount of soak time required for a given assembly depends on the thermal variation across the board and how quickly the oven's convection can overcome those differences.

Zone 3: The Reflow Zone. During the reflow zone, the temperature immediately crosses the 183°C horizontal line. This is the significant point at which the solder powder melts, coalesces and turns into one liquid solder mass.

In the reflow zone, the temperature is spiked up from 180°C to a peak temperature of 210° to 225°C. The board temperature is ramped up at the rate of 1.3° to 1.6°C per second at the entry of the reflow zone. In most cases, the solder remains in the liquidus phase (above 183°C) for 45 to 75 seconds. While the solder is liquidus, solder joints are formed as the solder reacts with the base metals in the components and the board to form the intermetallic layers. Post-reflow, the soldered assemblies exit the reflow oven and are allowed to cool to room temperature.

Figure 1. Manufacturers recommend a "generic" profile.

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Beyond the Generic Profile

The generic profile is a good starting point when setting up a reflow profile for an assembly. If all board assemblies had the same board thickness and component densities, and all oven capabilities were the same, it would be easy to come up with an ideal profile. In reality, boards vary in size and component density, and ovens vary in length and thermal capabilities.

After dialing in the generic profile, it is suggested to make adjustments to the profile based on the mix of defects common to each individual assembly. The three defects most commonly attributed to profiling parameters are solder balls, poor wetting and tombstoning.

Solder Balls. The presence of small solder balls on the board is a cosmetic and functional defect. There are several potential causes for solder ball generation. For this article's purpose, the focus will be on the reflow profile's role in solder ball generation.

If the initial preheat (Zone 1) is too rapid, the solvent can be forced to boil off too rapidly and promote solder balling. A ramp up of 2.5°C per second generally is the upper limit for the preheat ramp up. If the board is thin or has a small thermal mass, the ramp up rate may have to be reduced down to 1.5° or 2.0°C per second. Solder balls generated by this root cause will be random and not necessarily entrapped by flux residues.

There is another way to generate solder balls as a result of the reflow profile. If the soak zone is too long, the paste can slump. A ring of solder balls can be formed around the solder fillet as a result of the slumped paste. The paste flux bleeds out, then small solder balls break off from the main body of the solder fillet as the solder melts and tries to pull up into a single bead.

Poor Wetting. This occurs when flux activity is reduced and can no longer remove the oxides from the board and component surfaces effectively. In this situation, the solder beads up and appears to draw back from a wetted surface. Long profiles — 75+ seconds above reflow or unnecessarily long soak temperatures — can contribute to poor wetting. Conversely, a shorter reflow profile will mean less oxidation and, therefore, better wetting and improved shininess.

It is recommended to design a profile that is as short as possible, based on hitting the appropriate milestones within the profile for the specified amount of time. Any unnecessary time in the preheat or soak zone will add oxidation to the entire system, making it more difficult for the flux to fully promote acceptable wetting to all of the metallized surfaces.

Tombstoning. Tombstoning is a phenomena in which an individual component does not attach to the board metallization; instead, one end solders to the one side, the component stands upright and the opposite metallization is left standing up in the air facing away from the board.

Figure 2. Schematic presection of the tombstoning.

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The mechanism for tombstoning is when the solder paste on one end of the component reflows before the metallization on the other end of the component reflows. The solder starts to wet the first side and the surface tension of the liquid solder pulls the component off the opposite pad before the solder on that pad has a chance to liquefy (Figure 2). Although this is not the most common mechanism for tombstoning, which is inaccurate component placement, this article focuses only on profiling and how to optimize the profile to eliminate defects.

Figure 3. The reflow profile showing two lines to represent the warmest and coolest temperatures on the board.

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Many factors contribute to tombstoning, including solder paste volume and pad geometry. A primary consideration for reducing tombstoning involves considering the slope of the line as it crosses the melting point of the alloy. To illustrate the relation of the time of the reflow slope to tombstoning, it is easier to look at the slope as two lines, not one (Figure 3). This is important because all the components and areas of a circuit board have different heat capacities. The whole board and the board's components do not heat uniformly. The two lines will represent the warmest (W) and the coolest (C) temperatures on a given board. As the board approaches the liquidus temperature of the solder alloy, these two lines will pass through the liquidus temperature at different times. The point at which the warmest part of the assembly passes through the liquidus will be designated TW. The point where the coolest temperature passes through the liquidus will be designated TC. The distance between TW and TC can be expressed as ΔT. An example of a large ΔT and small ΔT is shown in Figure 4.

Figure 4. Examples of a small ΔT and a large ΔT.

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To minimize tombstone incidence, it is important to minimize the ΔT as the board passes through the liquidus temperature. Increasing the ΔT gives a larger temperature gradient across the board, which in turn increases the likelihood of one component termination reflowing before another, resulting in increased tombstoning.

Conclusion

The reflow profile is an important tool for process optimization and defect minimization. A generic profile is a solid place to start; however, it must be modified to achieve optimum solderability and the fewest defects for a given assembly. The reflow profile may be modified based on the presence of specific defects.

Dave Scheiner, senior technical service engineer, may be contacted at Northrop Grumman Corp. — Kester, 515 East Touhy Ave., Des Plaines, IL 60018; (847) 699-5543; Fax (847) 699-4980; E-mail: dscheiner@kester.com.