3.3.1 True Time Above Liquidus (TAL). IPC IPC-7530A

IPC-7530A Draft Document for Industry Consensus Only August 2016

Figure 3-10 Curing Profile

Soak Time: The time is monitored to ensure temperature equilibrium across the assembly. This portion of the profile also allows time to drive off volatile ingredients within the solder paste and activate the paste’s flux to remove oxides.

CAUTION: Use of a soak zone in a reflow profile is helpful for reducing voids in BGA solder joints but may increase the incidence of HoP in BGAs. Ramp to peak profile should be considered to minimize HoP. which the solder alloy is liquidus. The assembly must spend enough time in this state to ensure all areas of the assembly reach soldering temperature. TAL for eutectic solder may be shorter than TAL for noneutectic solder since noneutectic alloys have both solidus and liquidus temperatures but eutectic solders have the same solidus and liquidus temperatures.

Peak/Spike Temperature: Peak temperature is the maximum temperature recorded by the thermocouple for the monitored location. Excessive temperatures could result in component and PWB damage.

Cure Period: Cure period is monitored to ensure proper curing of the adhesive.

3.3.1 True Time Above Liquidus (TAL)

It is important to understand the difference between TAL and true TAL to minimize incidence of Head on Pillow in

BGAs. See Figure 3-11 and Figure 3-12. When we attach thermocouples to the inner and out balls of BGA, it is common to find that the temperature of the outer row of BGA balls are higher than the temperature of inner row balls, resulting in time delay (LTD, Figure 3-11) between outer and inner row balls melting.

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IPC-7530A Draft Document for Industry Consensus Only August 2016

Figure 3-11 Role of Liquidus Time Delay in Head on Pillow

Note: True TAL (in green) is less than total TAL. Liquidus time delay is the time that some balls were not above liquidus temperature.

Figure 3-12 Standard TAL vs True TAL

Note: True TAL is less than TAL.

Another way to describe true TAL is the time at which every component, no matter how large, is above the melting point of solder. True TAL is also the time when all the points of measure are above liquidus at the same time, or the smaller number. This is critical, especially with large BGAs, in which outer balls have longer TAL and inner balls may have lower TAL.

It is only during this time that the component is free to self-align. Any one location's TAL does not measure the actual time the entire component is free to self-align, hence the need for true TAL measurement.

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