One of the most popular ways to determine if a thermal profile of an electronic assembly is within specification is to consider the limits placed on four measurements or parameters: Initial Ramp Slope, Soak time, Time Above Liquidus and Peak temperature. Keep these four parameters within the specified (solder paste) limits and you can be assured that you are soldering the parts without damaging them.
Figure 1: Typical thermal profile with the four traditional parameters within spec
There are other ways to look at a profile which can be helpful in determining if the profile may threaten components and showing if it is consistent, both across solder joints, and over time.
In the profile example above, the Time Above Liquidus (TAL)on solder joints 1 and 3 are within 2 seconds, yet channel 3 (from the data; plot not shown for visual clarity) had more readings at higher temperatures. This means that although this part may have the same time above 183ºC, more readings were at temperatures higher than channel 1; higher risk of damage. Also note that the peak temperatures were not far apart; 222.2ºC vs. 223.5ºC.
So we added a new measurement to the MAP software to not only show Time Above Liquidus, but also consider the temperature values during the TAL portion of the profile. This new measurement has several names: "Total Heat," "Area Under the Curve," or "Stress Integral." It combines the time element of Time Above Liquidus with the temperature measurements during that time to give the Total Heat the component experienced, expressed in degree-seconds.
Figure 2: Total Heat measurements (component 1 only shown for clarity)
In this case, even though the Time Above Liquidus values are within 2 seconds and the peak temperature is less than 2 degrees apart, the Total Heat values are 2278º-sec and 2628 º-sec which differ by 350 º-sec! This clearly points out that component 3 had to withstand more Total Heat than component 1 and this simple parameter can now be examined in an instant, using the latest; version 2.18j of MAP software.