In my previous post on this topic, Stages of a Reflow Profile: Part II, I mentioned a few profile-induced soldering defects, including graping, tombstoning, and voiding. In this post, we will talk about the time-above-liquidus (TAL) stage.During the reflow process, it is important to consider the time that the solder's temperature is above the liquidus temperature (time-above-liquidus or TAL) as this is the hottest point during the profile, and it is also where the intermetallic formation occurs between the solder and the substrates. If the TAL is too long, it can lead to de-wetting of the solder (solder wicking back due to over-oxidation) and flux exhaustion or charring. If the TAL is too short, it will not allow enough time for the solder and intermetallic formation to wet. An ideal TAL is between 45-75 seconds. The TAL should be proportionate to the peak temperature. For example, if the peak temperature of the profile is 245°C (a hotter peak temperature), then the TAL should be shorter, around 45 seconds. Conversely, for a cooler peak temperature, like 230°C, the TAL should be longer, closer to 75 seconds.
Stay tuned for the final stage of the reflow profile, the cool down.