Ioan,

   We use a kind of Universal reflow profile, where I can pass mixed
boards with same overal results. The trick is based on long pre heating
time ( 145 oC to 155 oC ) where all components temperatures achieve same
level prior peak reflowing, reducing in this way the peak difference on
last zone.This profile takes about 6 minutes inside oven ( seven zone
Conceptronics ) and you can use for a 150 components board or a 500
components board without changes.
   Sometimes is impossible follow solder paste recomendations for reflow
profile due to different heat transfer across components, the results
are big differences in peak temperature ( > 10 Celcius degree ) and some
colder joints can happen or component overheating , this occur at reflow
profiles of about 3 minutes.
   For high mix production the better is reduce the number of profiles
used to optimize line changeover and reduce mistakes from erroneous
profile load.
    If you have different solder paste, different boards this could be a
nigthmare.

Thanks

Jorge Dourado de Santana
Maintenance / Process Engr
Microtec - Brazil

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