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July 25, 2017

Reflow oven profiles

What is the meaning of RAMP PROFILE & SOAK PROFILE in reflow soldering? What is the purpose of taking these two profiles?



V.V.

Experts Comments

A ramp profile also referred to as Ramp-to-Spike profile, is a reflow profile that increases to the reflow temperature in a near linear fashion. A soak profile also referred to as Ramp-Soak-Spike profile, ramps to a certain temperature at which point it levels off to a large degree before again ramping to the reflow temperature. 

The soak profile has traditionally been the most commonly used profile. It was influenced by the solder paste specifications that required a relatively long duration in a certain temperature range in order to activate the flux. This profile also tends to minimize the delta T across the product. The temperatures of all the areas of the product, which have different thermal mass, will have enough time for equalization during the soak period. The main disadvantage of the soak profile is that the PCB will potentially spend a long time above its glass transition temperature (Tg), thereby raising concerns of material degradation.

The ramp profile primarily came into existence to accommodate newer flux formulations, which are considerably free of volatile organic compound. These pastes do not require the extended soak time to dry the paste. The flux activation would be taking place even during the linear ramp to the reflow temperature.

The main objective behind either of the profiles is to ensure proper performance of the solder paste and process all areas of the product and components within their specifications. Establishing the correct process window to achieve this, and setting up the process within those specifications is critical.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.

To a large degree, this one goes back to the old days of IR (Infra Red) Reflow soldering. In those days, the machines did not have forced hot air and IR was the dominant heat source. IR panels or lamps were used and these heat sources had a tendency to create large thermal gradients across the board. This was especially true of large and densely populated boards where deltas could go as high as 25-30 ° C.

The soak portion of the profile was primarily used to allow the temperatures across the board to equilibrate. In other words, if you hold the product at one stable temperature for a while, the colder portions of the board will "catch up" in temperature and the ÄT will be reduced. And this worked to a degree. You would then spike up to reflow and while the ÄT would increase a bit during the spike, it was the best way to optimize the technology that was available at the time.

With the advent of forced hot air convection technology, the ÄT came down significantly and was fairly close throughout the board's excursion through the oven. Folks looking to increase their throughput realized that they could just go screaming right through the soak zone and straight up to reflow and knock off 20-40 seconds from their reflow profile while achieving the same soldering results.

In fact, in some ways the straight ramp or "tent" profile gave better results - as in some cases the extra time at temperature in soak zone depleted the flux. The tent profile reclaims that soak time and provides for better flux activity at reflow. 

And even today some boards are so densely populated and/or thick that the soak profile is still needed to overcome the ÄT associated with the particular product.

Certainly, the follow on question to this is: OK, so which profile is better or preferred?

And the answer is simple: whatever one your paste manufacturer tells you is better or preferred. 

(The flip side to that is – never listen to a Reflow oven vendor about what profile is best or preferred. Oven folks should only tell you that they can or cannot achieve whatever profile you need and that the oven has or doesn't have the flexibility to meet the requirements. Good soldering is about chemistry and metallurgy and time at temperature. Oven guys are not metallurgists or chemists and do not have enough intimate familiarity with all the fluxes and chemistry and metals used in of all the pastes out there to be a reliable source of info on "good" profiling)

The paste folks know their materials and how these materials will best react. They have done hours and hours of testing in their labs. Typically a paste spec sheet will have a recommended profile graph and that is the best starting point. From there you may find that your paste vendor can recommend certain tweaks or changes to the base graph to provide for greater speeds or improved results based on the particular application and results.

So work with the vendor of the paste for the starting point and recommendations.

(And for sure, once the target profile is established the oven guy CAN help you tweak the oven to meet the parameters so don't completely write off oven guy)

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Marc Peo
President
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

All reflow profiles can be simplified as 3 stages, RAMP/SOAK /SPIKE (or reflow).

The RAMP profile or RAMP TO SPIKE (RTS) as it is commonly referred to; is a profile that simply ramps from ambient temperature upto a peak temperature at a constant heating rate. In a RTS profile the heating rate is constant through each stage. I.e. the ramp or heating rate exhibited on the assembly by each zone of the reflow oven is the same, typically 0.5 to 1 Degrees C/Second.

The RAMP profile is very commonly specified in solder paste datasheets, but it cannot always be used. I will explain why.

If the assembly being soldered has a number of larger thermal mass components then it may be necessary to use a SOAK style profile, commonly referred to as the RAMP-SOAK-SPIKE (RSS) profile. In this style of profile we use different heating rates in the different stages of the profile.

If we use a simple RTS profile we may not transfer enough heat into the assembly to equalise the temperature through all thermal masses when we reach the final SPIKE or REFLOW stage. If we have a large variation of temperatures when we reach reflow we will have problems with establishing peak temperature & time above liquidus parameters within their given working range.

The difference between the coolest and hottest parts of the assembly when we reach the reflow stage is known in the profiling world as DeltaT.

To minimise this DeltaT we use the RAMP-SOAK-SPIKE profile, normally a faster heating rate is used in the RAMP phase, then the rate reduced in the SOAK phase, then increased in the SPIKE phase.

What we are doing is driving more heat into the assembly in the 1st stage of the profile, (being careful not to exceed the maximum rates of components etc.), we then SOAK the assembly at a slower rate to allow the assembly temperatures to equalise, equal temperatures = low DeltaT. When this is achieved we can reflow the assembly successfully within all given limits specified by the paste manufacturers.

I hope that's useful, the software supplied with SolderStar thermal profiling products makes this "black art" a breeze.

If you have any further questions, or wish some further reading on this subject, please contact me directly by email, marks@solderstar.com

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Mark Stansfield
Founder / Director
SolderStar Ltd
Co-founder and M.D of UK based thermal profiling equipment manufacturer SolderStar Ltd. He has software and electronic design experience specifically in the development of thermal profiling solutions for the electronics industry.

A Ramp profile is also referred to as a Linear or Tent profile. Its shape resembles a tent where it ramps directly to peak at a rate of about 2-2.5 degrees C per second and then slowly cools back to room temperature. This type of profile is becoming more common with todays modern convection ovens where an assembly equilibrium is easily achieved with the circulating hot air.

The Soak Profile (sometimes called the ramp, soak, spike profile) includes a zone of "leveling off" or "soaking" before the assembly goes into the spike or peak zone. This generally happens around 150-180 degrees C. The intention is to give the assembly a chance to reach an equilibrium across the board prior reflow. This profile was originally developed for IR ovens where higher mass areas of an assembly heated at a greatly different rate than low mass areas.

A ramp profile is much easier to set up than a soak profile. It takes a lot less oven manipulation. Experiment with both. If you can get away with a ramp profile, run with it.

Richard Burke
National Sales Manager
Datapaq
Mr Burke currently has eight years of thermal profiling experience in the Electronics Assembly industry including SMT, Wave, Curing, Wafer Bumping, Ceramics and a host of other thermal processes. He is a Graduate of Indiana University of Pennsylvania School of Business.
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