AR# 34867


FF/FFG1136 - temperature differential between the package and the solder balls


According to theImplementation and Solder Reflow Guidelines for Pb-Free Packages Application Note (XAPP427), the temperature differential between the top surface of the component and the solder balls should be maintained at less than 7C during the critical region of the cooling phase of the reflow process. With larger devices like XC5VSX50T-1FFG1136C, the component's package is made of metal so it cools down very fast, but the cooling of the solder balls is relatively slow.
How do I achieve the requirements?


The biggest impact to the temperature differential is the belt speed and how high the peak temperature of their profile is.
If the reflow furnace has only a few reflow zones then transition to the cooldown zone goes from the peat temp reflow zone directly to a fan cooled zone with no heater.This is very difficult to manage an adequate reflow temperature differential.
If the peak temp is too high, this transition to the cooldown zone is even more difficult to manage because the temperature has to drop over a larger range.
The best way to manage the cooldown is to use a transition zone from peak temp to the normal cooldown zone.This can only be done of larger reflow furnaces.
This cooldown transition can be performed by shutting off all or a few of the cooldown fans. The fans drive cool air onto the board as soon as it enters the cooling zone and causes the top of the package to cool down too quickly.
AR# 34867
Date 12/15/2012
Status Active
Type General Article
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