The experts have spoken well, but here is just one more thing to be aware of. Assemblies going through a reflow oven are like sponges, they soak up the heat at a rate based on their mass. Light, thin PWBs made of polyimide with a minimum of thin copper soak up relatively little heat, larger multilayer FR-4 PWBs soak up more heat. The PWBs you describe are massive, and with components added even more so. and they will soak up a LOT of heat when passing through the oven.
So keep this in mind: the more of these massive boards going through your oven, the more the thermal loading that is going to occur. So when creating your profile, you need to check it with just the one thermo-coupled board, and again with several more assemblies in front and behind the TC board. That way you can see just how much thermal loading your oven will undergo DURING PRODUCTION, and you can hopefully come up with a nominal profile somewhere in the center of this range. Run your test board first by itself, again with a full oven, and then average it out to get the best nominal profile settings.
You may have to run a limited number of assemblies during production in order for the oven to be able to stay in the nominal range. But I think any good 7-zone oven can handle it provided it is not overloaded with massive assemblies during production. I wish you good luck, but with good profiling you won't need it!
Richard D. Stadem
Advanced Engineer/Scientist, General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
You should be able to do this with a 7-zone oven if it is a pretty good convection oven. You may have to slow it down a bit. You can help get the heat in by going a little warmer on the early zones than you would normally do.
Due to the mass this assembly will likely heat slower and you will see a similar early profile to typical FR4 boards. Also note that it will take this one longer to cool down, so you will need to watch the back end of the profile to make sure you aren't too long on the duration.
These will be hotter than normal coming out of the oven so watch the handling. Be sure to get thermocouples on any solder joints on ground planes as they will likely heat slower than other solder joints.
Kevin Mobley
PCBA Engineering Liaison, General Atomics Electromagnetic Systems Group
Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.
A portion of the Solder Reflow Fundamentals class that I teach at SMTAI describes how to choose a starting recipe.
It lists the following methods:
- SWAG - based on skill, experience and luck;
- Start with a known recipe for a board and solder paste that is similar to the new one;
- Have someone else do it;
- Use predictive software that is included with many thermal profilers;
- Use software that is provided with some reflow ovens - such as BTU's Recipe Pro.
In all cases it is imperative that a thermal profile of the actual board as it goes through the oven is obtained to verify the results.
Fred Dimock
Manager, Process Technology, BTU International
Mr. Dimock is the manager of Process Technology at BTU International. His extensive experience in thermal processing includes positions at Corning, GE, and Sylvania. He has authored numerous articles on lead free processing and process control, taught classes at SMTAI, and participated in the IPC Reflow Oven Process Control Standard committee.
The heavy copper plane (0.040") will certainly act as a heat sink resulting in lower measured reflow temperatures than what would be expected from a typical (non-copper plane) circuit board. I recommend a slow reflow profile with approximately 5 minutes from ambient to peak temperature.
I also suggest running a soak type profile with 60-120 seconds of soak time between 150-200C for lead free soldering. It is important to place the profile thermocouples on cool and hot spots across the circuit board surface. Your solder paste supplier will be able to recommend reflow conditions and advise on what the solder paste can handle if it is necessary to run reflow outside of normal recommendations.
Tony Lentz
Field Applications, FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.
A thermal profile will be the best tool to help you find the right oven recipe.
One thing for sure, no matter the recipe you end up with, a thermal profile of the assembly will be needed to determine if you are reaching the required temperatures and times. The hardest part will be heating the copper core, since it has layers of insulation (PCB material), which will slow the heat flow to the inner core. Try a "standard recipe" to start with and run a thermal profile to see where you stand. Because the PCB layers of insulation will slow the temperature rise of the core, more than likely you will need to reduce the conveyor speed a bit to make up for the thermal flow slow down.
If decreased conveyor speed is not possible, increasing the convection fans or air flow/pressure may also help, if your oven has this capability. But remember, increased convection rate will NOT be as effective, because of the insulation layers over the core will still slow the heat flow to the copper core. Increasing the zone temperatures is your last resort, but this can begin to risk the temperature sensitivity of the component, which are exposed (no insulation) directly to the oven's heated zones.
Again, thermal profile this assembly after each oven recipe change to make sure you get it right.
Paul Austen
Senior Project Engineer, Electronic Controls Design Inc
Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.
The short answer is you will need to increase your zone temperatures and/or lengthen the heating time (slowing the conveyor speed). There are several thermal profiling packages that include software prediction features that can help you with developing a robust reflow recipe (like ECD's M.O.L.E. MAP software), and these would be a good place to start.
Knowing the copper weight of the PCB material is only a small part of the equation though, as the layers between the copper are insulators. Other variables to take into consideration are component densities and types, potentially thermally sensitive components, solder paste recommendations, your specific oven's ability to heat (regardless of zone counts), etc..
The only way to KNOW your thermal profile is correct is to measure it on an actual assembly. If you don't have a sacrificial assembly to use (ie: the customer won't allow or too costly), then you should attempt to replicate the assembly with like type board and components. This won't be exact, but at least this will be a better representative than a bare board.
Mark Waterman
M.O.L.E. Line Product Manager, Electronic Controls Design, Inc. (ECD)
Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.
You are right, given you have 7 zones, you have a fighting chance. You will most likely need to set up a Ramp-Soak-Spike profile, committing typically Zone 1/2 for Ramp section, Z3,4,5 for soak and Z6/7 for reflow. This allows you to drive a good amount of heat into the board in the Ramp, then time to Soak/equalize as best as possible minimizing any temperature differences across the assembly before reflowing. Selecting a paste with a generous working window would also help, if you have wildly differing thermal mass components or any temperature sensitive components it will be a challenge.
Ultimately you will need to perform some thermal profiling, if you are currently using linear ramp profiles you are going to have to make a best guess – otherwise start with a ramp soak spike shaped profile and go from there. It is likely you are going to have to slow the conveyor speed from what you normally use – this will allow temperature settings for the zone to be more achievable.
Adjacent zone in a reflow oven should not exceed typically < 40C, as the two zones will struggle to maintain differentials, this maximum zone delta value may vary oven to oven.
Your oven manufacturer/supplier is best person to suggest a starting point zone for temperature/speed – as they are most likely to have seen similar boards and all ovens perform differently - but it will only be a starting point.
In Solderstar profiling tools we have automatic prediction tools to help establish the best profile settings, once we have captured a profile from a test PCB and the oven – our software creates a mathematical model which can simulate oven and speed changes and drive towards more optimal solutions. We also have the ability to define individual process specifications for each measurement channel/components – so if you have a special/sensitive component we can define special limits for those areas.
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.
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