| Hello all, | | Our guys here still have some questions on the "optimized" profile which sees a more moderate ramp up straight to reflow temperatures versus the conventional profile which has a plateau at around 150 Celsius. If the optimized profile can really solve so many process problems, why was the convention profile used in the first place? What's your point of view on this issue. | | BTW, we're using a convection assisted IR oven, or if you look at it the other way around, an IR assisted convection oven. We can't seem to adjust to the new profile without sacrificing on throughput speed. Is this type of oven suitable for this? | | Thanks, | Joe |
In my opinion, the "soak Zone" profile that plateaus around 150 C is a throwback to the days of straight IR. That soak zone was needed to stabilize board temperatures before going into the spike zone. So all the paste manufacturers made pastes with fluxes that activated in this range. You'd hold for 60 to 90 seconds to activate the fluxes.
Then, one morning, someone woke up and siad "Hey, we got convection now. Why are we still heatsoaking?" and developed fluxes that don't need to dwell to work. And the benefits of this development are quicker throughput times and less thermal exposure to the assemblies.
So I went out and got me one of them "modern" pastes with the straight ramp profile. And in my convection ovens, my profile time dropped from about five minutes to about 3.5. The pastes in this "modern" category can ramp from ambient to peak at rates of 1 - 2 degrees C per second, which is the same ramp that you use in prehat and spike. So the new profile basically cuts out the soak zone.
I've realized higher throughput with the linear ramp. Boards that used to run at 80cm/sec belt speed are screaming through at 100 or 105 cm/sec in my 8-zone convection oven.
Now, about your oven. It is convection/IR assisted? This means IR panels and heat blowers? If the thermal transfer capability of the oven cannot heat the boards at reasonable belt speeds, then you are stuck with a soak zone. But on the bright side, you may be able to shorten the soak zone down to (mqaybe) 30 seconds instead of 60 to 90. And these newfangled pastes can take it. An Indium guy told me the flux in his SMQ92 will go up to 8 minutes without exhausing. So it doesn't matter what profile you use, although it does make a difference in the spread of residues.
So even if you can't run with the linear ramp, you can still upgrade your paste and reap all the printability, solderability, and testability benefits of this generation. And with a paste capable of linear ramps, you'll be able to cost-justify a new oven based on throughput increases.
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