There is a useful free maths model excel macro for reflow profiles that you can download from the ECD website, into which you plug the solder paste characteristics and oven specifications. Its a while since I used it, but as I recall some common types are already present to select in the model and you can play with these to get a feel for what will work or tweak these to suit, or even enter your own specs. You don't need an ECD mole to use this, its a stand alone utility.

< http://www.ecd.com/products/profileplanner/index.asp >

With lead-free there is no single recipe. As well as oven capability and paste used, it depends on the PCB thickness and thermal mass and the types of parts being soldered. Do some research and find any critical parts in the design with low maximum temperature, such as LEDs, electrolytics etc. and others with high thermal mass like BGAs or power semis. Then if you can take a sacrificial assembly with thermocouples attached to those parts and run a few times through the oven with slight variations to the oven recipe to balance out and ensure those parts are seeing the temperatures they need.

I'm sure others will have a favourite pb-f reflow recipe they know works for them, but a starting point might be:

ramp to 140C (2-4C/s), 
then 90-120s soak to 180C, 
then ramp to peak at 240C, with 20-40s max above 217C liquidus (assuming SAC paste) and max 60s above 200C
and then cool at about 4C/s

Hope this helps

Nigel Burtt
www.enjaybee.com

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