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Time above liquidus

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Time above liquidus | 9 May, 2007

Re: 63/37 paste reflow

Is there a standard reccomended time (ball park) above liquidus? What are the negative ramifications if the TAL is too short or too long?

Thanks in advance

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Time above liquidus | 10 May, 2007

30 to 90 seconds TAL is typical.

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Time above liquidus | 10 May, 2007

There is no standard for time above liquidus. It is described rather broadly by the solder paste supplier.

Reflow Zone: If the temperature is too high, boards may char or burn. If the temperature is too low, cold and grainy solder joints will result. Extended duration above the solder melting point will damage temperature sensitive components. It also results in excessive intermetallic growth that makes the solder joint brittle and reduces solder joint fatigue resistance. [R Prasad]

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Time above liquidus | 18 May, 2007

I have been TAL around 45 to 90 seconds. I prefer the longer TAL so, I set my profiles around 80 seconds. In the old days (Before RoSH), the longer times would help with wetting. But, since Lead-Free, well. We all know the story about wetting now. I just kept the longer TAL just out of habbit. Been working great for me.

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Time above liquidus | 25 May, 2007

TAL Too Long - dull solder joints, dewetting. TAL Too Short - cold solder, insufficient wetting.

You want TAL to be as short as possible (but not lower than 30 seconds), but long enough for every solder joint to be above TAL for at least 30 seconds. Variance in thermal mass of the board, and heating uniformity of the oven govern this. Essentially, the low mass stuff reflows first, and stays in TAL longer, while the opposite applies to the high mass areas. Best way to set up is the mount a few thermocouples to you high mass areas, and set the recipe to them, shooting for the 30 second TAL. All the low mass parts will reflow with a longer TAL. With some of the other themocouples on the low mass parts, you can make sure they do not exceed the 90 second TAL.

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Time above liquidus | 28 May, 2007

Look here:

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