The Benefits of a Ramp-to-Spike Reflow Profile

Estimated reading time: 10 minutes

The pains often associated with profiling may be reduced greatly if certain guidelines are followed and if there is a strong understanding of the variables that can be encountered during the reflow process.

David Suraski

Many older-style ovens were prone to heating different parts of an assembly at varying rates, depending on the color and texture of the parts and substrates being reflowed. Some areas of an assembly could reach much higher temperatures than other areas. This temperature variation is referred to as the assembly`s λT. With a large λT, some areas of an assembly may receive excessive heat, while other areas may receive insufficient heat. This could cause a variety of solder defects, including solder balling, non-wetting, damaged devices, voiding and charred residues.

Newer Reflow Ovens

Most newer-style reflow ovens, known as the forced convection type, blow warm air on and around assemblies. One advantage of this type of oven is that they are able to provide heat to an assembly gradually and uniformly, regardless of part color or texture. Although the heat absorption may vary slightly because of the varying thickness and component population, forced convection ovens generally provide heat in such a manner that the λT is not significant. In addition, the maximum temperature and temperature rate of a given profile can be strictly controlled with these ovens. What they offer is greater zone-to-zone stability, providing a more controlled reflow process.

Why and When to Soak

The sole intention of the soak zone is to reduce/eliminate a large λT. The soak should bring the temperature of all parts of the assembly to equilibrium before the assembly reaches the solder reflow temperature so all parts of the assembly reflow simultaneously. Because the soak zone is unneeded, the profile may be altered into a linear ramp-to-spike (RTS) profile.

It is important to note that the soak zone generally is not needed to activate the solder paste flux chemistry. This is a common misconception in the industry, and it bears correcting. Most solder paste chemistries demonstrate adequate wetting activity when processed in a linear RTS profile. In fact, using the RTS profile generally will improve wetting.

Ramp-soak-spike

The ramp-soak-spike (RSS)1 profile may be used with RMA or no-clean chemistries, but generally is not recommended for use with water-soluble chemistries, as the RSS soak zone profile may break down the paste activators prematurely and result in inadequate wetting. The sole purpose of using the RSS profile is to eliminate or reduce a large λT.

As seen in Figure 1, the RSS profile begins with a steep ramp up to approximately 150°C within a target time of 90 seconds at a maximum rate of 2° to 3°C per second. Following the ramp, the profile soaks the assembly between 150°to 170°C within a target time of 90 seconds; the assembly should achieve thermal equilibrium by the completion of the soak zone. After the soak, the assembly will enter the spike area, where the assembly will be reflowed above 183°C for a target time of 60 (±15) seconds.

The entire profile should last between 3.5 and 4 minutes from 45°C to a peak temperature of 215°C (±5°C). The profile cool-down rate should be controlled within 4°C per second. In general, a faster cool-down rate will result in a finer grain structure and a stronger and shinier solder joint. However, exceeding 4°C per second could result in thermal shock.

Ramp-to-spike

The RTS profile may be used with any chemistry or alloy, and is preferred for use with water-soluble solder pastes and difficult-to-solder alloys and parts. If a large λT exists on the assembly, such as with processes using fixturization or those using an inefficient reflow oven, the RTS may not be the appropriate profile choice.

RTS has several advantages over the RSS profile. RTS generally will result in brighter and shinier joints and fewer problems with solderability, because the solder paste reflowed in a RTS profile keeps its flux vehicle throughout the preheat stage. This also will promote better wetting; therefore, the RTS should be used on difficult-to-wet alloys and parts. Because the RTS profile ramp rate is so controlled, there is much less chance of solder defects or thermal shock. In addition, the RTS profile is more economical because of the reduced heating energy used in the first half of the oven. Furthermore, troubleshooting the RTS tends to be a relatively simple process, and operators with experience troubleshooting the RSS profile should have no difficulty in adjusting the RTS profile to achieve optimum profiling results.

Setting Up the RTS Profile

As seen in Figure 2, the RTS profile is simply a gradual linear ramp from ambient to peak temperature. The RTS profile ramp zone serves as the preheat zone for the assembly, wherein the flux is activated, the volatiles are driven off, the assembly is prepared for reflow and thermal shock is prevented. The typical ramp rate for a RTS profile is 0.6° to 1.8°C per second. The first 90 seconds of the ramp should be kept as linear as possible.

A simple rule of thumb for the ramp rate of the RTS profile is that two-thirds of the profile should be below 150°C. After this temperature, the activation systems of most solder pastes begin to break down quickly. Therefore, keeping the front end of the profile cool will preserve the activator longer, resulting in better wetting and shinier solder joints.

The RTS profile spike zone is the stage where the assembly reaches the solder reflow temperature. After reaching 150°C, the peak temperature should be reached as quickly as possible. The peak temperature should be controlled at 215°C (±5°C), with time above liquidus (183°C) for 60 (±15) seconds. This time above liquidus will reduce flux entrapment and voiding, and increase pull strength. As with RSS, the RTS profile length should be a maximum of 3.5 to 4 minutes from ambient to peak temperature, and the cool-down rate should be controlled within 4°C per second.

Certain board coatings may require an increase in the profile peak temperature. If soldering to gold-over-nickel coated pads, a peak temperature of at least 220°C should be met; this will prevent post-reflow thermal reliability issues, as tin and gold form a secondary eutectic at 217°C. If soldering to pads coated with an organic surface protectant (OSP), peak temperatures of up to 225°C may be required to penetrate the coating completely. These peak-temperature adjustments are necessary if using either profile.

