Pb-Free Reflow Soldering

Versal ACAP Packaging and Pinouts Architecture Manual (AM013)

Document ID
AM013
Release Date
2020-07-16
Revision
1.0 English

Xilinx uses SnAgCu solder balls for commercial-grade (XC) and automotive-grade (XA) BGA packages. In addition, suitable material are qualified for the higher reflow temperatures (250°C maximum, 260°C for dry rework only) required by Pb-free soldering processes.

Xilinx does not support soldering SnAgCu BGA packages with SnPb solder paste using a Sn/Pb soldering process. Traditional Sn/Pb soldering processes have a peak reflow temperature of 220°C. At this temperature range, the SnAgCu BGA solder balls do not properly melt and wet to the soldering surfaces. As a result, reliability and assembly yields can be compromised.

The optimal profile must take into account the solder paste/flux used, the size of the board, the density of the components on the board, and the mix between large components and smaller, lighter components. Profiles should be established for all new board designs using thermocouples at multiple locations on the component. In addition, if there is a mixture of devices on the board, then the profile should be checked at various locations on the board. Ensure that the minimum reflow temperature is reached to reflow the larger components and at the same time, the temperature does not exceed the threshold temperature that might damage the smaller, heat sensitive components.

The following tables and figure provide guidelines for profiling Pb-free solder reflow. In general, a gradual, linear ramp into a spike has been shown by various sources to be the optimal reflow profile for Pb-free solders. This profile has been shown to yield better wetting and less thermal shock than conventional ramp-soak-spike profile for the Sn/Pb system. SnAgCu alloy reaches full liquidus temperature at 235°C. When profiling, identify the possible locations of the coldest solder joints and ensure that those solder joints reach a minimum peak temperature of 235°C for at least 10 seconds. Reflowing at high peak temperatures of 260°C and above can damage the heat sensitive components and cause the board to warp. Users should reference the latest IPC/JEDEC J-STD-020 standard for the allowable peak temperature on the component body. The allowable peak temperature on the component body is dependent on the size of the component. The table lists the peak package reflow body temperature information. In any case, use a reflow profile with the lowest peak temperature possible.

Table 1. Pb-Free Reflow Soldering Guidelines for Package Sizes Up to 45 mm x 45 mm
Profile Feature Convection, IR/Convection
Preheat ramp-up rate 30°–150°C 2°C/s maximum

1°C/s maximum for lidless packages with stiffener ring

Preheat temperature soak time 150°–200°C 60–120 seconds
Temperature maintained above 217°C 60–150 seconds (60–90 seconds typical)
Time within 5°C of actual peak temperature 30 seconds maximum
Peak temperature (lead/ball) 230°C–245°C typical (depends on solder paste, board size, component mixture)
Maximum peak temperature (body) 240°C–250°C, package body size dependent (see the specific Versal ACAP Data Sheet).
Ramp-down rate 2°C/s maximum
Time 25°C to peak temperature 3.5 minutes minimum, 5.0 minutes typical, 8 minutes maximum
Table 2. Pb-Free Reflow Soldering Guidelines for Package Sizes Greater than 45 mm x 45 mm and Up to 55 mm x 55 mm
Profile Feature Convection, IR/Convection
Preheat ramp-up rate 30°–150°C 0.5°C/s–1.5°C/s
Preheat temperature soak time 150°–190°C 65–70 seconds
Temperature maintained above 217°C 50–60 seconds
Maximum peak temperature (body) 234°C–238°C, package body size dependent (see the specific Versal ACAP Data Sheet).
Ramp-down rate 240°–125°C 1°C/s–2°C/s
Table 3. Pb-Free Reflow Soldering Guidelines for Package Sizes Greater than 55 mm x 55 mm
Profile Feature Convection, IR/Convection
Preheat ramp-up rate 30°–150°C 0.5°C/s–1.5°C/s
Preheat temperature soak time 150°–190°C 76–81 seconds
Temperature maintained above 217°C 77–93 seconds
Maximum peak temperature (body) 231°C–240°C, package body size dependent (see the specific Versal ACAP Data Sheet).
Ramp-down rate 240°–185°C 0.7°C/s–0.8°C/s
Ramp-down rate 185°–125°C 1.6°C/s–1.75°C/s
Figure 1. Typical Conditions for Pb-Free Reflow Soldering