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July 28, 2017

Lead-Free Reflow Soldering

We have a 1997 reflow oven which has 5 heating & 2 cooling zones.

We have gathered profiles from various paste manufacturers, but we don't have data on what temperature profiles should be set for heating and cooling zones?

For lead free soldering what should be our baseline for low/medium density boards?



H.M.

Experts Comments

This is a tough one without knowing more information about the variables including

--the details of the profile you are trying to match
--the heated length of the oven you are using
--The size of the heated zones
--The heat transfer properties of the oven
--etc, etc.

And rather than start a blog I would suggest using your current tin /lead profile parameters as a starting point. In a very broad sense, the lead free profile will be "similar" to the lead free profile in terms of residence time and shape but with higher peak temperatures. (Again, in a VERY broad sense).

So, starting with the profile you now have on the oven simply raise the zone temperatures an amount equal to the temperature differential between the two profiles. In other words, if the lead free profile wants to see a 240 degree peak and your current profile hits 220 deg C peak then raise the reflow zone temps by 20 degrees.

This will get you close without over temping the product. Then you can tweak from there based on the profile graph you see. This will also give you a feel for the heat transfer capability and relationship of setpoint to board temperature of the oven.

For example, If you raise the zone temp by 20 degrees and the peak board temperature goes up by 10 degrees, then you know that you need to raise by another 20 degrees to hit your desired peak temperature.

You may also want to think about getting a KIC or ECD or other predictive profiling system. With these systems you can input the target profile info and their software can give you some starting points. They are fairly good at this and worth the investment in cases like this.

If there are any questions or if more info is needed, please feel free to ask.

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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.

For most lead free reflow, 7 zones are required. This is not to say you cannot run with 5, but 7 are better.

Product density and layout have a lot to do with this, the more uniform the density is of the board, the less delta T there is to worry about, requiring fewer zones.

If you give me the make of your oven I can give you some starting set points for a medium density board. However if pallets are used or there is not a uniform lay out of the board, this may not be your final set up.

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Karl Seelig
Vice President of Technology
AIM
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

When you say you've gathered profiles from paste manufacturers, I assume you mean you have their data sheets with recommended profiles. That's a good starting point and it's one of the pieces of the puzzle required in order to set up your oven. But in order to establish your heating and cooling zone set temperatures, you need to run some profiles.

Attach some thermocouples to your assemblies and run them through the oven. It's the only way. For this, you will need something that will record time/temperature data of your assemblies as they pass through the oven. My biased opinion would be to purchase a profiler. A profiler will not only take the time and temperature date of the oven, but it will also analyze the data for you. It will tell you how you did with your slopes, soak, time above liquidus, peak temp and cooling zones.

Basic profilers start at less than $3,500 and will do predictions to help you through this process. Our more advanced Rapid Oven Setup System will automatically give you a starting point and fine tune the recipes for you.

You might ask why we just can't give you some zone set points to use. Well, this would be very dangerous. We have no idea how well your oven transfers heat nor how your assemblies would be affected by different heating rates or line speeds. And lead-free pastes have much tighter tolerances than lead-based pastes. So don't take any chances. Run some profiles.

Feel free to contact me directly at rburke@datapaq.com if I can be of any assistance.

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.

The profiles specified in the solder paste manufacturers datasheets cannot be used per say. These normally give the information on the process window that will give good soldering quality at product level. 

They are commonly specified as 3 curves on the same graph which shows the lowest, middle and fastest process times that can be used and also have the key parameters that must be adhere to also encoded within them. For example min/max. Time Above Liquidus time, soak time and heating rates.

Peak temperatures are normally specified separately. If curves are not used then normally the parameters and their working ranges are specified as tabular data.

It is not possible for the solder paste manufacturer to recommend reflow heater set points as oven vary widely in their performance and zone configuration, plus every electronic assembly has differing thermal mass and required careful profile tuning, especially when dealing with lead free.

The job of the process engineer is to establish these optimised settings for each electronic assembly processed on the reflow oven, you then may be able to rationalise the profiles established into a low/medium density scenario, assuming the profiles can be aligned within a common window of set points. This is mainly restricted by how widely your thermal masses vary from PCB to PCB and also how good your oven actually can perform. This was common practice with SnPb soldering as the process window was much wider and many assemblies could be grouped under one profile.

With Pb Free this is much more difficult; ultimately an engineering process of establishing the correct profile must be performed.

A number of tools are available to make this job a much quicker process, at SolderStar we produce a range of instruments specifically designed to pass through your reflow oven with your electronic assemblies and show you exactly what profile at product level is being achieved.

We also integrate Solder Paste libraries to allow the software to quickly show if your product profile is in or out of paste specification, if outside limits we have software simulation tools to allow the optimal reflow profile to be established without the need for continual trial and error.

To summarise, it is unlikely you will find heating & cooling zone recommendations for the reasons described above. If you know somebody using the same oven as yourselves then they of course could point you in the right direction if they have already moved over to the lead free process.

If you have any other questions, please feel free to contact me directly at 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.

The first thing to do is to determine whether or not your oven has the capacity to adequately reflow your PCBAs. Take your most complex PCBA (in terms of surface geometry and mass) and instrument it with thermocouples (high-temp soldered or conductive epoxied to the key solder joints - hottest and coldest points).

Choose one of the profiles from the data sheets you received from the lead-free solder paste manufacturers and profile the board. You will want to assure that all the TCs see a miinum of 225 deg C.

To accomplish this, what is the highest temperature your PCBA experienced (assuming you;'ve done your best to work to reduce the gradient across the board) ? What is the compononet that is most thermally vurnerable on the assembly? Did you exceed the maximum process temperature recommended by the component manufacturer or not?

Finally, assuming you've achieved a proper thermal profile, what speed is your conveyor now running and is it adequate to support the throughput from your pick and place machine?

Pay attention to the solder paste manufacturer's recommended rate of heating, too.

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Phil Zarrow
Principal Consultant
ITM Consulting
Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.
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