Troubleshooting the RTS Profile

The same rules are applied to troubleshooting the RSS and RTS profiles: Adjust the temperature or time at temperature of the profile as needed to achieve optimum results. Often, this requires trial and error, with slight temperature increases or decreases made and observing the results. The following is a summary of common reflow problems encountered with the RTS reflow profile and remedies to resolve them.

Solder Balling

Solder balling is recognized by numerous tiny solder balls trapped along the peripheral edge of the flux residue after reflow. In a RTS profile, this is usually the result of too slow a ramp rate, wherein metal oxidation occurs as a result of the flux vehicle being burned off ahead of reflow. This problem normally can be corrected by slightly increasing the profile`s ramp rate. Solder balling also may be a result of too rapid a ramp rate. This, however, is unlikely with the RTS profile, because of the relatively slow and steady ramp.

Solder Beading (Satellites)

Often confused with solder balling, solder beading is a defect recognized by one or a few larger balls, generally located around chip caps and resistors (Figure 3). Although this normally is the result of an excessive paste deposit during printing, it sometimes can be resolved with a profile adjustment. As with solder balling, solder beading that occurs during the RTS profile is normally a result of too slow a ramp rate. In this case, the slow ramp rate causes capillary action to draw the unreflowed paste away from the solder deposit to beneath the component. During reflow, the paste forms a solder bead that is squeezed out to the side of the component as the solder surface tension pulls the component to the board. As with balling, the solution to solder beading is to raise the ramp rate until the problem is resolved.

Poor Wetting

Poor wetting (Figure 4) often is the result of time and temperature ratios. The activators contained in solder pastes consist of organic acids that degrade with time and temperature. If a profile is too long, joint wetting can be compromised. Because paste activators normally survive longer with the RTS profile, poor wetting with this profile is less likely than with RSS. If poor wetting is experienced with the RTS profile, steps should be taken to ensure that the first two-thirds of the profile occur below 150°C. This will extend the life of the paste activator and result in improved wetting.

Solder Deficients

Solder deficients are often the result of uneven heating or an excessive heating ramp, which causes component leads to get too hot and solder to wick up the leads. The leads will appear thicker after the profile, and the pads will have an insufficient amount of solder. Reducing the ramp rate or otherwise ensuring the even heating of the assembly will help to prevent this defect.

Tombstoning

Tombstoning normally is the result of nonequal wetting forces, which causes a component to stand on end after reflow (Figure 5). In general, the slower the heating and more stable a board is, the less this will occur. Reducing the ramp rate as the assembly passes through 183°C will help to remedy this defect.

Voiding

Voiding is a defect found commonly with an X-ray or cross-section inspection of a solder joint. Voiding is recognized by the appearance of tiny "bubbles" in the joint (Figure 6). These may be air or flux entrapment. Voiding is generally caused by one of three profile errors: insufficient peak temperature, insufficient time at temperature or excess temperature during the ramp stage. As the RTS profile ramp rate is closely controlled, voiding normally is the result of the first or second error, which generally results in nonvolatized flux entrapment in the joint. To correct voiding in this case, a profile should be taken at the point where the voiding occurs and adjusted appropriately until the problem is resolved.

Dull and Grainy Joints

A relatively common reflow defect is dull and grainy joints (Figure 7). This defect may be merely aesthetic, but it could be the sign of a weak joint. To correct it in the RTS profile, the two zones before the spike should be reduced by 5°C; the peak temperature then should be raised by 5°C. If this is not successful, the temperature should continue to be adjusted slightly in this manner until the desired results are achieved. These adjustments will prolong the life of the paste activator, reducing the paste`s exposure to oxidation and improving wetting ability.

Charred Residue

Charred residues, although not necessarily a defect of functionality, may be experienced using the RTS profile. To correct this, the temperature or time of the spike zone may have to be reduced. If a recommended RTS profile is being followed, this normally is a simple matter of a slight (5°C) temperature decrease.

Conclusion

The RTS profile is not a cure-all for every reflow profile soldering issue. Also, the RTS profile cannot be used with all ovens or all assemblies. However, the implementation of the RTS profile can reduce energy costs, increase efficiency, reduce solder defects, improve wetting and simplify the reflow process. This is not to state that the RSS profile has become obsolete, or that the RTS profile never can be used with an older-style oven. However, engineers should be cognizant that there may be a better reflow profile style available.

ACKNOWLEDGEMENT

The author would like to thank Karl Seelig of AIM for his invaluable technical contributions to this article.

REFERENCE

1 All profiles pertain to the Sn63/Pb37 alloy, which has a eutectic melting point of 183°C.

DAVID SURASKI may be contacted at AIM, 25 Kenney Drive, Cranston, RI 02920; (401) 463-5605; E-mail: dsuraski@aimsolder.com; Web site: www.aimsolder.com.

Figure 1. A typical ramp-soak-spike reflow profile.

Figure 2. The typical ramp-to-spike reflow profile. Note a gradual linear ramp from ambient to peak temperature.

Figure 3. Solder beads can be seen next to these resistors.

Figure 4. Poor wetting can be caused by time and temperature ratios.

Figure 5. Nonequal wetting forces cause components to tombstone.

Figure 6. Voiding is caused by air or flux entrapment.

Figure 7. Dull and grainy joints can indicate a weak joint.